+
21212121
BK PRECISION
®
INSTRUCTION
MODELS 2120B
MANUAL
and 2125A
30 MHz DUAL-TRACE OSCILLOSCOPES
+
®
BK PRECISION
TEST INSTRUMENT SAFETY
WARNING
Normal use of test equipment exposes you to a certain amount of danger from electrical shock because
testing must often be performed where exposed high voltage is present. An electrical shock causing
10milliampsofcurrentto pass throughtheheart will stop most human heartbeats. Voltageas low as35 volts
dc or ac rms should be considered dangerous and hazardous since it can produce a lethal current under
certain conditions. Higher voltage poses an even greater threat because such voltage can more easily
produce a lethal current. Your normal work habits should include all accepted practices that will prevent
contact with exposed high voltage, and that will steer current away from your heart in case of accidental
contact with a high voltage. You will significantly reduce the risk factor if you know and observe the
following safety precautions:
1. Don’t expose high voltage needlessly in the equipment under test. Remove housings and covers only when necessary.
Turn off equipment while making test connections in high-voltage circuits. Discharge high-voltage capacitors after
removing power.
2. If possible, familiarize yourself with the equipment being tested and the location of its high voltage points. However,
remember that high voltage may appear at unexpected points in defective equipment.
3. Use an insulated floor material or a large, insulated floor mat to stand on, and an insulated work surface on which to
place equipment; make certain such surfaces are not damp or wet.
4. Use the time-proven “one hand in the pocket” technique while handling an instrument probe. Be particularly careful to
avoid contacting a nearby metal object that could provide a good ground return path.
5. When using a probe, touch only the insulated portion. Never touch the exposed tip portion.
6. When testing ac powered equipment, remember that ac line voltage is usually present on some power input circuits such
as the on-off switch, fuses, power transformer, etc. any time the equipment is connected to an ac outlet, even if the
equipment is turned off.
7. Some equipment with a two-wire ac power cord, including some with polarized power plugs, is the “hot chassis” type.
This includes most recent television receivers and audio equipment. A plastic or wooden cabinet insulates the chassis
toprotect the customer. When the cabinet is removed for servicing, a serious shock hazard exists if the chassis is touched.
Not only does this present a dangerous shock hazard, but damage to test instruments or the equipment under test may
result from connecting the ground lead ofmost test instruments (includingthis oscilloscope) to a “hot chassis”. To make
measurements in “hot chassis” equipment, always connect an isolation transformer between the ac outlet and the
equipment under test. The B+K Precision Model TR-110 or 1604A Isolation Transformer, or Model 1653A or 1655A
AC Power Supply is suitable for most applications. To be on the safe side, treat all two wire ac powered equipment as
“hot chassis” unless you are sure it has an isolated chassis or an earth ground chassis.
8. Never work alone. Someone should be nearby to render aid if necessary. Training in CPR (cardio-pulmonary
resuscitation) first aid is highly recommended.
Instruction Manual
22820 Savi Ranch Parkway, Yorba Linda, CA 92887
for
+
BK PRECISION
®
Models 2120B,
2121 and 2125A
30 MHz
Dual-Trace Oscilloscopes
©2002 B+K Precision Corp.
This symbol on oscilloscope means “refer to instruction manual
for further precautionary information”. This symbol appears in the
manual where the corresponding information is given.
+
®
TABLE OF CONTENTS
Page Page
TEST INSTRUMENT SAFETY . . . . . . inside front cover
FEATURES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
SPECIFICATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
CONTROLS AND INDICATORS . . . . . . . . . . . . . . . . . . 7
General Function Controls . . . . . . . . . . . . . . . . . . . . . . 7
Vertical Controls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Horizontal Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Triggering Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Rear Panel Controls . . . . . . . . . . . . . . . . . . . . . . . . . . 10
OPERATING INSTRUCTIONS. . . . . . . . . . . . . . . . . . . 11
Safety Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Equipment Protection Precautions. . . . . . . . . . . . . . . 11
Operating Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Initial Starting Procedure . . . . . . . . . . . . . . . . . . . . . . 12
Single TraceDisplay . . . . . . . . . . . . . . . . . . . . . . . . . 12
Dual Trace Display. . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Triggering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Magnified Sweep Operation . . . . . . . . . . . . . . . . . . . 15
OPERATING INSTRUCTIONS (Continued)
X−Y Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Video Signal Observation . . . . . . . . . . . . . . . . . . . . . 15
Application Guidebook. . . . . . . . . . . . . . . . . . . . . . . 15
Delayed Sweep Operation. . . . . . . . . . . . . . . . . . . . . 15
Component Test Operation . . . . . . . . . . . . . . . . . . . . 16
MAINTENANCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Fuse Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Line Voltage Selection. . . . . . . . . . . . . . . . . . . . . . . . 20
PeriodicAdjustments. . . . . . . . . . . . . . . . . . . . . . . . . 20
Calibration Check . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Instrument Repair Service. . . . . . . . . . . . . . . . . . . . . 21
APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Important Considerations for Rise Time and
Fall Time Measurements . . . . . . . . . . . . . . . . . . . . 22
2121 Specifications & Operating Instruction .. . .. . ..23
Service Information. . .. . .. . .. . .. . .. . .. . .. . .. . ..............26
“Guidebook To Oscilloscopes”
Availability. .. . .. . .. . .. . .. . .. . ..insideback cover
2
FEATURES
LOW COST,HIGH PERFORMANCE
B+K Precision’sentry leveloscilloscopes, Models 2120B
and 2125A, are economically priced to equal the competition, but offer higher performance than the competition.For
example, most competitor’sentry level oscilloscopes have a
20 MHz bandwidth, while B+K Precision’s Models 2120B
and 2125A offer a 30 MHz bandwidth. These oscilloscopes
are built by and backed by B+K Precision, a company that
has been selling reliable, durable, value priced test instruments for over 50 years.
CRT FEATURES
Rectangular CRT
Rectangular CRTwith large 8 x 10 centimeterviewing
area. On Model 2125A, graticule is equipped with
variable scale illumination.
Convenience
Tracerotation electrically adjustable from front panel.
0%, 10%, 90%, and 100% markersfor rise time measurements.
DUAL TRACEFEATURES
Dual Trace
Models2120Band2125Aeachhavetwovertical input
channels for displaying two waveforms simultaneously. Selectable single trace (either CH 1 or CH 2) or
dual trace. Alternate or chop sweep selectable at all
sweep rates.
Sum and Difference Capability
Permits algebraic addition or subtraction of channel 1
and channel 2 waveforms, displayedas a single trace.
Useful for differentialvoltageand distortion measurements.
HIGH FREQUENCY FEATURES
Wide Bandwidth
Conservatively-rated −3 dB bandwidth is dc to 30
MHz.
Fast Rise Time
Rise time is less than 12 ns.
Fast Sweep
Maximum sweep speed of 10 ns/div(with X10 MAG)
assures high frequencies and short-duration pulses are
displayed with high resolution.
VERTICAL FEATURES
High Sensitivity
5 mV/div sensitivityfor full bandwidth. High-sensitivity 1 mV/div and 2 mV/div using PULL X5 gain
control.
Calibrated VoltageMeasurements
Accurate voltage measurements (±3%) on 10 calibrated ranges from 5 mV/div to 5 V/ div. Vertical gain
fully adjustable between calibrated ranges.
SWEEP FEATURES
Calibrated Time Measurements
Accurate (±3%) time measurements. The main sweep
has 23 calibrated ranges from 2 S/div to 0.1 µS/div.
The delayed sweep on the Model 2125A has 23 calibrated ranges from 2 S/div to 0.1 µS/div. Sweep time
is fully adjustable between calibrated ranges.
X10 Sweep Magnification
Allows closer examination of waveforms, increases
maximum sweep rate to 10 nS/div.
DUAL TIME BASE FEATURES (Model 2125A)
Dual Sweep Generators
Main sweep gives normal waveform display, delayed
sweepmaybe operated atfastersweep speedtoexpand
a portion of the waveform.
Four Sweep Modes
Choice of main sweep only, delayed sweep only,
main sweep and delayed sweep sharing the trace
(percentage of main /del aye d swee p adjustable), or
X−Y.
Adjustable Start Of Delayed Sweep
DELAYTIME POSition control allows adjustment of
delayed sweep starting point.
TRIGGERING FEATURES
Two TriggerModes
Selectable normal (triggered) or automatic sweep
modes.
Triggered Sweep
Sweep remains at restunless adequate trigger signal is
applied. Fully adjustable trigger level and (+) or (−)
slope.
3
FEATURES
AUTO Sweep
Selectable AUTO sweep provides sweep without trigger input, automatically reverts to triggered sweep
operation when adequate trigger is applied.
Five TriggerSources
Five trigger source selections, including CH 1, CH 2,
alternate, EXT, and LINE.
Video Sync
Frame(TV V) or Line (TV H) triggering selectablefor
observingcomposite video waveforms.TV-Hposition
can also be used as low frequency reject and TV-V
position can be used as high frequency reject.
Variable Holdoff
Trigger inhibit period after end of sweep adjustable.
Permits stable observation of complex pulse trains.
OTHER FEATURES
X−Y Operation
Channel 1 can be applied as horizontal deflection
(X-axis) while channel 2 provides vertical deflection
(Y-axis).
Built-in Probe Adjust Square Wave
A 2 V p-p, 1 kHz square wave generator permits probe
compensation adjustment.
Component TestFunction (Model 2125A)
Built-inX−Ytypecomponenttesterapplies fixedlevel
ac signal to components for display of signature on
CRT.
Channel 2 (Y) Output (Model 2125A)
A buffered 50Ω output of the channel 2 signal is
available at the rear panel for driving a frequency
counter or other instruments. The output is 50 mV/div
(nominal) into 50Ω.
Z-Axis Input (Model 2125A)
Rear panel Z-Axis input allows intensity modulation.
Supplied With Two Probes
4
SPECIFICATIONS
CRT:
Type: 6-inch rectangular with integral graticule,
P31 phosphor.
DisplayArea:8 x 10 div (1 div = 1 cm).
AcceleratingVoltage:2kV.
Phosphor:P31.
Trace Rotation: Electrical, front panel adjustable.
Scale Illumination: Continuously variable (Model 2125A).
Beam Finder (Model 2125A).
VERTICAL AMPLIFIERS (CH 1 and CH 2)
Sensitivity: 5 mV/div to 5 V/div, 1 mv/divto 1 V/divat
X5 MAG.
Attenuator:10 calibrated steps in 1-2-5 sequence. Vernier
control provides fully adjustable sensitivity between
steps; range 1/1 to at least 1/3.
Accuracy:±3%, 5 mV to 5 V/div;5%, at X5 MAG.
Input Resistance:1MΩ±2%.
Input Capacitance: 25 pF ±10 pF.
Frequency Response:
5 mV/div to 5 V/div:
DC to 30 MHz (−3 dB).
X5 MAG:
DC to 10 MHz (−3 dB).
Rise Time:
12 nS; 35 nS at X5 MAG.
Overshoot:Less than 5%.
Operating Modes:
CH 1: CH 1, single trace.
CH 2: CH 2, single trace.
DUAL: CH 1 and CH 2, dual trace.
Alternate or Chop selectable at
any sweep rate.
ADD: Algebraic sum of CH 1 + CH 2.
Chop Frequency:Approximately 500 kHz.
PolarityReversal:CH 2 invert.
MaximumInput Voltage:400 V (dc + ac peak).
HORIZONTAL AMPLIFIER
(Input through channel 1 input)
X−Y mode:
CH 1 = X axis.
CH 2 = Y axis.
Sensitivity: Same as vertical channel 2.
Input Impedance: Same as vertical channel 2.
Frequency Response:
DC to 1 MHz (−3 dB).
X-Y Phase Difference: 3° or less at 50 kHz.
MaximumInputVoltage: Same as verticalchannel 1.
SWEEP SYSTEM
Operating Modes:
Model 2125A: Main, Mix (both main and delayed
sweep displayed), Delay (only delayed sweep
displayed), X−Y.
Model 2120B: Main only.
Main Time Base: 0.1 µS/div to 2.0 S/div in 1-2-5
sequence,23 steps. Vernier control provides fully
adjustablesweep time betweensteps.
DelayedTime Base (Model 2125A only): 0.1 µS/div to
2.0 S/div in 1-2-5 sequence, 23 steps.
Accuracy:±3%, except ±6% on 0.2 S/div and ±20% on
0.1 µS/div.
Sweep Magnification: X10 ±10%.
Holdoff: Continuously adjustable for main time base from
NORM to 5 times normal.
Delay Time Position: Control sets percentageof display
that is devoted to main and delayed sweep.
Delay Jitter: 1/10,000 of full scale sweep time.
TRIGGERING
Trigger Modes:
AUTO (free run), NORM, TV-V, TV-H.
Trigger Source:
CH 1, CH 2, Alternate, EXT, LINE.
Slope:
(+) or (–).
5
SPECIFICATIONS
Trigger Coupling:
AUTO: Sweep free-runs in absence of
suitable trigger signal.
NORM: Sweep triggered only by adequate
trigger signal.
TV-V: Video vertical sync pulses are
selected. Also usable for high
frequency reject.
TV-H: Video horizontal sync pulses are
selected. Also usable for low
frequency reject.
Trigger Sensitivity:
Auto: 1.5 div (internal)
≥0.5 Vp-p (external)
100 Hz – 40 MHz (2125A)
100 Hz – 30 MHz (2120B)
Norm: 1.5 div (internal)
≥0.5 Vp-p (external)
100 Hz – 40 MHz (2125A)
DC – 30 MHz (2120B)
TV-V: 1.0 div (internal)
≥0.5 Vp-p (external)
DC – 1 kHz (2125A)
20 Hz – 1 kHz (2120B)
TV-H: 1.0 div (internal)
≥0.5 Vp-p (external)
1 kHz– 100 kHz
MaximumExternalTriggerVoltage: 300 V (dc + ac
peak).
COMPONENTTESTER (Model 2125A)
ComponentsTested: Resistors, capacitors, inductors, and
semiconductors.
TestVoltage: 6 V rms maximum(open).
TestCurrent:11 mA maximum (shorted).
TestFrequency:Line frequency (60 Hz in USA).
OTHER SPECIFICATIONS
Cal/ProbeCompensation Voltage:2 V p-p ±3% square
wave,1 kHz nominal.
CH 2 (Y) Output (Model 2125A):
Output Voltage: 50 mV/div (nominal into 50 ohm
load).
Output Impedance: Approximately 50 ohms.
Frequency Response:20 Hz to 30 MHz, −3 dB.
IntensityModulation (Model 2125A)
Input Signal: TTL level,intensity increasing with
more positive levels, decreased intensity with more
negative levels.
Input Impedance: Approximately 50 kΩ.
Usable Frequency Range: DC to 5 MHz.
Maximum Input Voltage: 30 V (dc + ac peak).
Power Requirements: 100–130 VAC or 200–260 VAC,
50/60 Hz, 38 watts.
Dimensions (H 3 W 3 D):
5.2″ 3 12.8″3 15.7″
(132 3 324 3 398 mm).
Weight: 16.8 lbs (7.6 kg).
Environment:
Within Specified Accuracy: +10° to +35° C, 10–80%
relative humidity.
Full Operation: 0° to +50° C, 10–80% relative
humidity.
Storage: −30° to +70° C, 10–90% relative humidity.
ACCESORIES SUPPLIED:
Two Switchable X1/X10 Probes.
Instruction Manual.
AC Line Cord.
6
18
17
6
1
5
2
4
33
32
31
30
28
27
26
25
24
34
13
14
15
7
8
16
10
12
11
19
20
9
21
22
23
CONTROLS AND INDICATORS
18
17
6
4
1
13
5
3
14
15
16
9
33
12
32
10
31
19
30
20
21
28
22
27
26
33
34
Fig. 2. Model 2120B Controls and Indicators.
ADD:
The inputs from channel 1 and channel 2 are
summed and displayed as a single signal. If the
Channel 2 POSition/PULL INVert control is
pulled out, the input from channel 2 is subtracted
from channel 1 and the difference is displayed as a
single signal.
13. CH1 AC-GND-DC Switch. Three-position lever
switch with the following positions:
AC:
Channel 1 input signal is capacitively coupled; dc
component is blocked.
GND:
Opens signal path and grounds input to vertical
amplifier. This provides a zero-volt base line, the
position of which can be used as a reference when
performing dc measurements.
DC:
Direct coupling of channel 1 input signal; both ac
and dc components of signal produce vertical de-
flection.
14. CH1 (X) Input Jack. Vertical input for channel 1.
X-axis input for X-Y operation.
15. CH1 (X) VOLTS/DIV Control. Vertical attenuator
for channel 1. Provides step adjustment of vertical
sensitivity. When channel 1 VARiable control is set
to CAL, vertical sensitivity is calibrated in 10 steps
from 5 mV/div to 5 V/div in a 1-2-5 sequence. When
the X-Y mode of operation is selected, this control
provides step adjustment of X-axis sensitivity.
16. CH1 VARiable/PULL X5 MAG Control:
VARiable:
Rotation provides vernier adjustment of channel 1
vertical sensitivity. In the fully-clockwise (CAL)
position, the vertical attenuatoris calibrated. Counterclockwise rotation decreases gain sensitivity. In
X-Y operation, this control becomes the vernier
X-axis sensitivity control.
PULL X5 MAG:
When pulled out, increases vertical sensitivity by a
factor of five. Effectively provides two extra sensitivity settings: 2 mV/div and 1 mV/div. In X-Y
mode, increases X-sensitivity by a factor of five.
17. CH1 POSition/PULL ALT TRIGger Control:
POSition:
Adjusts vertical position of channel 1 trace.
PULL ALT:
Used in conjunction with the Trigger SOURCE
switch to activate alternate triggering. See the description under the Trigger SOURCE switch.
18. CH2 POSition/PULL INVert Control:
POSition:
Adjusts vertical position of channel 2trace.In X-Y
operation, rotation adjusts vertical position of X-Y
display.
8
CONTROLS AND INDICATORS
PULL INVert:
When pushed in, the polarity of the channel 2 signal
is normal. When pulled out, the polarity of the
channel 2 signal is reversed, thus inverting the
waveform.
19. CH2 VOLTS/DIV Control. Vertical attenuator for
channel 2. Provides step adjustment of vertical sensitivity. When channel 2 VARiable control is set to
CAL, vertical sensitivity is calibratedin 10 steps from
5 mV/div to 5 V/div in a 1-2-5 sequence. When the
X-Y mode of operation is selected, this control provides step adjustment of Y-axis sensitivity.
20. CH2 VARiable/PULL X5 MAG Control:
VARiable:
Rotation provides vernier adjustment of channel 2
vertical sensitivity. In the fully-clockwise (CAL)
position, the vertical attenuatoris calibrated. Counterclockwise rotation decreases gain sensitivity. In
X-Y operation, this control becomes the vernier
Y-axis sensitivity control.
PULL X5 MAG:
When pulled out, increases vertical sensitivity by a
factor of five. Effectively provides two extra sensitivity settings: 2 mV/div and 1 mV/div. In X-Y
mode, increases Y-sensitivity by a factor of five.
21. CH2 (Y) Input Jack. Vertical input for channel 2.
Y-axis input for X-Y operation.
22. CH2 AC-GND-DC Switch. Three-position lever
switch with the following positions:
AC:
Channel 2 input signal is capacitively coupled; dc
component is blocked.
GND:
Opens signal path and grounds input to vertical
amplifier. This provides a zero-volt base line, the
position of which can be used as a reference when
performing dc measurements.
DC:
Direct coupling of channel 2 input signal; both ac
and dc components of signal produce vertical deflection.
HORIZONTAL CONTROLS
23. Main Time Base TIME/DIV Control. Provides step
selection of sweep rate for the main time base. When
theVARiableSweep control is settoCAL,sweeprate
iscalibrated.This control has23steps,from 0.1 µS/div
to 2 S/div, in a 1-2-5 sequence.
24. 2125A Only. DELAY Time Base TIME/DIV Control. Provides step selection of sweep rate for delayed
sweep time base. This control has 23 steps, from 0.1
µS/div to 2 S/div, in a 1-2-5 sequence.
25. 2125A Only. DELAY TIMEPOSitionControl. Sets
starting point of delayed sweep. Clockwise rotation
causes delayed sweep to begin earlier.
26. VARiable Sweep Control. Rotation of control is ver-
nier adjustment for sweep rate. In fully clockwise
(CAL) position, sweep rate is calibrated. On the
Model 2125A, this control is the vernier adjustment
for both the main and delayed time bases.
27. POSition/PULL X10 MAG Control.
POSition:
Horizontal (X) position control.
PULL X10 MAG:
Selects ten times sweep magnification when pulled
out, normal when pushed in. Increases maximum
sweep rate to 10 nS/div.
28. 2125A Only. Sweep Mode Switch. Selects sweep
(horizontal) mode. Four-position rotary switch with
the following positions:
MAIN:
Only the main sweep operates, with the delayed
sweep inactive.
MIX:
The main and delayed sweep share a single trace;
main sweep occupies the left portion of thedisplay;
delayed sweep occupies the right portion of the
display. The DELAY TIME POSition control de-
termines the percentage of display that is main
sweep and the percentage of display that is delayed
sweep (main sweep is usually brighter than the
delayed sweep). Delayed sweep speed cannot be
slower than main sweep speed.
DELAY:
Only delayed sweep operates, while main sweep
stays inactive. DELAY TIME POSition control
determines the starting point of the delayed sweep.
X-Y:
Used with the VERTicalMODE switch and Trigger SOURCE switch to sel ect X-Y operating
mode. The channel 1 input becomes the X-axis and
the channel 2 input becomes the Y-axis. Trigger
source and coupling are disabled in this mode.
29. 2120B Only. X-Y Switch. Used with the VERTical
MODE switch and Trigger SOURCE switch to se-
lect X-Y operating mode. The channel 1 input becomes the X-axis and the channel2 input becomes the
Y-axis. Trigger source and coupling are disabled in
this mode.
9
CONTROLS AND INDICATORS
TRIGGERING CONTROLS
30. HOLDOFF/PULL CHOP Control.
HOLDOFF:
Rotation adjusts holdoff time (trigger inhibit period
beyond sweep duration). When control is rotated
fully counterclockwise, the holdoff period is MIN-
inum (normal). The holdoff period increases progressively with clockwise rotation.
PULL CHOP:
When this switch is pulled out in the dual-trace
mode, the channel 1 and channel 2 sweeps are
chopped and displayed simultaneously (normally
used at slower sweep speeds). When it is pushed in,
the two sweeps are alternately displayed, one after
the other (normally used at higher sweep speeds).
31. Trigger SOURCE Switch. Selects source of sweep
trigger. Four-position lever switch with the following
positions:
CH1/X-Y/ALT
CH1:
Causes the channel 1 input signal to become the
sweep trigger, regardless of the VERTical
MODE switch setting.
X-Y:
Used with two other switches to enable the X-Y
mode — see the Operating Instructions under
“XY Operation”.
ALT:
Used with the channel 1 POSition/PULL
ALTernate TRIGger control to enable alternate
triggering. Alternate triggering, used in dualtrace mode, permits each waveform viewed to
become its own trigger source.
CH2:
The channel 2 signal becomes the sweep trigger,
regardless of the VERTicalMODE switch setting.
LINE:
Signal derived from input line voltage (50/60 Hz)
becomes trigger.
EXT:
Signal from EXTernal TRIGger jack becomes
sweep trigger.
32. Trigger COUPLING Switch. Selects trigger cou-
pling. Four-position lever switch with the following
positions:
AUTO:
Selectsautomatic triggering mode. Inthismode,the
oscilloscope generates sweep (free runs) inabsence
of an adequate trigger; it automatically reverts to
triggeredsweepoperationwhenanadequatetrigger
signal is present. On the Model 2125A, automatic
triggering is applicable to both the main sweep and
delayed sweep.
NORM:
Selects normal triggered sweep operation. A sweep
is generated only when an adequatetrigger signal is
present.
TV-V:
Used for triggering from television vertical sync
pulses. Also serves as lo-pass/dc (high frequency
reject) trigger coupling.
TV-H:
Used for triggering from television horizontal sync
pulses.Alsoserves as hi-pass(lowfrequency reject)
trigger coupling.
33. TRIGger LEVEL/PULL (-) SLOPE Control.
TRIGger LEVEL:
Trigger level adjustment; determines the point on
the triggering waveform where the sweep is triggered. Rotation in the (-) direction (counterclockwise) selects more negative triggering point;
rotation in the (+) direction (clockwise) selects
more positive triggering point.
PULL (—) SLOPE:
Two-position push-pull switch. The “in” position
selectsapositive-goingslope and the “out” position
selectsa negative-goingslopeas triggeringpointfor
main sweep.
34. EXTernal TRIGger Jack. External trigger input for
single- and dual-trace operation.
REAR PANEL CONTROLS (not shown)
35. Fuse Holder/Line Voltage Selector. Contains fuse
and selects line voltage.
36. Power Cord Receptacle.
37. 2125A Only. CH 2 (Y) SIGNAL OUTPUT Jack.
Output terminal where sample of channel 2 signal is
available.Amplitudeofoutput is nominally 50 mV per
division of vertical deflection seen on CRT when
terminated into 50 Ω. Output impedance is 50 Ω.
38. 2125A Only. Z-Axis Input Jack. Input jack for inten-
sity modulation of CRT electron beam. TTL compatible (5 V p-p sensitivity). Positive levels increase
intensity.
39. Handle/Tilt Stand.
40. Feet/Cord Wrap.
10
OPERATING INSTRUCTIONS
NOTE
All operating instructions in this chapter
apply equally to both Models 2125A and
2120B, exceptforthesectionson “Delayed
Sweep Operation” and “Component Test”,
which apply only to the Model 2125A.
Other differences are noted when necessary.
SAFETY PRECAUTIONS
WARNING
The following precautions must be observed to help prevent electric shock.
1. When the oscilloscope is used to make measurements
in equipment that contains high voltage, there is always acertainamountofdangerfromelectricalshock.
The person using the oscilloscope in such conditions
should be a qualified electronics technician or otherwise trained and qualified to work in such circumstances. Observe the TEST INSTRUMENT SAFETY
recommendations listed on the inside front cover of
this manual.
2. Donotoperatethisoscilloscopewith the case removed
unless you are a qualified service technician. High
voltage up to 2100 volts is present when the unit is
operating with the case removed.
3. The ground wire of the 3-wire ac power plug places
the chassis and housing of the oscilloscope at earth
ground. Use only a 3-wire outlet, and do not attempt
to defeat the ground wire connection or float the oscilloscope; to do so may pose a great safety hazard.
4. Specialprecautionsarerequiredtomeasureorobserve
line voltage waveforms with any oscilloscope. Use the
following procedure:
a. Do not connect the ground clip of the probe to
either side of the line. The clip is already at earth
ground and touching it to the hot side of the line
may “weld” or “disintegrate” the probe tip and
cause possible injury, plus possible damage to the
scope or probe.
b. Insert the probe tip into one side of the line voltage
receptacle, then the other. One side of the receptacleshouldbe“hot” and produce thewaveform.The
other side of the receptacle is the ac return and no
waveform should result.
EQUIPMENT PROTECTION
PRECAUTIONS
Thefollowing precautionswillhelpavoid
damage to the oscilloscope.
1. Never allow a small spot of high brilliance to remain
stationary on the screen for more than a few seconds.
The screen may become permanently burned. A spot
will occur when the scope is set up for X−Y operation
and no signal is applied. Either reduce the intensity so
the spot is barely visible, apply signal, orswitch back
to normal sweep operation. It is also advisable to use
low intensity with AUTO triggering and no signal
applied for long periods. A high intensity trace at the
same position could cause a line to become permanently burned onto the screen.
2. Do not obstruct the ventilating holes in the case, as this
will increase the scope’s internal temperature.
3. Excessive voltage applied to the input jacksmay damage the oscilloscope. The maximum ratings of the
inputs are as follows:
CH 1 and CH 2:
400 V dc + ac peak.
EXT TRIG:
300 V dc + ac peak.
Z-AXIS INPUT (Model 2125A):
30 V ( dc and ac peak).
4. Always connect a cable from the ground terminal of
the oscilloscope to the chassis of the equipment under
test. Without this precaution, the entire current for the
equipment under test may be drawn through the probe
clip leads under certain circumstances. Such conditionscouldalsoposea safety hazard, which theground
cable will prevent.
5. The probe ground clips are at oscilloscope and earth
ground and should be connected only to the earth
ground or isolated common of the equipment under
test. To measure with respect to any point other than
the common, use CH 2 – CH 1 subtract operation
(ADD mode and INV 1), with the channel 2 probe to
the point of measurement and the channel 1 probe to
the point of reference. Use this method even if the
reference point is a dc voltage with no signal.
11
OPERATING INSTRUCTIONS
OPERATING TIPS
The following recommendations will help obtain the best
performance from the oscilloscope.
1. Always use the probe ground clips for best results,
attached to a circuit ground point near the point of
measurement.Donotrelysolelyon an external ground
wire in lieu of the probe ground clips as undesired
signals may be introduced.
2. Avoid the following operating conditions:
a. Direct sunlight.
b. High temperature and humidity.
c. Mechanical vibration.
d. Electrical noise andstrongmagnetic fields,suchas
near large motors, power supplies, transformers,
etc.
3. Occasionally check trace rotation, probe compensation,andcalibrationaccuracy of theoscilloscopeusing
the procedures found in the MAINTENANCE section
of this manual.
4. Terminate the output of a signal generator into its
characteristic impedance to minimize ringing, especially if the signal has fast edges such as square waves
or pulses. For example, the typical 50 Ω output of a
square wave generator should be terminated into an
external 50 Ω terminating load and connected to the
oscilloscope with 50 Ω coaxial cable.
5. Probe compensation adjustment matches the probe to
the input of the scope. For best results, compensation
should be adjusted initially, then the same probe alwaysusedwiththesamechannel. Probe compensation
should be readjusted when a probe from a different
oscilloscope is used.
INITIAL STARTING PROCEDURE
Until you familiarizeyourself with the use of allcontrols,
the settings given here can be used as a reference point to
obtain a trace on the CRT in preparation for waveform
observation.
1. Set these controls as follows:
On both models:
VERTical MODE to CH1.
CH1AC/GND/DC to GND.
Trigger COUPLING to AUTO.
Trigger SOURCE to CH1.
All POSition controls and INTENSITY control centered (pointers facing up).
Main Time Base control to 1 mS/div.
On the Model 2125A:
Sweep Mode switch to MAIN.
2. Press the red POWER pushbutton (Model 2120B), or
rotate the POWER control clockwise away from
“OFF” (Model 2125A).
3. A trace should appear on the CRT. Adjust the trace
brightness with the INTENSITY control, and the
trace sharpness with the FOCUS control.
NOTE
On the Model 2125A, you can use the
BEAM FINDER pushbutton to locate a
trace that has been moved off the screenby
the POSition controls. When the button is
pushed, a compressed version of the trace
is brought into view which indicates the
location of the trace.
SINGLE TRACEDISPLAY
Either channel 1 or channel 2 maybeusedforsingle-trace
operation. To observe a waveform on channel 1:
1. Perform the steps of the “Initial Starting Procedure”.
2. Connect the probe to the CH 1 (X) input jack.
3. Connect the probe ground clip to the chassis or common of the equipment under test. Connect the probe
tip to the point of measurement.
4. Move the CH1 AC/GND/DC switch out of the GND
position to either DC or AC.
5. If no waveforms appear, increase the sensitivity by
turning the CH 1 VOLTS/DIV control clockwise to a
position that gives 2 to 6 divisions vertical deflection.
6. Position the waveform vertically as desired using the
CH1 POSition control.
7. Thedisplayonthe CRT may be unsynchronized.Refer
to the “Triggering” paragraphs in this section for procedures on setting triggering and sweep time controls
to obtain a stable display showing the desired number
of waveforms.
DUAL TRACEDISPLAY
In observing simultaneous waveforms on channel 1 and
2, the waveforms are usually related in frequency, or one of
the waveforms is synchronized to the other, although the
basic frequencies are different. To observe two such related
waveforms simultaneously, perform the following:
1. Connect probes to both the CH 1 (X) and CH 2 (Y)
input jacks.
2. Connect the ground clips of the probes to the chassis
or common of the equipment under test. Connect the
tips of the probes to the two points in the circuit where
waveforms are to be measured.
12
OPERATING INSTRUCTIONS
3. To view both waveforms simultaneously, set the
VERTical MODE switch to DUAL and select either
ALT (alternate) or CHOP with the PULL CHOP
switch.
4. In the ALT sweep mode (PULL CHOP switch
pushed in), one sweep displays the channel 1 signal
and the next sweep displays the channel 2 signal in an
alternating sequence. Alternate sweep is normally
used for viewing high-frequency or high-speed waveforms at sweep times of 1 ms/div and faster, but may
be selected at any sweep time.
5. In the CHOP sweep mode (PULL CHOP switch
pulled out), the sweep is chopped (switched) between
channel 1 and channel 2. Using CHOP, one channel
does not have to “wait” for a complete swept display
of the other channel.Therefore, portions of both channel’swaveformsaredisplayedwith the phase relationship between the two waveforms unaltered. Chop
sweep is normally used for low-frequency or lowspeed waveforms at sweep times of 1 ms/div and
slower; or where thephaserelationship between channel 1 and channel 2 requires measurement.
If chop sweep is used at sweep times of 0.2ms/divand
faster, the chop rate becomes a significant portion of
the sweep and may become visible in the displayed
waveform. However, you may select chop sweep at
any sweep time for special applications.
6. Adjust the channel 1 and 2
▲
POSition controls to
▼
place the channel 1 trace above the channel 2 trace.
7. Set the CH 1 and CH 2 VOLTS/DIV controls to a
position that gives 2 to 3 divisions of vertical deflection for each trace. If the display on the screen is
unsynchronized, refer to the “Triggering” paragraphs
in this section of the manual for procedures for setting
triggering and sweep time controls to obtain a stable
display showing the desired number of waveforms.
8. WhentheVERTicalMODEswitchissettoADD,the
algebraic sum of CH 1 + CH 2 is displayed as a single
trace. When the PULL INV switch is pulled out, the
algebraic difference of CH 1 – CH 2 is displayed.
9. Iftwowaveformshavenophaseorfrequencyrelationship, there is seldom reason to observe both waveforms simultaneously. However, these oscilloscopes
do permit the simultaneous viewing of two such unrelated waveforms, using alternate triggering. Refer to
the paragraphs on “Triggering - Trigger SOURCE
Switch”, for details on alternate triggering.
TRIGGERING
The Models 2125A and 2120B Oscilloscopes provide
versatility in sync triggering for ability to obtain a stable,
jitter-free display in single-trace, or dual-trace operation.
The proper settings depend upon the type of waveforms
being observed and the type of measurement desired. An
explanation of the various controls which affect synchronization is given to help you select the proper setting over a
wide range of conditions.
Trigger COUPLING Switch
1. In the AUTO position, automatic sweep operation is
selected. In automatic sweep operation, the sweep
generator free-runs to generate a sweep without a
trigger signal. However, it automatically switches to
triggered sweep operation if an acceptable trigger
source signal is present. The AUTO position is handy
when first setting up the scope to observe a waveform;
itprovidessweep for waveformobservationuntil other
controls can be properly set. Once the controls are set,
operation is often switched back to the normal triggering mode, since it is more sensitive. Automatic sweep
must be used for dc measurements and signals of such
low amplitude that they will not trigger the sweep.
2. The NORM position provides normal triggered
sweep operation. The sweep remains at rest until the
selected trigger source signal crosses the threshold
level set by the TRIG LEVEL control. The trigger
causes one sweep to be generated, after which the
sweep again remains at rest until triggered. In the
normal triggering mode, there will be no trace unless
an adequate trigger signal is present. In the ALT
VERTICAL MODE of dual trace operation with the
SOURCE switch also set to ALT, there will be no
trace unless both channel 1 and channel 2 signals are
adequate for triggering. Typically, signals that produce even one division of vertical deflection are adequate for normal triggered sweep operation.
3. The TV H and TV V positions are primarily for
viewing composite video waveforms.Horizontal sync
pulses are selected as trigger when the trigger COU-
PLINGswitchissettothe TV H position, and vertical
sync pulses are selected as trigger when the trigger
COUPLING switch is set to the TV V position. The
TV H and TV V positions may also be used as low
frequency reject and high frequency reject coupling,
respectively. Additional procedures for observing video
waveforms are given later in this section of the manual.
13
OPERATING INSTRUCTIONS
Trigger SOURCE Switch
The trigger SOURCE switch (CH 1, CH 2, etc.) selects
the signal to be used as the sync trigger.
1. If the SOURCE switch is set to CH 1 (or CH 2) the
channel 1 (or channel 2) signal becomes the trigger
source regardless of the VERTICAL MODE selection.CH 1,orCH2areoftenused as the triggersource
for phase or timing comparison measurements.
2. By setting the SOURCE switch to ALT (same as
CH1) and PULL ALT TRIG pulled, alternating triggering mode is activated. In this mode, the trigger
source alternates between CH 1 and CH 2 with each
sweep. This is convenient for checking amplitudes,
waveshape, or waveform period measurements, and
even permits simultaneous observation of two waveforms which are not related in frequency or period.
However,thissetting is notsuitablefor phaseortiming
comparison measurements. For such measurements,
both traces must be triggered by the same sync signal.
Alternate triggering can only be used in dual-trace
mode (VERT MODE set to DUAL), and with alternate sweep only (PULL CHOP not engaged).
3. In the LINE position, triggering is derived from the
input line voltage (50/60 Hz) and the trigger
SOURCE switch is disabled. This is useful for measurements that are related to line frequency.
4. In the EXT position, the signal applied to the EXT
TRIG jack becomes the trigger source. This signal
must have a timing relationship to the displayed waveforms for a synchronized display.
TRIG LEVEL/PULL (–) SLOPE Control
(Refer to Fig. 3)
A sweep trigger is developed when the trigger source
signalcrossesapresetthreshold level. Rotation oftheTRIG
LEVEL control varies the threshold level. In the + direction
(clockwise), the triggering threshold shifts to a more positive value, and in the − direction (counterclockwise), the
triggering threshold shifts to a more negative value. When
the control is centered, the threshold level is set at the
approximate average of the signal used as the triggering
source. Proper adjustment of this control usually synchronizes the display.
The TRIG LEVEL control adjusts the start of the sweep
to almost any desired point on a waveform. On sine wave
signals, the phase at which sweep begins is variable. Note
that if the TRIG LEVEL control is rotated toward its
extreme + or − setting, no sweep will be developed in the
normal trigger mode because the triggering threshold exceeds the peak amplitude of the sync signal.
When the PULL (–) SLOPE control is set to the + (“in”)
position, the sweep is developed from the trigger source
waveform as it crosses a threshold levelin a positive-going
direction. When the PULL (–) SLOPE control is set to the
− (“out”) position, a sweep trigger is developed from the
trigger source waveform as it crosses the threshold level in
a negative-going direction.
MAIN TIME BASE Control
Set the Main Time Base TIME/DIV control to display
the desired number of cycles of the waveform. If there are
too many cycles displayed for good resolution, switch to a
faster sweep time. If only a line is displayed, try a slower
sweep time. When the sweep time is faster than the waveform being observed,only part of it will be displayed, which
may appear as a straight line for a square wave or pulse
waveform.
HOLDOFF Control
(Refer to Fig. 4)
A “holdoff”period occurs immediately after the completion of each sweep, and is a period during which triggering
of the next sweep is inhibited. The normal holdoff period
varies with sweep rate, but is adequate to assure complete
retrace and stabilization before the next sweep trigger is
permitted. The HOLDOFF control allows this period to be
extended by a variable amount if desired.
Slope “–” Range
Slope “+” Range
+
Level
–
Fig. 3. Function of Slope and Level Controls.
A. Holdoff not used
B. Holdoff used
Fig. 4. Use of HOLDOFF Control.
14
OPERATING INSTRUCTIONS
This control is usually set to the MIN position (fully
counterclockwise) because no additional holdoff period is
necessary. The HOLDOFF control is useful when a complex series of pulses appear periodically such as in Fig. 4B.
Improper sync may produce a double image as in Fig. 4A.
Such a display could be synchronized with the VAR
SWEEP control, but this is impractical because time measurements are then uncalibrated. An alternate method of
synchronizing the display is with the HOLDOFF control.
The sweep speed remains the same,but the triggering of the
next sweep is “held off” for the duration selected by the
HOLDOFF control. Turn the HOLDOFF control clockwise from the MIN position until the sweep starts at the
same point of the waveform each time.
MAGNIFIED SWEEP OPERATION
Since merely shortening the sweep time to magnify a
portion of an observed waveform can result in the desired
portion disappearing off the screen, magnified display
should be performed using magnified sweep.
Using the POSition control, move the desired portion
of waveformto the centerof the CRT. Pull out the PULL X10
knob to magnify the display ten times. For this type of display
the sweep time is the Main Time Base TIME/DIV control
setting divided by 10. Rotation of the POSition control can
then be used to select the desired portion of the waveforms.
X−Y OPERATION
X−Y operation permits the oscilloscope to perform many
measurements not possible with conventional sweep operation. The CRT display becomes an electronic graph of two
instantaneous voltages. The display may be a direct comparison of the two voltages such as stereoscope display of
stereo signal outputs. However, theX−Y mode can be used
to graph almost anydynamic characteristic if a transducer is
used to change the characteristic (frequency, temperature,
velocity, etc.) into a voltage. One common application is frequencyresponse measurements, wheretheY axis correspondsto
signal amplitude and the X axis corresponds to frequency.
1. On the Model 2125A, set the SWEEP MODE switch
to the X−Y position. On the Model2120B, depress the
X−Y switch. On both models, set the Trigger Source
and VERTical MODE switches to X−Y.
2. In this mode, channel 1 becomes the X axis input and
channel 2 becomes the Y axis input. The X and Y
positionsarenowadjustedusingthe POSitionand
the channel 2 POSition controls respectively.
3. Adjust the amount of vertical (Y axis) deflection with
the CH 2 VOLTS/DIV and VARIABLE controls.
4. Adjust the amount of horizontal (X axis) deflection
with the CH 1 VOLTS/DIV and VARIABLE controls.
VIDEO SIGNAL OBSERVATION
Setting the COUPLING switch to the TV-H or TV-V
position permits selection of horizontal or vertical sync
pulses for sweep triggering when viewing composite video
waveforms.
When the TV-H mode is selected, horizontal sync pulses
are selected as triggers to permit viewing of horizontal lines
of video. A sweep time of about 10 µs/div is appropriate for
displaying lines of video. The VAR SWEEP control can be
set to display the exact number of waveforms desired.
When the TV-Vmode is selected, verticalsync pulses are
selected as triggers to permit viewing of vertical fields and
frames of video. A sweeptime of 2 ms/div is appropriate for
viewing fields of video and 5 ms/div for complete frames
(two interlaced fields) of video.
At most points of measurement, a composite video signal
is of the (−) polarity,that is, the sync pulses arenegativeand
the video is positive. In this case, use (− ) SLOPE. If the
waveform is taken at a circuit point where the video waveform is inverted, the sync pulses are positive and the video
is negative. In this case, use (+) SLOPE.
APPLICATIONS GUIDEBOOK
B+K Precision offers a “Guidebook to Oscilloscopes”
which describes numerous applications for this instrument
and important considerations about probes. It includes a
glossary of oscilloscope terminology and an understanding
of how oscilloscopes operate. It may be downloaded free of
charge from our Web site, www.bkprecision.com.
DELAYED SWEEP OPERATION (Model 2125A)
(Refer to Fig. 5)
Delayed sweep operation is achieved by use of both the
main sweep and the delayed sweep and allows any portion
of a waveform to be magnified for observation. Unlike X10
magnification, delayed sweep allows selectable steps of
magnification.
1. SettheSweepMode switch to the MAIN positionand
adjust the oscilloscope for a normal display.
2. Set the Sweep Mode switch to the MIX position. The
display will show the main sweep on the left portion
(representing the MAIN Time Base control setting)
and the delayed sweep on the right portion (representing the DELAY Time Base control setting). The
MAIN Time Base portion of the trace usually will be
brighter than the delayed time base portion. Fig. 5
shows a typical display for the MIX display mode.
3. Shift the percentage of the display that is occupied by
the main sweep by adjusting the DELAY TIME
POSition control. Counterclockwise rotation causes
more of the display to be occupied by the main sweep
15
OPERATING INSTRUCTIONS
Delayed Sweep
100
90
Main
10
0
Sweep
Fig. 5. MIX SWEEP MODE Display.
and clockwise rotation causes more of the display to
be occupied by the delayed sweep.
4. Set the Sweep Mode switch to the DELAY position
to display only the magnified delayed sweep portion
of the display.
NOTE
In order to obtain meaningful results with
delayed sweep, the DELAY Time Base
control must set be set to a faster sweep
speed than the MAIN Time Base control.
Because of this, the oscilloscope automatically prevents (electrically) the DELAY
Time Base from being set to a slower
sweep speed than the MAIN Time Base.
Forexample,if the MAIN TimeBaseisset
to0.1ms/div, the slowestpossibleDELAY
TimeBasesweep speed is also 0.1ms/div,
even if the control is set slower.
COMPONENT TEST OPERATION
(Model 2125A)
Do not apply an external voltage to the
COMP TEST jacks. Only non-powered
circuits should be tested with this unit.
Testing powered circuits could damage
the instrument and increase the risk of
electrical shock.
The component test function produces a component “signature” on the CRT by applying an ac signal across the
device and measuring the resulting ac current. The display
represents a graph of voltage (X) versus current (Y). The
component test function can be used to view the signatures
of resistors, capacitors, inductors, diodes, and other semiconductor devices. Devices may be analyzed in-circuit or
out-of-circuit and combinations of two or moredevicesmay
be displayed simultaneously. Each component produces a
different signature and the components can be analyzed as
outlined below.
Component Test mode is activated by depressing the
COMPonent TEST switch. The SWEEP MODE switch
must not be in the DELAY position.
Resistors
A purely resistive impedance produces a signature that is
a straight line. A short circuit produces a verticalline and an
open circuit causes a horizontal line. Therefore, the higher
the resistance, the closer to horizontal the trace will be.
Values from 10 Ω to about 5 kΩ are within measurement
range. Values below 10 Ω will appear to be a dead short
while values above 5 kΩ will appear to be an open circuit.
Fig. 6 shows some typical resistance signatures.
To test a resistor, insert one of the resistor’sleads into the
white COMP TEST jack, and the other into the GND jack
(make sure that the leads touch the metal walls inside the
jacks). To test in-circuit, a pair of test leads can be used to
connect the COMP TEST and GND jacks to the compo-
nent(s).
Capacitors
Besureto dischargecapacitors(byshorting the leads together) before connecting
to the COMP TEST jack. Some capacitors can retain a voltage high enough to
damage the instrument.
A purely capacitive impedance produces a signature that
is an ellipse or circle. Value is determined by the size and
shape of the ellipse. A very low capacitance causes the
ellipse to flatten out horizontally and become closer to a
straight horizontal line and a very high capacitance causes
the ellipse to flatten out vertically and become closer to a
straight vertical line. Values from about 0.33 µF to about
330 µF are within measurable range. Values below 0.33 µF
will be hard to distinguish from an open circuit and values
above330µFwillbehardtodistinguishfromashortcircuit.
Fig. 7 shows several typical capacitance signatures.
To test a capacitor, insert the capacitor’s positive lead into
the white COMPTEST jack, and the negativelead into the
GND jack (make sure that the leads touch the metal walls
inside the jacks). To test in-circuit or to test a capacitor with
leadsthatare tooshortto fitintotheCOMPTESTandGND
jacks, a pair of test leads can be used to connect the COMP
TEST and GND jacks to the component(s).
16
OPERATING INSTRUCTIONS
Fig. 6. Typical Resistive Signatures.
17
OPERATING INSTRUCTIONS
100
90
10
0
0.33 F Capacitorµ
100
90
Inductors
Like capacitance, a purely inductiveimpedance produces
asignaturethat is an ellipse or circle and valueisdetermined
by the size and shape of the ellipse. A very high inductance
causes the ellipse to flatten out horizontally and a very low
inductance causes the ellipseto flatten out vertically.Values
from about 0.05 H to about 5 H are within measurement
range. Values below 0.05 H will be hard to distinguish from
a short circuit and values above 5 H will be hard to distinguish from an open. Fig. 8 shows several typical inductance
signatures.
To test an inductor, insert one of the inductor’s leads into
the white COMP TEST jack, and the other into the GND
jack (make sure that the leads touch the metal walls inside
the jacks). To test in-circuit or to test an inductor with leads
that are too short to be inserted into the COMP TEST and
GND jacks, a pair of test leads can be used to connect the
COMP TEST and GND jacks to the component(s).
100
90
10
100
90
10
0
10
0
4.7 F Capacitorµ
1 Henry Inductor
100
90
0
300 F Capacitorµ
10
0
5 Henry Inductor
Fig. 7. Typical Capacitive Signatures.
Fig. 8. Typical Inductive Signatures.
18
Semiconductors
Purelysemiconductordevices(such as diodes and transistors) will produce signatures with straight lines and bends.
Typical diode junctions produce a single bend with a horizontal and vertical line as shown in Fig. 9. Zener diodes
produce a double bend with two vertical and one horizontal
line as shown in Fig.10 (value is determined by thedistance
of the leftmost vertical component from the center graduation on the CRT). The maximum Zener voltage observable
on this feature is about 15 V. It is also possible to test
transistors and IC’s by testing one pair of pins at a time.
NOTE
When testing diodes it is important to
connect the diode’s cathode to the white
COMP TEST jack and the anode to the
GNDjack.Reversingthe polaritywillnot
damage the devicebut the horizontal and
vertical components of the signature will
appear in different q uadrants of the
display.
To test semiconductors, insert the diode’s or transistor’s
leads (only two at a time) into the COMP TEST and GND
jacks (make sure that the leads touch the metal walls inside
the jacks). To test in-circuit or to test IC’s or devices with
leads too short to insert into the COMP TESTand GND
jacks, a pair of test leads can be used to connect the COMP
TEST and GND jacks to the component(s).
OPERATING INSTRUCTIONS
100
90
10
0
Silicon Diode
Fig. 9. Typical P-N Junction Signature.
100
90
Combinations of Components
Using the component test feature it is also possible to
observe the signatures of combinations of components.
Combinations cause signatures that are a combination of the
individual signatures for each component. For example, a
signature for a resistor and capacitorinparallelwill produce
a signature with the ellipse of the capacitor but the resistor
would cause the ellipse to be at an angle (determined by the
valueof the resistor). When testing combinations ofcomponents it is important to make sure that all the components
being connected are within measurement range.
In-CircuitTesting
The component test feature can be very effective in locating defective components in-circuit, especially if a “known
good” piece of equipment is available for reference. Compare the signatures from the equipment under test with
signatures from identical points in the reference unit. When
10
0
10 V Zener Diode
Fig. 10. Typical Zener Signature.
signaturesareidentical or verysimilar,the testedcomponent
is good. When signatures are distinctively different, the
tested component is probably defective.
19
MAINTENANCE
n
n
n
WARNING
The following instructions are for use by
qualifiedservicepersonnelonly. Toavoid
electricalshock,donot perform any servicing other than contained in the operating instructions unless you are qualified
to do so.
Highvoltageup to 2000 V is presentwhen
covers are removed and the unit is operating. Remember that high voltage may
be retained indefinitely on high voltage
capacitors. Also remember that ac line
voltage is present on line voltage input
circuits any time the instrument is
plugged into an ac outlet, even if turned
off. Unplug the oscilloscope and discharge high voltage capacitors before
performing service procedures.
FUSE REPLACEMENT
If the fuse blows, the “ON” indicator will notlightand the
oscilloscope will not operate. The fuse should not normally
open unless a problem has developed in the unit. Try to
determine and correct the cause of the blown fuse, then
replace only with the correct value fuse. For 110/125 V line
voltage operation, use an 800 mA, 250 V fuse. For 220/240
Vlinevoltageoperation, use a 600 mA, 250 V fuse.Thefuse
is located on the rear panel adjacent to the power cord
receptacle.
PERIODIC ADJUSTMENTS
Probe compensation and trace rotation adjustments
should be checked periodically and adjusted if required.
These procedures are given below.
Probe Compensation
1. Connect probes to CH 1 and CH 2 input jacks. Per-
form procedure for each probe, one probe at a time.
2. Set the probe to X10 (compensation adjustment is not
possible in the X1 position).
3. Touch tip of probe to CAL terminal.
4. Adjust oscilloscope controls to display 3 or 4 cycles of
CAL square wave at 5 or 6 divisions amplitude.
5. Adjust compensation trimmer on probe for optimum
square wave (minimum overshoot, rounding off, and
tilt). Refer to Fig. 11.
Correct
Compensatio
Over
Compensatio
Insufficient
Compensatio
Remove the fuseholder assembly as follows:
1. Unplug the power cord from rear of scope.
2. Insert a small screwdriver in fuseholder slot (located
between fuseholder and receptacle). Pry fuseholder
away from receptacle.
3. When reinstalling fuseholder, be sure that the fuse is
installedso that the correct line voltage is selected(see
LINE VOLTAGE SELECTION).
LINE VOLTAGE SELECTION
To select the desired line voltage, simply insert the fuse
and fuse holder so that the appropriate voltage is pointed to
by the arrow.Be sure to use the proper valuefuse (see label
on rear panel).
Fig. 11. Probe Compensation Adjustment.
Trace Rotation Adjustment
1. Set oscilloscope controls for a single trace display in
CH 1 mode, and with the channel 1 AC-GND-DC
switch set to GND.
2. Use the channel 1 POSition control to position the
trace over the center horizontal line on the graticule
scale. The trace should be exactly parallel with the
horizontal line.
3. Usethe TRACE ROTATION adjustmentonthefront
panel to eliminate any trace tilt.
20
MAINTENANCE
CALIBRATION CHECK
A general check of calibration accuracy may be made by
displaying the output of the CAL terminal on the screen.
Thisterminalprovidesasquarewaveof 2 V p-p. This signal
should produce a displayed waveform amplitude of four
divisions at .5 V/div sensitivity for both channel 1 and 2
(with probes set for direct). With probes set for X10, there
should be four divisions amplitude at 50 mV/div sensitivity.
The VARIABLE controls must be set to CAL during this
check.
NOTE
The CAL signal should be used only as a
general check of calibration accuracy,not
as a signal source for performing recalibration adjustments; a voltage standard
calibrated at several steps and of 0.3% or
better accuracy is required for calibration
adjustments.
The CAL signal should not be used as a
time base standard.
INSTRUMENT REPAIR SERVICE
Because of the specialized skills and test equipment re-
quired for instrument repair and calibration, many customers prefer to rely upon B+K Precision for this service. To
use this service, even if the oscilloscope is no longer under
warranty, follow the instructions given in the SERVICE
INFORMATION portion of this manual. There is a flat rate
charge for instruments out of warranty.
21
APPENDIX
IMPORTANT CONSIDERATIONS FOR RISE TIME
AND FALL TIME MEASUREMENTS
Error in Observed Measurement
The observed rise time (or fall time) as seen on the CRT
is actually the cascaded rise time of the pulse being measured and the oscilloscope’sownrisetime.Thetwo rise times
are combined in square law addition as follows:
T
observed
=
2
(T ) +(T )
pulse
scope
2
The effect of the oscilloscope’s rise time is almost negli-
gible when its rise time is at least 3 times as fast as that of
the pulse being measured. Thus, slower rise times may be
measured directly from the CRT. However, for faster rise
time pulses, an error is introduced that increases progressively as the pulse rise time approaches that of the oscilloscope. Accurate measurements can still be obtained by
calculation as described below.
Direct Measurements
The Models 2125A and 2120B oscilloscopeshavea rated
rise time of 12 ns. Thus, pulse rise times of about 36ns or
greater can be measured directly. Most fast rise times are
measured at the fastest sweep speed and using X10 magnification. For the Models 2125A and 2120B, this sweep rate
is 10 ns/div. A rise time of less than about four divisions at
this sweep speed should be calculated.
Calculated Measurements
For observed rise times of less than 36 ns, the pulse rise
time should be caluclated to eliminate the error introduced
by the cascaded oscilloscope rise time. Calculate pulse rise
time as follows:
T
pulse
=
(T ) +(T )
observed
2
scope
2
Limits of Measurement
Measurements of pulse rise times that are faster than the
scope’srated rise time are not recommended because a very
small reading error introduces significant error into the
calculation. This limit is reached when the “observed” rise
time is about 1.3 times greater than the scope’s rated rise
time, about 16 ns minimum for the Models 2125A and
2120B.
Probe Considerations
Forfastrise time measurementswhichapproachthelimits
of measurement, direct connection via 50 Ω coaxial cable
and 50 Ω termination is recommended where possible.
When a probe is used,its rise time is also cascadedinsquare
law addition. Thus the probe rating should be considerably
faster than the oscilloscope if it is to be disregarded in the
measurement.
22
23
2121 Specification and operating instruction
2121 model Oscilloscope is identical to the model 2120B except for the built in Frequency Counter
module. This portion contains specific information regarding the built in Frequency Counter.
2
4
5
6
Model 2121 Controls and Indicators
Counter Controls
1. TRIGger LEVEL control.
Trigger level adjustment selects the point
of triggering on measured signal.
PULL (-) SLOPE
Two position push/pull switch. The “in”
position selects a positive–going slope and
“out” selects a negative going slope
triggering point for Counter.
2. Trigger SOURCE Switch. Selects the
Counter input. Could be set to CH1. CH2.
LINE or External.
3. 5 digits Counter display.
4. Hz. LED. Is indicates when the display in
Hz mode
5. KHz. LED. Is indicates when the display
in KHz mode.
7
7. Universal Counter input Jack.
2121 Universal Counter Specifications.
DISPLAY:
5 Digits, 0.36” Red LED, Display at “Hz” or
“KHz” auto range.
AUTO SELECT:
Gate time is auto select from 10 to 0.25
seconds.
AUTO DETECT:
The Gate time LED will “FLASH” when
input signal is detected and synchronized and
will be “OFF” without input signal or if
trigger level was incorrectly set.
AUTO RESET:
After input signal was disconnected the
Counter Display will hold frequency reading
for 10 sec. only before performing auto reset.
6. Time/div frequency selector.
24
DISPLASY RESOLUTION:
Auto select from 0.001Hz to 1KHz depending
on the frequency.
MAX COUNTER RANGE:
0.1Hz to 50MHZ
MAXIMUM EXTERNAL VOLTAGE
300V dc + ac peak
ACCURACY:
+0.01% + 1 digit or 1/99999 +1 digit
Time Base:
18,432MHz + 10ppm ( 23°C±5°C )
SENSITIVITY:
NOTE:
1. The Counter must be set at ”DC
COUPLING” operation then the input signal
is less than 10Hz.
2. The counter is operated by the “Trigger
Source” CH1, CH2, or EXT.
NOTE: If input signal is not synchronized
correctly on CRT display Frequency counter
may have incorrect measurements.
To check power line frequency with the 2121
set Trigger SOURCE switch to LINE position.
There is no manual synchronization necessary
in this mode, Counter will show Line
frequency automatically.
To activate the dedicated frequency counter
input, separate from Oscilloscope cannels, set
Time /Div switch to any range under red
FREQ.≥100KHz label. Set SOURCE switch
to EXT position. Now 2121 is set to Universal
counter mode. Use the Trigger Level knob to
select correct Counter trigger level. Flashing
red LED on the top left corner of counter
display is indicating the correct trigger level is
set.
NOTE: If trigger level is not set correctly on
and red Led is not ON or flash Frequency
counter may have incorrect measurements.
MODE RANGE SENSITIVITY
2Hz~40MHz ≥1Div
INT
EXT
1Hz~45MHz ≥2Div
0.2Hz~50MHz ≥3Div
10Hz~50MHz ≥200mVrms
1Hz~50MHz ≥400mVrms
2121 Internal Frequency Counter
operating instructions.
The signal from both Oscilloscope channels
(CH1 and CH2.) could be used for frequency
measurements input.
Set VERT and SOURCE switches to selected
channel and synchronize measured signal on
CRT display. Counter will auto sense and
register signal frequency on the counter red
digital display. Flashing red dote in top left
corner of the first digit is Gate indicator. It is
light up every time during frequency counter
is update.
SERVICE INFORMATION
Warranty Service:Please return the product in the original packaging with dated proof of purchase to the address below.
Clearly state in writing the performance problem and return any leads,connectors, and accessories that you are using with the
device.
Non-Warranty Service: Return the product in the original packaging to the address below. Clearly state in writing the
performance problem and return any leads, connectors, and accessories that you are using with the device. Customers not on
open account must include payment in the form of a money order or credit card. For the most current flat-rate repair charges,
contact the factory before shipping the product.
Return all merchandise to B+K PrecisionCorp. with pre-paidshipping. The flat-rate repair charge includes return shipping
to locations in North America. For overnight shipments and non-North America shipping fees,contact B+K Precision Corp.
IMPORTANT
Includewiththe instrument your completereturnshipping address, contactname,phone number,and description ofproblem.
Service Department
B+K Precision Corp.
22820 Savi Ranch Parkway
Yorba Linda, CA 92887
Phone(714) 921-9095
Fax(714) 921-6422
E-mail: service@bkprecision.com
25
LIMITED WARRANTY
B+K Precision Corp. warrantsto the original purchaser that its product, and the component parts thereof, will be free from
defects in workmanship and materials for a period of three years from the date of purchase (Model 2125A) or two years from
the date of purchase (Model 2120B).
B+K Precision Corp. will, without charge, repair or replace, at its option, defective product or component parts upon
delivery to an authorized B+K Precision service contractor or the factory service department, accompanied by proof of the
purchase date in the form of a sales receipt.
To obtain warranty coverage in the U.S.A., this productmustbe registeredby completing and mailing theenclosedwarranty
registrationcardtoB+KPrecision,22820 Savi Ranch Parkway Yorba Linda,CA92887 withinfifteen(15)daysfromthe
date of purchase.
Exclusions: This warranty does not apply in the event of misuse or abuse of the product or as a result of unauthorized
alterations or repairs. It is void if the serial number is altered, defaced or removed.
B+K Precision Corp. shall not be liable for any consequential damages, including without limitation damages resulting
from loss of use. Some states do not allow limitation of incidental or consequential damages, so the above limitation or
exclusion may not apply to you.
This warranty gives you specific rights and you may also have other rights which vary from state to state.
26
INFORMATION
One of the best tutorials on oscilloscopes in the industry. Valuable to those with little knowledge
of oscilloscopes as well as the experienced technician or engineer who wishes
to refresh their memory or explore new uses for oscilloscopes.
Download your FREE copy
from our Web site
www.bkprecision.com
22820 Savi Ranch Parkway, Yorba Linda, CA 92887
©2003 B+K Precision Corp.
481-312-9-002 Printed in Taiwan
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