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Model 8901B
08901-90114V2
VOLUME
Service Sheet 1-A53
Service Sheet 2-A53
Service Sheet 3-A32 Power Reference Oscillator Assembly
Service Sheet 4-A15
Service Sheet &A17 Input Mixer. A16 Buffer Amplifier. and A18
Service Sheet 6-A55
Service Sheet 7-A54
Service Sheet 8-A6
Service Sheet 9-A6
Service Sheet 10-A4 FM Demodulator Assembly (Limiters)
Service Sheet ll-A4 FM Demodulator Assembly (Discriminator)
Service Sheet 12-A2 Audio Filters Assembly
Service Sheet 13-A3 Audio De-emphasis and Output Assembly
Service Sheet 14-A5 Voltmeter Assembly (Audio Detectors)
Service Sheet 15-A5 Voltmeter Assembly (Voltmeter)
Service Sheet 16-A52 Audio Counter and Distortion Analyzer Assembly
Service Sheet 17-A19
Service Sheet
Service Sheet 19-A22 Low Frequency VCXO and
Filter Assemblies
Service Sheet 20-A20
Service Sheet 20-A20
Service Sheet 21-A20
Service Sheet 21-A20
Service Sheet 22-All Counter Assembly (Time Base)
Service Sheet 23-A11 Counter Assembly (Counter)
Service Sheet 24-A13 Controller Assembly
Service Sheet 25-A1 Keyboard Assembly (Keyboard)
Service Sheet 26-A1 Keyboard Assembly (Displays)
Service Sheet 27-A1 Keyboard and Display Assembly (Annunciators)
Service Sheet 28-A14 Remote Interface Assembly
Service Sheet 29-A51 FM Calibrator Assembly
Service Sheet 30-A50
Service Sheet 31-A10 Power Supply Regulators. A26 Power Supply Motherboard.
and A29 Series Regulator Heat Sink Assemblies
Service Sheet 32-A10 Power Supply Regulators. A26 Power Supply.
and A29 Series Regulator Heat Sink Assemblies
Service Sheet 33-A72
Service Sheet 34-A71
18-A23
RF
Power Assembly (Sensor Input Circuits)
RF
Power Assembly (Control Circuits)
R.F
Input Assembly
IF
Channel Filter Assembly (Option Series 030)
IF
AmplifierAIetector Assembly (Option Series 030)
AM
Demodulator Assembly (ALC Loop)
AM
Demodulator Assembly (Control Circuits)
LO
Divider Assembly
Sampler and A24 High Frequency VCO Assemblies
LO
Control Assembly
LO
Control Assembly
LO
Control Assembly (Digital Circuits) (2314A
LO
Control Assembly (Digital Circuits)
AM
Calibrator Assembly
IF
Channel Filter Assembly (Option Series 030) (2642A
IF
Amplifier Detector Assembly
2
TABLE
OF
CONTENTS
.....................
.........................
........................
...................................
IF
Amplifier Assemblies
W14A
to
2636A)
W14A
........................
....................
........................
.....................
................................
......................
........................
...........................
................
................................
..............
A21
Low Frequency VCXO
(Analog
(Analog
Circuits)
Circuits)
(2314A
(2642A
(2627A
to2622A)
and
to2622A)
and
..........................
............................
................................
...........................
...........................
.................
............................
..............................
..............................
..................
(Option
Series 030K2642A
.......
Above)
Above)
and
......
........
to
2636A)
......
:
....
...........
..........
...........
..........
..........
Above)
and
.....
Above)
...
.8
F.l
8F-9
8F-15
8F-17
8F-23
8F-33
8F-37
8F-41
8F-51
8F-55
8F-57
8F-63
8F-69
8F-79
8F-87
8F-93
8F-101
8F-107
8F-119
8F-123
8F-129
8F-135
8F-139
8F-143
8F-147
8F-157
8F-167
8F-173
8F-177
8F-181
8F-197
8F-205
8F-213
8F-217
8F-221
8F-225
rev.08JUL91
Model 8901B Service
ASSEMBLY
0
A53
PRINCIPLES
General
The load impedance of the power-sensing element dissipates the RF input power applied
power sensor (in tile Sensor Module).
output
and convert the signal back
Amplifier
The ac signal, which
in the external power sensor and the RF Power circuits. Amplifier
external power sensor form a low-noise, high-gain operational amplifier. See Figure
of the amplifier
VR1,
non-linear output of the power sensor’s power sensing element. (The efficiency of the power-sensing
element
shaping network reduces the gain of Amplifier
R16 (RNG
Service
RF
Power (Sensor Input Circuits)
OF
OPERATION
of
the power-sensing element
to
1
is
proportional
is
approximately
and
VR2 and associated components are
is
slightly impaired when the RF input
5)
permit fine adjustment
to
dc for measurement by the Voltmeter.
to
700.
Sheet
A
sampling gate (chopper) in the power sensor converts the dc
a 220
Hz
ac signal. The RF Power circuits amplify the ac signal
the
RF
input power, is amplified
DC
bias
is
set by R2,
part
to
1
to give a linear overall response.
of
the gain of the shaping circuit for high levels.
1
to
the external
by
tuned ac amplifier stages
1
and the output amplifier of the
8F-1.
The ac gain
R3,
R4,
R5,
and R6. Diodes CR1, CR2,
of
a shaping network which compensates for the
the power sensor is near maximum power.) The
R13
(RNG
4)
and
I
SENSOR MODULE
h
Figure
8F-1.
-,Y
I
I
POHER
First Amplifier Stage
A53 POWER METER ASSEMBLY
SENSOR
CABLE
of
the RF Power Meter
-
Service Sheet
1
8F-1
Service Model 8901B
The combination of C10, R9, and R10 determines, in part, the upper cutoff frequency (240
of
the bandpass response
frequency (200
Hz).
the ac amplifiers; C14, R19, and R20 determine, in part, the lower cutoff
C1,
C5,
and C13 filter line noise.
Ground Regulator
U17 and U1B are connected as a voltage follower between the GND
Sensor Module and ANALOG GND
1
of the RF Power circuits. This circuit ensures that a minimum
voltage difference exists between the grounds to eliminate error-creating voltage difference between
measurement ground and instrument ground.
Attenuators 1 and 2 and Amplifiers 2 and
Attenuators 1 and 2 operate as shown in Table
Zuble
8F-1.
Range
1
2
3
4
5
Attenuation
’
Attenuator
3
8F-1.
(The attenuation indicated
us.
Range
Attenuation
1
Attenuator2
0 0
0
0
40
40
20
40
20
40
of
Attenuators
(dB)
REGULATOR
1
and
I
Total
I
0
20
40
60
80
line from the
is
voltage attenuation.)
2
Hz)
of
Amplifier 2 has a gain
R31
C27 and
determine, in part, the upper cutoff frequency (240
ac amplifiers; C19, R24, R25, and R26
Hz).
(200
of
21. Amplifier 3 has a gain of 19. The two combinations of
Hz)
of the bandpass response of the
and
C22
and
R27 determine, in part, the lower cutoff frequency
C18
and R21 and of
Synchronous Detector
The 220
gate (chopper)
Q6
is
-1.
ac signal originating in the power sensor’s chopper, the output
dc level of the signal from
the external power sensor.
220
Hz
The 220
(Q4)
Qll and Q12 are drivers. R40 (FREQ) allows fine adjustment of the multivibrator’s frequency. R40 is
adjusted for minimum phase shift through the bandpass filter response of the ac amplifiers. The point
of minimum phase shift gives the maximum and most drift-immune power indication.
Hz
to
the gate of
When Q4
Multivibrator drives the Synchronous Detector in the same way
in
the external power sensor. The 220
FET
Q4 which causes Q4 to turn on and off. When Q4 is on, the gain of amplifier U3
is
off,
the gain
of
U3
U3
is proportional
is
+l.
Since the 220
to
Hz
switching signal is applied through Q5 and
Hz
drive signal is in synchronism with the
of
U3
is
full-wave rectified. The average
the power dissipated in the power-sensing element of
it
drives the sampling
Multivibrator
Hz
Multivibrator drives both the chopper in the external power sensor and the gain switch
of
the Synchronous Detector. Q8, Q9, and associated components form an astable multivibrator.
8F-2
Service Sheet
1
Model
8901B
TROUBLESHOOTING
General
Procedures for checking the RF Power Assembly are given below. The circuits to check are marked
on the schematic diagram by a hexagon with a check mark and a number inside, for example,
In
addition, any points outside the labeled circuit area that must be checked are
signals are also shown on the schematic inside a hexagon, for example,
Extend the board assembly where necessary to make measurements. Extending the assembly will require
freeing the
the cables will require disconnecting several RF cables. The RF cables need not be reconnected while
troubleshooting the assembly.
two
multi-conductor cables atop the assembly
Service
also
identified. Fixed
(+1.9
TO
+2.1
VDC)
.
so
that they may be reconnected. Freeing
@
.
Tighten
SMC
connectors to
tors is insufficient. Hand-tightened connectors can
reduced performance
Equipment
Oscilloscope
Range Calibrator.
(ZJ
Amplifier 1 and Ground Regulator
1.
Remove any connection to the SENSOR connector.
2.
Connect one channel of a high-impedance, dc coupled oscilloscope
other channel to pin 7 of UlB. Connect pin 3 of
indicated in the table below. After connecting the resistor to the supply, briefly short pin
U17 to chassis ground and observe the oscilloscope display. The voltages should be as indicated
in Table
......................................................................
................................................................
Failure of the Ground Regulator may make Amplifier 1 appear to fail.
8F-2.
0.6
N.
or
malfunctions.
Check
m
(5
NOTE
in.
lb). Hand tightening of connec-
work
loose and cause
to
pin 6 of U17. Connect the
U17
through a
100
kR resistor to the supply
HP
1740A
HP 11683A
2
of
NOTE
U17
and
UlB
will be operating with nearly short-circuited outputs, which
they should be able to withstand indefinitely. It
as
brief
as
possible. The +15V Supply is easily accessed at the + end
-15V
Supply at the - end of
Table
Connection on U17 Pin
-15V
8F-2.
is
Voltage Limits for
Limits (Vdc) Limits (Vdc)
+0.2
to
+1
-1
.o
to
-0.2
removed, the outputs of U17 and U1B will normally
Hint:
to be
C2,
the
of
If
the short on pin 2 of U17
towards the corresponding supply and, for the
is
a good practice, however,
C4.
(./1>
Step
6
.o
+15V
on U1B Pin 7
+0.1
to
+0.3
-0.3
to
-0.1
Supply, will begin pulsing.
2
Service Sheet
If
this change
1
drift
8F-3
Service
Model
8901B
does not occur, the Ground Regulator is working properly but
Analog Ground
3.
Remove the short from pin 2 of U17. Connect the oscilloscope to A53TP6
the 100
and
4.
Ground pin 3 of U1A. (Shorting
5.
Connect the oscilloscope to pin 1 of U1A. The voltage should be between
Hint:
is
6. Connect a
be between
7.
Remove all jumpers. Leave the oscilloscope connected
8.
Connect the range calibrator to the SENSOR input. Set the calibrator’s range
to calibrate, and polarity to normal.
9.
Press RF POWER. The waveform on the oscilloscope display should appear as shown in
Figure 8F-2.
kS1
f50
mVdc.
UlA,
high enough to drive UlA into limiting by the
1)
requires more current than the regulator
resistor to chassis ground. The level at A53TP6 should slowly
R1
with a cliplead may be the simplest way to do this.)
R2,
and R6 form an inverting amplifier with the
-15V
Supply.
42.2
kR
-0.5
resistor between the + end
and
+0.5
Vdc. (Ignore any ac ripple.)
4.2
to
of
4.9
C5
and the -15V Supply. The voltage should
to
ms
+
its
load (the devices connected
is
able to supply.
+15V
Supply
pin 1 of U1A.
(A
GND).
drift
to between
-15
and
as
the input. The gain
to
1
Connect
-12
Vdc.
mW, function
to
-50
ov
Figure
Hint:
If
the period
the Zeroing Control circuit can alter the input to Amplifier
this is suspected, short R65 (see Service Sheet
10.
Set the calibrator’s range to
waveform should appear as in the figure for step
Hint:
If
faulty, check the components associated with the Range 4 shaping circuit (R13,
CR2, etc.).
is
out of limits, see
10
8F-2.
mW.
Waveform
@
Reduce the oscilloscope’s vertical gain by a factor of
for
220
Hz
2)
and recheck the waveform.
9
with the voltage limits multiplied by
(v’l)
Step
9
Multivibrator Check. Improper operation of
1
and cause an erroneous output. If
10.
The
10.
CR1,
8F-4
Service Sheet
1
Model 8901B Service
11.
Set the calibrator’s range to 100 mW. Reduce the oscilloscope’s vertical gain by a factor of 10.
The waveform should appear as in the figure for step 9 with the voltage limits multiplied by 100.
Hint:
If
faulty, check the components associated with the Range 5 shaping circuit (R16, VR1,
VR2, etc.).
@
Attenuator
1.
Connect the range calibrator to the SENSOR input. Set the calibrator’s range to 1 mW, function
to calibrate, and polarity to normal.
2. Connect a high-impedance, ac coupled oscilloscope to pin
shown in the figure for step 9 of the
Hint:
1,
Amplifier
If faulty, perform
2,
Attenuator
(./1>
above.
2,
and Amplifier 3 Check
1
of U1A. The display should be as
(J1)
Amplifier 1 and Ground Regulator Check.
3. Connect the oscilloscope to pin 2 of U4A. Key in 0.209 SPCL to switch U4A on and U4B
The waveform on the oscilloscope display should appear as shown in Figure 8F-3.
-4
4.2
to
4.9
ms
+
ov
off.
Figure
Hint:
Pin 1 of U4A should be a TTL low. Pin 8 of U4B should be high.
4. Key in 0.208 SPCL to switch U4A
The waveform should appear
Hint:
Pin 1 of U4A should be high. Pin 8 of U4B should be low.
5. Connect the oscilloscope to pin
SPCL to switch U4C on and U4D and U8A
8F-4.
Hint:
Pin 9 of U4C should be low. Pin 16
8F-3.
as
in the figure for step
11
of U4C. Switch the calibrator’s range to 1 mW. Key in 0.202
Waveform
off
and
for
@
Step
3
U4B on. Switch the calibrator’s range to 100 mW.
3.
off.
The waveform should appear as shown in Figure
of
U4D and pin
1
of
U8A should be high.
Service Sheet
1
8F-5
Service
I
ov
-+
4.2
to
4.9
ms
+
Model
8901B
Figure 8F-4.
ov
+
Waveform for
4.2
to
4.9
ms
@
+
Step
5
8F-6
Service Sheet
Figure 8F-5.
1
Waveform for
@
Step
8
Model 8901B Service
6.
Key in 0.204 SPCL to switch U4D on and U4C and U8A
mW. The waveform should appear as in the figure for step
Hint:
Pin 16
7.
Key in 0.208 SPCL to switch U8A on and U4C and U4D
mW. The waveform should appear as in the figure for step
8.
Connect the oscilloscope to A53TP1 (AC). The waveform should appear as shown
of
U4D should be low. Pin 9 of U4C and pin
off.
Switch the calibrator’s range
5.
1
of
U8A should be high.
off.
Switch the calibrator’s range
5.
in
Figure 8F-5.
to
to
10
100
Hint:
approximately
@
Synchronous Detector Check
1.
Connect the range calibrator to the
to calibrate, and polarity to normal. Press
2.
Connect a high-impedance, ac coupled oscilloscope to A53TP1 (AC). The display should be as
shown in the figure for step
3 Check.
Hint:
3.
DC couple the oscilloscope and connect
shown in Figure
Hint:
between 4.2 and 4.9
as Q4 switches from
of
The waveform at pin 7 of
600
mVpp.
If
faulty, perform
8F-6.
The collector of Q6 should be switching between
u3.
U2B should have a similar shape and an amplitude of
SENSOR
8
of the
(J2)
above.
ms.
The gain
off
to on. Non-uniform half-cycles indicates unequal gain for the
@
of
operational amplifier based on U3 switches from
4.2
input. Set the calibrator’s range
RF
POWER.
Amplifier
it
to A53TP2
to
4.9
1,
Attenuator 2, Amplifier 2, and Amplifier
(a
DET). The waveform should appear as
0
and
approximately -15V with a period
ms
+-
to
1
mW, function
$1
two
to
-1
states
ov
Figure
8F-6.
Waveform
for
(J3)
Step
e-
3.4
to
3.8
v
e-
ov
3
Service Sheet
1
8F-7
Service
@
Model
220
Ht
Multivibrator
1.
Connect a high-impedance, dc coupled oscilloscope to A53TP4 (MULTVIB or MV1). The
waveform should be a square wave switching between
4.2
and
4.9
ms.
Check
OV
and 10 to
11V
with a period between
8901B
Hint: A more accurate way of measuring the multivibrator’s frequency and adjusting it
is found in
2.
Connect the oscilloscope to
Adjustment 20-Power Meter.
A53TP5
(MV2)
and
repeat step
1.
if
necessary
8F-8
Service Sheet
1
Model
8901B
Service
ASSEMBLY
0
A53
RF
Power
(Control Circuits)
PRINCIPLES OF OPERATION
General
This Service Sheet documents the circuits that filter the signal from the Synchronous Detector, zero
the Power Sensor, and control the Power Meter and external Sensor Module.
Noise Filter
The dc voltage from the Synchronous Detector (see Service Sheet
improve measurement readability. U13 and associated components form an active, three-pole, low-pass
filter. The noise bandwidth
is
bandwidth
1.
(In addition, in range 1, the Controller effectively increases filtering by averaging several successive
readings.)
The Voltmeter reads the Noise Filter output (RF PWR/SENSOR TYPE) via switch U8C to display RF
power. The Voltmeter also reads the Noise Filter output via
to
the front-panel SENSOR input connector. By reading the voltage developed at the junction of R68,
R69, and the SENSOR-TYPE READBACK line the type of sensor
has a unique resistor value with one end of the resistor connected
Module, the resistor is 1.62
used in ranges 5,4, and 3 (refer
Service
is
10 Hz with switch U8B closed and 1 Hz with U8B open. The 10 Hz
Sheet
to
Table
2
8F-1);
U8D
1)
is
filtered to remove noise and
the 1 Hz bandwidth
to identify the type of sensor connected
is
determined. Each sensor type
to
ground. (In the HP 11722A Sensor
is
used in ranges 2 and
kn.)
Zeroing Control
Power offsets (positive
come primarily from the Power Sensing Element in the external Power Sensor. The offset
by the Zeroing Control DAC (U10) when the Power Meter
Controller removes the power from the external Power Sensor using the Input Switch in the Sensor
Module.
manually removed before zeroing
takes an RF power reading and adjusts the Zeroing Control DAC
attempt may be required to bring the reading within limits.
The Zeroing Control DAC outputs a current proportional
current, flowing through R65 develops a negative voltage. Another current, originating from Q15, flows
through R67 to develop a constant positive voltage across R65. The combination of the currents from
the DAC and Q15 thus permits a positive
Sensing Element of the external Power Sensor. To minimize
VR4, and VR5 are biased by thermally compensated current sources (Q14 and Q15) to produce the
reference voltage to the DAC and R67.
(If
a sensor
or
negative readings from the Power Meter when no power
is
zeroed. To zero the Power Meter, the
is
used which
is
not part of a Sensor Module, the RF power source must be
is
attempted.) With no power into the Power Sensor, the Controller
to
cancel the reading; more than one
to
the binary-weighted input. This output
or
negative offset. The offsetting voltage
drift
of the offset, reference diodes VR3,
is
actually applied)
is
fed to the Power
Sensor Module Switch Control
The Switch Drive One-Shot permits control of latching-type RF switches in the Sensor Module which
can have either automatic breaking
U16 provides an energizing pulse for
Switch Q3 actually energizes the RF switch solenoid. The collector resistor (R77) of Q3 provides
sufficient drive current for the types of RF switches commonly used in the external Sensor Module.
or
non-breaking solenoid drive contacts. For either case, one-shot
30
ms which
is
sufficient to throw the switch’s plunger.
is
cancelled
Service Sheet 2
8F-9
Service Model 8901B
CR3 suppresses the emf generated by the
is
driven from U16 via Q13.
Relays K1 and K2, activated by U14A and U14B, route the switch drive to the proper solenoid contacts.
The Controller then activates the RF switch via the Switch Drive One-Shot.
RF
switch solenoid when the drive current
is
interrupted. Q3
Power Reference Oscillator Control
The
RF
When
Power Reference Oscillator (see Service Sheet
Q2
is on, -15V is supplied to the reference circuitry.
3)
is activated
by
the Controller via Q7 and
Frequency Offset Control
Gates U15A and U15B provide a three-level
down-converter the status and frequency range of frequency offset.
information.)
(0,
+3,
and
+5V)
output
to
indicate to
(See Table 8D-4 for status
Select Decoder and Data Latches
Refer to the general discussion under
Power-Up Reset circuit that sets the front-end components of the instrument to the most-protected
state at power-up; that is, input attenuation
switched out.
Instrument
is
set
Bus
in Service Sheet BD5. R53 and C35 for the
to
maximum and the external Power Sensor
an
Q2.
external
is
8F-10
Service Sheet
2
Model
8901B
TROUBLESHOOTING
General
Procedures for checking the
on the schematic diagram by
In addition, any points outside the labeled circuit area that must be checked are also identified. Fixed
signals are also shown on the schematic inside a hexagon, for example,
Extend the board assembly where necessary to make measurements. Extending the assembly will require
freeing the
the cables will require disconnecting several
troubleshooting the assembly.
two
RF
Power Assembly are given below. The circuits to check are marked
a
hexagon with a check mark and a number inside, for example,
multi-conductor cables atop the assembly
RF
cables. The
(+I
.9
TO
+2.1
VDC)
.
so
that they may be reconnected. Freeing
RF
cables need not be reconnected while
Service
(J3)
.
Equipment
Oscilloscope
Range Calibrator.
(XJ
Noise Filters Check
1.
Connect the range calibrator
to calibrate, and polarity to normal. On the Measuring Receiver, press RF POWER.
2.
Connect a high-impedance, dc coupled oscilloscope to A53TP2
The waveform should appear as in Figure 8F-7.
Hint:
If
I
Tighten
tors
reduced performance
SMC
connectors to
is
insufficient. Hand-tightened connectors can
or
0.6
N.
m
malfunctions.
(5
in.
lb).
Hand tightening of connec-
work
loose and came
......................................................................
................................................................
to
the SENSOR input. Set the calibrator’s range
(a
DET). (See Service Sheet
the waveform
is
faulty, see Service Sheet 1 and check the Synchronous Detector.
4
4.2
to
4.9
ms
+
3.4
to
1
mW,
to
HP
3.8
HP 1740A
11683A
function
1.)
v
Figure
8F-7.
Waveform for
(J1)
Step
2
Service Sheet
ov
2
8F-11
Service Model 8901B
0
3. Connect the oscilloscope
The waveform should be dc
Hint:
Pin 8 of U8B should be a TTL high.
4. Key in 0.230 SPCL to set the Noise Filters to narrow. The waveform should be between
900
mV.
Hint:
Pin 8 of U8B should be a TTL low.
RF
Power and Sensor-Type Readback Switches Check
1.
Connect the range calibrator to the SENSOR input. Set the calibrator’s range to 1 mW,
to calibrate, and polarity to normal. On the Measuring Receiver, press RF POWER.
2.
Connect a high-impedance, dc coupled oscilloscope to A53TP3 (FLTR). The waveform should be
at
a
dc
Hint:
3. Key in 0.21A SPCL to enable the RF Power Readback Switch. Key in 49.F SPCL to read back
the output
Hint:
4. Key in 0.219 SPCL to enable the Sensor-Type Readback Switch. Key in 49.F SPCL. The display
should read between -0.01 and +0.01. (The sensor-type resistor in the range calibrator
circuit.)
level between
If
the waveform
of
the switch. The display should read between
Pin
9 of U8C should be a TTL low. Pin 16 of
to
A53TP3 (FLTR). Key in 0.231 SPCL to set the Noise
at
a
700
and
900
is
faulty, perform
level between
mV.
700
and 900 mV.
above.
U8D
0.7
and 0.9.
should be a
TTL
high.
Filters
is
to
wide.
700
function
a short
and
Hint:
Pin 9 of U8C should be a TTL high. Pin 16 of U8D should be a TTL low.
@
Power Reference Oscillator Control Check
1.
Connect a high-impedance, dc coupled oscilloscope
2. Key in 0.21E SPCL to switch Q2 on. The dc waveform should be between
Hint:
Pin 10 of U6 should be a TTL high.
3. Key in 0.21A SPCL to switch
Hint:
Pin 10 of U6 should be a TTL low.
(J4)
Sensor Module Switch Control Check
1.
Disconnect any connection to the SENSOR input.
2.
Connect a high-impedance, dc coupled oscilloscope to pin 6 of U16.
to
3. Key in 0.24 SPCL
be a square wave with a period of approximately 60 ms and a level alternating between a TTL
high and a TTL low.
Hint:
Pin 1 of U16 should be low-going TTL pulses with a width of approximately 30 ms and a
period of approximately 60 ms.
4.
Connect the oscilloscope to pin 14 of K1. Connect a 10 kR resistor between pin 14 of K1 and
ground. The waveform should be a square wave with a period of approximately 60 ms and a level
alternating between approximately
trigger the Switch Drive One-Shot. The waveform on the oscilloscope should
Q2
off.
-
to
the collector of Q2.
The dc waveform should be between
15 and
OV.
-15
and -14 Vdc.
+1
and
+2
Vdc.
5.
Move the oscilloscope and resistor to pin 8 of the relay indicated in Table 8F-3. Key in the
Direct Control Special Functions indicated in the table. For each step, the waveform should be as
described.
8F-12 Service Sheet
If
2
faulty,
also
check the logic level
on
the pin on U14 indicated.
Model 8901B Service
@
Zeroing Control Check
1.
Connect the range calibrator to the
2.
On the Measuring Receiver, press RF POWER.
3.
Connect a high-impedance, dc coupled oscilloscope
voltage should be between +6.0 and +6.4 Vdc. (The two voltages may differ from each other.)
Hint:
If
the voltage at the collector of Q15
OV, there may be
5.
Connect the oscilloscope to pin
Functions to clear the latches of U11: 0.220 SPCL, 0.222 SPCL, 0.224 SPCL, 0.226 SPCL,
0.228 SPCL, 0.22A SPCL, 0.22C SPCL, and 0.22E SPCL. (Note that each suffix
oscilloscope should read approximately 15 mVdc.
Hint:
Pins 4,
Direct Control
Relay
K1
K1
K2
K2
a
fault in the Ground Regulator. (See Service Sheet
5,
6,
7,
9, 10,
Special Waveform
Functions Pin 2 Pin
0.212, 0.24
0.21A, 0.24
0.21A, 0.24
0.212, 0.24
SENSOR
4
of
U10. Successively key in the following Direct Control Special
11,
and
12
as
in
step
4
0
Vdc
as
in step
0
4
Vdc
input. Set the calibrator’s function to standby.
to
the collectors of Q14 then Q15. The dc
is
faulty, check the anode of VR5;
of U11 should be TTL low.
Level
L
H
H
L
(TTL)
at U14
7
H
L
L
H
1.)
if
the voltage
is
is
not
even.) The
6. Successively key in 0.221 SPCL, 0.223 SPCL, 0.225 SPCL, 0.227 SPCL, 0.229 SPCL, 0.22B
SPCL, 0.22D SPCL, and 0.22F SPCL.
Special Function
is
entered, the voltage should drop
is approximately -15 mVdc.
Hint:
As each Special Function is entered, the pins of U11 mentioned in the previous hint should
successively go to a TTL high.
@
Select Decoder and Data Latches Check
1.
Key in the Direct Control Special Functions indicated in Table 8F-4. For each setting, check the
pins on U12 indicated.
Direct Control
Special
Function
0.200
0.21
0
0.220
0.230
0.240
(Note
that each suffix
is
successively larger
Level (TTL) at U12 Pin
11 12 13 14 15
H
H
H H
H H
H
H
H H
H
H H
is
odd.) As each Direct Control
H
H H
H H
H H
steps
until the voltage
Service Sheet
2
8F-13
Service Model
8901B
Table
Direct Control
Special
Function
0.230
0.23E
0.232
Direct Control
Function
4.
Key in the Direct Control Special finctions indicated
pins on
U6
indicated.
Direct Control
Function
2
L
H
L
lbble
Special
0.200
0.20F
lbble
Special
0.21
0
0.21
F
8F-5.
Levels at
Level (TTL) at U9 Pin
7
L
H
H
8F-6.
8F-7.
Levels at
H
Levels at
U9
10
L
H
L
Level (TTL) at U5 Pin
L
Level (TTL) at U6 Pin
and
US,
US,
U15,
15
L
H
L
@
L
in
@J
@
Step
Level at U15 Pin
3
TTL
L
7TL
H
2.6
to
3.2Vdc
Step
3
L
Table 8F-7.
Step
4
2
5
rrL
L
lTL
H
lTL
L
L
For
each setting, check the
5.
Key in
1
of
0.0
SPCL
U9
to
reset the latches. Measure all the
should be the same
0.200
in step
3,
to
disable the current Direct Control Special Function. Momentarily ground pin
IC
and
as
those given
0.210
in step
pins in steps
for
the Direct Control Special Function code
4.
2,
3,
and
4.
For
each pin, the level
0.230
in step
2,
8F-14 Service Sheet
2
Model 8901B
Service
ASSEMBLY
0
A32
PRINCIPLES
General
The Power Reference Oscillator generates a 50
over a wide range of environmental conditions. The oscillator
500
a
50
MHz Oscillator
The tank circuit of the
combination of C13 and C14. (Since C13 has much less capacitance than C14,
C14 in determining the frequency of oscillation.) At
shift. Another
tank circuit is fed to the base of Q1 through a capacitive voltage divider-C4 and varactor diode CR3.
Thus,
Service
Power Reference
OF
OPERATION
load.
180"
phase shift
at
50
MHz, an in-phase signal fed back to the base of Q1 re-enforces the oscillation.
Oscillator
50
MHz Oscillator
is
generated between the base and collector of Q1. The output of the
Sheet
MHz signal which maintains a constant output level
is
a pi-network consisting of C11, L1, and the series
3
is
adjusted to deliver 1 mW
50
MHz, the tank circuit produces 180" phase
(0
&m) into
it
predominates over
Voltage divider R12 and R13 sets the bias voltage
establish the quiescent emitter current. The frequency of oscillation
is
output of the oscillator
which sets the dc collector voltage of Q1 at ground potential.
taken from the tap of capacitive divider C13 and C14. L2
at
the base of Q1. Emitter resistors R14 and R15
is
adjusted by L1 (FREQ). The
is
an
ALC Loop
The Automatic Leveling Control (ALC) Loop is a negative-feedback loop which assures that the
oscillator output level remains constant. CR2 detects the positive peak of the signal at the output
is
of the tank circuit. The detected peak voltage
by comparator U2. If the detected signal level differs from the reference,
varactor diode CR3 which changes the junction capacitance of the diode and thus the division ratio of
the capacitive divider
bias), the positive feedback to the base of Q1 is increased and the output level of the oscillator
increased.
C9
and
CR3.
If
the capacitance of CR3 decreases (due
stored in C7 and is compared
U2
to
a
dc reference voltage
alters
the reverse bias on
to
an increase
DC Reference and Level Adjust
The reference to which the detected oscillator signal
voltage reference diode VR1. VR1, when biased with a specified current, has a breakdown voltage that
is constant with temperature. Q2 supplies
Q2
and has a temperature coefficient that tracks the base-emitter junction of Q2.
U1
inverts and slightly attenuates the reference voltage from VR1. R4 (LEVEL
adjustment of the reference and hence the output power of the oscillator. CR1 adds a temperature
coefficient to the reference input of U2 which matches the temperature coefficient
CR2. The Power Reference Oscillator
REF OSC ON/OFF -15V line.
a
constant current
is
switched
is
compared in the ALC Loop originates with
to
VR1. VR2 sets the emitter current of
ADJ)
permits fine
of
the detector diode
off
by interrupting the -15V supply via the PWR
RF choke
of
reverse
is
Service Sheet
3
8F-15
Service Model 8901B
TROUBLESHOOTING
General
Procedures for checking the Power Reference Oscillator Assembly are given below. The circuits to check
are marked on the schematic diagram by a hexagon with a check mark and a number inside, for example,
@
.
Fixed signals are also shown on the schematic inside a hexagon, for example,
Remove the assembly, the assembly cover, and RF cables where necessary
to
(+I
.9
TO
f2.1
VDC)
make measurements.
Tc....;;7
.
When removing the
supply terminals do not short against the metal
Tighten
tors is insufficient. Hand-tightened connectors can work loose and cause
reduced performance
If
the correct output level
measurement system.
19-Power Reference
SMC
connectors to
a fault in the Power Reference Oscillator
Equipment
Oscilloscope
.
. . . .
. .
.
. . .
.
. . . . . . . . . . .
Power Reference Oscillator Check
1.
Connect a high-impedance, dc coupled oscilloscope to
RF POWER, and switch CALIBRATE on and
+2
Vdc when
Hint:
If
2. Switch CALIBRATE on. Connect the oscilloscope to A32TP1. The voltage should read between
+3.5 and +5.5 Vdc.
off
and between
faulty, see Service Sheet 2 and check the Power Reference Oscillator Control.
RF
power calibrator assembly, take care that the power
0.6
N.
m
(5
in. lb). Hand tightening of connec-
or
malfunctions.
(1
mW)
must
be set
To
see what this entails, review
in
Section
. .
5.
. . . . . .
. . .
. . . . . . . .
C3
off.
The dc waveform should read between
-15
and -14 Vdc when on.
chassis
is
by
. .
. . . . . . . . . . .
parts.
isolated and repaired,
a very accurate power
Adjustment
.
.
. . . . .
(where the violet wire connects). Press
18
. . . .
OT
. . .
.
HP
1740A
+1
and
Hint:
The voltage at the collector of Q2 should be between
3.
Connect the oscilloscope
Hint:
If the oscillator does not oscillate and the voltage
U2 is working properly.
positive than
Hint:
Modulation
R10, R11,
8F-16 Service Sheet
or
3
to
A32TP2. The voltage should read between
If
the oscillator output
-1
Vdc, U2 is working properly.
or
spurious signals on the oscillator output may be caused by a fault in R9,
C8.
-6.4
and
-6.0
at
A23TP2 is more negative than
is
very large and the voltage
-5
Vdc.
and
at
-1
Vdc.
A23TP2
-5
is
Vdc,
more
Model 8901B Service
ASSEMBLIES
RF
Input (A15)
PRINCIPLES
General.
The RF Input Assembly contains the Input Attenuator, Overpower Protection, RF Level and Overpower
Detector, and 5.25 MHz High-Pass Filter. Together, these circuits provide a suitable input signal for
the Input Mixer (see Service Sheet
5.25
MHz High-Pass Filter.
The 5.25 MHz High-Pass filter must be switched in by entering user Special Function 3.3
Its function is
present along with a higher frequency input signal. The filter
termination
An example of such a signal
Input Mixer over the wide range of input frequencies. The filter
transistor Q2.
OF
OPERATION
to
to
all frequencies present at
prevent the
Service
5).
IF
from responding
its
is
the IF itself. The
Sheet
output (whether above
4
to
low-frequency, spurious signals which may be
is
a diplexer type which presents a
500
termination improves the RF flatness of the
is
switched in by relay K2 via driver
or
3.4 SPCL.
or
below the cutoff frequency).
500
Input Attenuator.
The Input Attenuator is composed of
10
dB
steps. Each pad
(R15 and R19) in parallel to handle the brunt of high-level RF power. The pads are switched in by
relays K3, K4, and K5 driven by transistors Q6, Q7, and Q8 respectively.
RF
Level Detector.
The
RF
Level Detector (CR1 and CR2) senses the positive peak of the input signal. The detected dc
voltage
LEVEL measurement mode is selected, and to de-activate the Overpower Protection relay. Because the
detector can introduce a slight amount of clipping on the input signal, it is switched slightly off
the instrument
when QlO is on.
is
used to initially set the Input Attenuator,
is
tuned
is
a resistive pi network. The
to
the input signal except when measuring Rf level. The detector
two
20
dB
pads and one 10
first
to
shunt
give
dT3
arm
an
indication
pad for a range of 0 to
of 20
dB
No. 1 has two resistors
of
RF level when the RF
Detector Amplifier.
U1 and U2 from a unity-gain amplifier and peak detector with offset. U2 detects the peaks of the
signal from the RF Level Detector when AM is present on the signal. Whenever the voltage at the
(+)
non-inverting
Note 2 on the schematic) turns on and charges C22 from
the input voltage at the inverting input plus the constant drop across CR7, R34, CR8, and R42. U1 is
a
simply
remains charged to its previous level. R39 and R41 slowly discharge C22 when the input signal level
is lowered
CR8 are hot carrier diodes whose offset voltage tracks that of CR1 and CR3 with temperature. Fine
adjustment of the offset is made with R42 which
is present.
unity-gain buffer amplifier. When the input voltage drops the output of U2 shuts off, and C22
or
input of U2 exceeds that
removed. CR7 and CR8 are biased on by R26 which acts as a current source. CR7 and
of
the inverting input
its
is
set for zero output from U1 when no input signal
(-),
the output transistor of U2 (see
emitter until the voltage across C22 equals
50
is
shut off
dT3
in
after
Service Sheet 4 8F-17
Service Model 8901B
Overpower Detector.
The Overpower Detector amplifier U3 senses when the output from the RF Level Detector and voltage
R54,
divider R36,
corresponds to
Protection relay K1 via the LO Control circuits (see Service Sheet
reset by the operator pressing any front-panel key.
and R37 exceeds +2.7V (set by
1W
of input power. The output of U3A then goes low and deactivates the Overpower
R43,
R44,
and hysteresis resistor R56) which
21.)
K1 remains de-activated until
The OVERPOWER(L) output line from U3A
perfroms two other functions. First the line is used to discharge the storage capacitor
Amplifier between
RF
when
a qausi-low is put on the line by the LO Control circuits. The low does not trip the overpower circuit
but
It
also turns
Detector can still sense
LEVEL is not being measured after the instrument
is
low enough
off
RF
LEVEL measurements. Second, the line
to
set the Detector Amplifier Discharge comparator
the
RF
Level Detector by turning on Q11, Q9, and QlO. In this
an
overpower condition and trip the Overpower Protection.
is
also
an input line from the LO Control circuit which
C22
of
is
used to turn off the RF Level Detector
is
tuned.
To
accomplish these
U3B
low which discharges C22.
state
the
Relay Drivers.
The drivers for the five relays are similar.
or
Q8)
turns the transistor on and energizes the relay. The relay contacts move in the direction of the
arrow.
and improve switching speed. Control
The capacitors across the relay coils suppress the flyback voltage when the coil
A
of
the relays
TTL
low at the base of a driver transistor (Ql, Q2, Q6, Q7,
is
de-energized
is
via the LO Control circuits (see Service Sheet 21).
the Detctor
two
tasks,
RF
Level
8F-18
Service Sheet
4
Model 8901B
TROUBLESHOOTING
General
Service
Procedures for checking
the schematic diagram by a hexagon with a check mark and a number inside, e.g.
any points outside the labeled area that must be checked are
on the schematic inside a hexagon, e.g.
and output cables where necessary to make measurements.
Tighten
connectors
came reduced performance, malfunctions,
he RF Input Assembly are given below. The circuits to check are marked on
also
identified. Fixed signals are shown
(+1.9
to
+2.1
Vdc)
.
Extend the board assembly
SMC
connectors to 0.6 N-m
is
insufficient. Hand tightened connectors can
(5
in.
lb).
Hand tightening
or
damage to the instrument.
Equipment
Oscilloscope
Power Supply..
Signal Generator.
Voltmeter
(J1)
Input Attenuator Check
1.
Set the signal generator to
ac coupled oscilloscope. Switch the input impedance of the oscilloscope to 500
input in 50R using a tee.
......................................................................
...................................................................
.................................................................
........................................................................
11
MHz CW at +13 dBm. Connect int Rf output to the input of an
work
@
of
loose and
or
.
In addition,
and
its input
HP 1740A
HP
6215A
HP 8640B
HP 3455A
terminate the
2.
Fine adjust the signal generator's level for
3.
Reconnect the signal generator's output to the Modulation Analyzer's INPUT. (The input cable,
W1
or
W3, should be connected
A15J2 (RF OUT).
4.
Key in 41.0
check the signals indicated in Table 8F-8.
Hint: If the oscilloscope display reads low for all above conditions, check the 5.25 MHz High-Pass
Filter, Overpower Protection, and input cable (W1
I
Check
Thru
SPCL
Table
Path
to
initialize the instrument. Key in the Direct Control Special Functions and
8F-8.
I
Direct
0.047
to
Levels at
Control
Function
A15J2 with an extender cable.) Reconnect the oscilloscope to
I
an
oscilloscope display of 3 Vpp.
or
XA15,
18
QSc, Q7c, and Q~c,
A28XA15
I
19 20
L I L
W36).
.
-a.
Level
I
I
I
L)
.~
Pin Trai
Q6-c
L
H
-a
ar
nsistor
c,
H I H
(J1)
_,
__
-
Step
4
I
Oscilloscope
I
2.75
to
3.00
I
Service Sheet
4
8F-19
Service
@
Model 8901B
5.25
MHz
High-Pass
1.
Set the signal generator to
ac coupled oscilloscope. Switch the input impedance of the oscilloscope to
Filter Check
5.25
MHz CW
at
+3
am. Connect
its
RF output
50R
to
the input
or
terminate the
input in 50R using a tee.
Fine adjust the signal generator’s level for an oscilloscope display of 1 Vpp.
2.
Reconnect the signal generator’s output to the Modulation Analyzer’s INPUT. (Connect the input
3.
cable,
W1
or
W36,
to
A15J1 with an extender cable.) Reconnect the oscilloscope
to
A15J2
OUT).
of
(RF
an
Key in 41.0 SPCL and 0.047 SPCL
4.
path. Key in the Direct Control Special Functions and check the signals indicated in Table 8F-9.
Hint:
Table
Check
Thru
Path
5.25
MHz
HPF
If
the oscilloscope display reads low for both above conditions, check the Input Attenuator,
8F-9.
Direct Control Level (TTL) at Oscilloscope
Special
Overpower Protection, and input cable (Wl
(J3)
Overpower Protection Check
1.
Set the signal generator to
11
Analyzer’s INPUT.
2.
Connect an ac coupled oscilloscope
oscilloscope to 500
3.
Key in 41.0 SPCL to initialize the instrument. Key in
0
dB.
Check the signals indicated in Table 8F-10 for the thru path only.
or
terminate the input in
Table
~~ ~
Check
Thru
8F-10.
Path
Overeower
to
initialize the instrument and set the attenuator to the thru
Levels at
Function
0.024
0.02c
or
W36).
MHz CW at
to
A15J2 (RF OUT). Switch the input impedance of the
Levels at
Level
A28XA15
L
H
XA15,
‘A28XA15 Pin 28 02-c Display (Vpp)
and
Q2c
@
Step
4
L
H
+13
am. Connect its RF output to the Modulation
H
L
0.76
0.59
to
to
1.00
0.73
50 R using a tee.
1.1
SPCL
to
set the input attenuation to
XA15
and
Qlc
(J3)
Step
3
(TTL)
Pin 17
at Oscilloscope
-
01-c
~ Display (Vpp)
H
L
2.75
to
0
3.00
Hint: If the oscilloscope display reads low, check the Input attenuator,
input cable (Wl
4. Remove signal generator from INPUT. Set power supply to
Modualtion Analyzer’s INPUT (the minus side should be at ground).
5.
Reconnect the signal generator. Check the signals indicated in the table under step 3 for the
condition
6. Repeat step
8F-20 Service Sheet 4
or
W36).
of
overpower. The display should also show
3
to check the recovery from the overpower condition.
E06.
5.25
MHz High-Pass Filter, and
20
Vdc. Touch the +20V lead to the
Model 8901B
(J4)
Overpower
NOTE
If step 4 is repeated,
dc blocking capacitor
Discharge
it
also after completing this check.
it
is usually necessary to first discharge the input
by
connecting a
Detector Check
1.
Key in 41.0 SPCL to initialize the instrument.
2.
Connect a high-impedance, dc coupled oscilloscope
node should be a TTL high.
5Ofl
termination to
or
a dc voltmeter to A28XA15 pin
the
INPUT.
Service
21.
The
(ZJ
3. Induce an overpower transient by touching the +20V lead
(the minus side should be at ground). The node should show
to
the Modulation Analyzer’s INPUT
a
momentary low. The display should
also show E06.
NOTE
If
steps 1 to 3 are to be repeated, it is necessary to first discharge the input
50
R
dc blocking capacitor by connecting a
termination to the INPUT.
Discharge it also after completing this check.
Detector
1.
Set the signal generator to
Analyzer’s INPUT. (Connect the input cable,
2.
Key in 41.0 SPCL to initialize the instrument. Press RF LEVEL.
Amplifier
Check
11
MHz CW at +13 dBm. connect its
Wl
or
W36,
RF
OUTPUT
to
A15J1 with an extender cable.)
to
the Modualtion
3. Key in 0.024 SPCL and 49.31 SPCL to turn the detector on and to connect the internal voltmeter
to the output of the Detector Amplifier. Change the level
Table
8F-11
and note the display. Alternatively, measure A15TP1 (RF DET) with a dc voltmeter.
%ble
RF
Level
8F-11.
1.05
0.27
Detector Amplifier Limits
Display
Limits
to
1.25
to
0.33
Voltage
at
A15TP1
1.05
0.27
of
the signal generator as indicated in
Limits
(Vdc)
to
1.25
to
0.33
(J6)
Hint:
If
the
off
condition above is slightly out of limit, perform the
RF
Level
1.
Remove the cable (Wl
2.
Measure the dc voltage at the junction
Detector
Offset
Check
or
W36) from A15J1. Key in 0.024 SPCL to turn the detector on.
of
CR4 and C13.
(The input impedance of the voltmeter must be at least 10 MR.)
Measure the dc voltage at pin
3.
read in step
4.
Key in 0.020 SPCL to turn the detector
Measure the dc voltage at the junction of CR4 and CR13.
5.
2.
2
of
U2.
It
should be 50 to
off.
$10.0 Vdc.
Measure the dc voltage at pin
6.
2
of U2.
It
should be between +13.0 and +14.5 Vdc.
RF
Detector Offset Adjustment.
It
should be between -0.5 and -0.2 Vdc.
70
mV more negative than the voltage
It
should be between +8.5 and
Service Sheet
4
8F-21
Service Model 8901B
(J7)
Detector Amplifier Discharge Check
1.
Set the signal generator to
Analyzer’s INPUT. (Connect the input cable, W1
2.
Key in 41.0 SPCL to initialize the instrument. Press RF LEVEL.
11
MHz CW at +13 dJ3m. Connect its RF output
or
W36, to A15J1 with
to
the Modulation
an
extender cable.
3. Key in
4.
5.
6.
@
SWRCheck
1.
0.020
SPCL to turn the detector
to the output of the Detector Amplifier. The display should read between
Key in
0.024
SPCL to turn the detector on. Key in 49.31 SPCL. The display should read between
off.
Key in 49.31 SPCL to connect the internal voltmeter
-0.0200
1.0000 and 1.3000.
Set
the signal generator for 50% AM at a
20
Hz rate. The display should vary no more than
fO.lV from its average value.
Press RF LEVEL. Switch the signal generator’s AM off, then turn the RF
as
Modualtion Analyzer’s display
0.010 -03 watts by the second reading after the RF
the reading decreases. The reading should decrease to less than
is
switched
off.
Perform the SWR portion of the RF Level Performance Test.
and
+0.0200.
off
and note the
8F-22
Service Sheet
4
Model 8901B
Service
ASSEMBLIES
0
A17 Input Mixer
0
A16 Buffer Amplifier
0
A18
IF
Amplifier
0
52
LO
Input Switch (Option Series
PRINCIPLES
OF
OPERATION
General-A17 Input Mixer Assembly
The Input Mixer Assembly down-converts the input signal to an intermediate frequency
2.5
input signals above
is
normally
MHz. Below 2.5 MHz the input signal passes directly through the Mixer into the
down-conversion.
The Input Mixer Assembly contains the Mixer, LO Amplifier, and two
Filter and a
Filter in the AM Demodulator Assembly which determines the frequency response of the
(see Service Sheet
1.5
MHz for frequencies above
4
MHz Low-Pass Filter). The 4 MHz Low-Pass Filter
MHz, the
8).
Service
030)
IF
is equal
10
Sheet 5
to
the LO frequency minus the signal frequency. The
MHz
and
455
kHz for frequencies between
IF
Amplifier without
IF
filters (a
is
followed by a
455
2.5
2.5
kHz Bandpass
MHz Low-Pass
1.5
(IF).
and 10
MHz
For
IF
IF
LO
Amplifier
The input
Assembly (see Service Sheet 17). The amplifier has a gain of approximately 10
port of the Mixer (Ul) at about
Q6,
which are actively biased by
notice that
choke.) The base of
Q5
is normally a junction drop above this. The collector of
Changes in the collector voltage of
voltage
The gain of each stage
to the collector load. C3 increases the gain slightly at high frequencies.
of
to
the
LO
Amplifier
for
dc levels the emitter of
Q5
is
fixed at the voltage determined by voltage divider
Q4.
is
inversely proportional
is
a
1.25
to
1301.5 MHz signal which comes from the LO Divider
+10
am. The amplifier has
Q5
and
Q7
respectively. Using
Q5
is
connected directly to the collector of
Q4
alter the collector current of
to
the total emitter resistance and directly proportional
dB
two
stages, stripline transistors
Q4
and
Q5
to
illustrate the biasing,
Q4.
R1
and
R2.
Q5
is
the source of dc base current for
Q5,
which regulates the collector
and drives the L
(L3 is an RF
The emitter of
Mixer
Mixer U1
but signals at the
pass into the
coupled out from the center tap of the same transformer. UlCl
that follow. UlTl optimizes the impedance seen by the
Mixer’s R port improves the flatness over the wide range of input frequencies by presenting a constant
impedance to the
is
a single-balanced type (that is, signals at the L port are balanced out at the R and I ports,
R
port are not balanced at the I port). This permits low-frequency input signals to
IF
without down-conversion. The LO signal
IF
at the R port. The Limiter adds protection to the Mixer.
is
coupled into the Mixer by UlT2.
is
the first element of the
IF
Amplifier. The Input Pad ahead
IF
Q4
IF
filters
of
and
Q4.
is
the
Service Sheet
5
8F-23
Service Model 8901B
IF Filters
IF
Filters
path. When Q12B goes on, Q12C goes on and
activates Q9 and
The seven-pole,
passband flatness to minimize incidental AM (AM generated in the
(23144
The 455 kHz Wide Bandpass Filter has seven poles and a 3
for best passband flatness to minimize incidental
L11 is adjusted primarily for best phase linearity vs. frequency to minimize FM distortion generated
in the IF. The filter
of U2B goes low. This also turns on DS1.
The 4
MHz
is switched in by
via the LO Control Assembly (see Service Sheet 21).
The
IF
is routed in one of
Option 030, the IF is routed directly to the A18
030 installed, the
(2642A
The down-converted
1.
Filter
is selected, C32
of
The amplifier has
2.3 to the input of Q8 (the through-path). C18, R13, and
response to compensate for slope in the
Transistor Q12B controls the routing of the
Q8 go off and switch the through-path out. This also turns on
Control Assembly (see Service Sheet 21).
adjusted primarily for best phase linearity vs. frequency to minimize FM distortion generated in the
(UlC2
the Input Mixer for low-frequency input signals. The
to
2636~)
AM (AM
is
switched in by Q3 and Q1 which forward-bias CR3 and CR6 when the output
Low-Pass Filter has three poles
Q2
which forward-biases CR4 and CR5 when U2A goes low. Control of the filters
two
ways depending upon the option installed. For instruments without
IF
is
routed
and
Above)
(IF)
in
the Input Mixer
is
switched in (by Q13 turning on CR8 and CR9)
a
gain
Q1,
and switches the 455
455
kHz
to
the A16 Buffer Amplifier Assembly.
signal from the Input Mixer is filtered by the three-pole, 4 MHz
is
the first reactive component of the filter.) When the 455
of
5
to
the input
Wide Bandpass Filter has
(23144
IF
Amplifier Assembly. For instruments with Option
of
Q9 (the input
4
MHz Filters 1 and
IF
to
the 455 kHz Wide Bandpass Filter
Ql2D
kHz
Wide Bandpass Filter in.
3
dB
bandwidth of 200 kHz.
generated in the
to
2332A)
IF
is then amplified by the First
to
goes
or
five poles
to
improve the conversion efficiency
the 455 kHz Wide Bandpass Filter) and
L7
add a slight slope
2.
off.
Ql2C forward-biases CR3 and CR4,
At
DS1.
Control of the filters
dB
bandwidth of 200
IF
kHz.
as
L8
IF
as the result of
(23334
to
IF
to
the
IF
or
the through
the same time CR5 and
is
L8
is
adjusted for best
the result of FM). L11
is
adjusted
FM).
2636A).
Low-Pass
kHz
Amplifier.
frequency
via the LO
IF.
It
is
IF
is
NOTE
In Selective Power (Option Series
kHz
IF
bandpass
measurements, the filter bandwidth depends
mode and option installed.
The IF
gain of 2.8 at the output
Low-Pass Filter
General
The signal from the Input Mixer is split and amplified by
amplifiers. One
the Opt. 030 circuitry.
IF
Buffer Amplifier
Transitors Q3, Q2, and
The non-inverting input
divider formed by R10 and R9 supplies base bias
l+(R14/R12). The output from the emitter of
by
is
routed to the A72
to
2
on to the
-
A16
IF
Buffer Amplifier Assembly (Opt.
is
a
0
dB
Q1
R17 and the input impedance of the following circuitry. This results in a overall gain
filter
is inserted into the
IF
Channel Filter Assembly through the Second IF Amplifier, which has a
54 and unity gain at the output
A18
IF
Amplifier Assembly. The 4 MHz Low-Pass Filter 2 has three poles.
gain buffer
form a non-inverting operational amplifier with an ac gain
is
the base
for
the A18
of
Q3 and the inverting input is the emitter of Q3. The voltage
030)
Q1
measurement modes, a second
IF
path.
For
Selective Power
on
the particular measurement
to
81.
The
IF
is also routed through 4 MHz
030
only)
two
IF
Amplifier and the other a 9
for
both Q3 and Q6. The gain of the amplifier is
is
divided by
2
(23144
descrete non-inverting operational
(6
dB)
by the voltage divider formed
455
to
2636A)
dB
gain amplifier for
of
of
2
(6
dB).
1
(0
dB).
83-24 Service Sheet
5
Model
8901B
IF
Amplifier
Service
Transistors
5.6
(15
determining resistors are
the following circuitry, which also divides the output from the emitter of
in a overall gain of 2.8
General-AI 8 IF
The signal from the Input Mixer, whether down-converted
The amplifier is a low-noise type with 33
distortion. The IF Amplifier has three stages.
IF
Input Amplifier
The first stage,
of feeding signal back to the input through R6, generates
by
a
strictly passive resistance. The input impedance
gain. The gain is approximately
Q6,
Q5,
and
Q4
form a non-inverting operational amplifier with
dB)
.
The operation of the amplifier
R5
and R3. Resistor R8 forms a voltage divider with the input impedance of
(9
&).
Amplifier Assembly
Q7
and
Q5,
is low noise and has 20
R9
divided by
is
identical to that of the
or
dB
of gain and a phase compensation network
dB
of gain.
a
lower source noise than would be generated
is
essentially equal to
R7.
Inverting Amplifier
The second stage
compensate for phase shifts generated in the
distortion. The IF shape can also be adjusted to minimize incidental AM.
A
simplified diagram
-1,
Q2
with a gain of
is
a unity-gain amplifier with a phase-shift characteristic that can be adjusted to
1.5
MHz
IF
system. This compensation improves FM
of
this stage is shown in Figure
+l.
The voltage gain
for
8F-8.
the circuit is
Q1
an
ac gain of approximately
IF
Buffer Amplifier. The gain
Q4
by 2
(6dB).
not,
is
amplified by the
An
active input impedance, the result
R6
divided by the amplifier
is
shown as
an
amplifier with a gain of
This results
IF
Amplifier.
to
reduce FM
V,
R-
_-
-
VI
which has a constant magnitude
C15,
and
C16.
R
is
L2,
optimum phase shift (minimum FM distortion) at
flatness (minimum incidental AM) at
formed by the combination of
v1
(1-1)
and a variable phase shift. The impedance
1.5
MHz.
jX
R+jX'
R17,
1.5
R23, and
MHz.
R24.
R
is
fine adjusted by
R19
fine adjusts the gain of
v2
jX
is
formed by L1,
Q1
R23
for
for best
Figure
8F-8.
Simplified Diagram
of
Phase Compensation Amplifier
Service Sheet
5
8F-25
Service Model
IF
Output
Amplifier
8901B
The third stage
plus R29 divided
is
by
a
13
dB
amplifier which drives the
R27.
AM
Demodulator.
Its
gain is approximately one
8F-26
Service Sheet
5
Model 8901B Service
TROUBLESHOOTING
General
Procedures for checking the Input Mixer and
check are marked on the schematic diagram by a hexagon with a check mark and a number inside, for
example,
Fixed signals are also shown on the schematic inside a hexagon, for example,
Extend the board assemblies and their input and output cables where necessary
@
.
In addition, any points outside the labeled area
Tighten
tors is insufficient. Hand-tightened connectors can work loose and
reduced performance, malfunctions,
SMC
connectors to
0.6
IF
Amplifier Assemblies are given below. The circuits to
that
must be checked are also identified.
(+I
.9
TO
to
make measurements.
N.
m
(5
in. lb). Hand tightening of connec-
cause
or
damage to the instrument.
Equipment
Oscilloscope
Signal Generator.
Spectrum Analyze
Test Probe
Voltmeter
(J1)
LO
Amplifier General Check
......................................................................
.................................................................
..........................................................
...................................................................
........................................................................
NOTE
HP 8559A/182T
HP 1250-1598
+2.1
VDC)
HP 1740A
HP 8640B
HP 3455A
.
This test checks only the
localize a catastrophic failure.
and will detect more subtle failures; however, more time and equipment are
required,
1.
Key in 5 and press MHz
2.
Connect a high-impedance oscilloscope to the base of A17Q4. The waveform should be a square
wave with a period of approximately 180 ns and an amplitude of
ringing.
Hint:
If the signal
Sheet 17).
3. Connect the oscilloscope to the collector of A17Q4. The waveform should be a square wave of
1.2
Vpp
or
greater excluding ringing.
4.
Connect the oscilloscope to the collector of A17Q6. The waveform should be a square wave of
1
Vpp
or
greater excluding ringing overshoot on the falling edge.
to
is
faulty, check the output from the A19
set
LO
Amplifier at a low frequency but will easily
@)
LO
the
LO
to
5.455 MHz.
Amplifier Check
is
more thorough
0.5
LO
Divider Assembly (see Service
Vpp
or
greater excluding
Service Sheet
5
8F-27
Service Model 8901B
(./2)
LO
Amplifier Check
NOTE
To check for a catastrophic failure
Amplifier General Check
1.
Unplug A17U1 Mixer. Gently pry
2. Set the spectrum analyzer to measure a +20 dBm,
8
pins
3.
Key in 57.0 SPCL to cause the LO to sweep sequentially across bands DBLR through
signal should sweep slowly from above 1300 MHz
five bands. As the low end of a band
then continue to sweep. Throughout the sweep, the fundamental of the
and +15 dBm.
Hint:
Hint:
LO frequency.
4. Set the spectrum analyzer to view a 0 to 40 MHz signal.
5. Key in 56.0 SPCL to cause the LO
sweep slowly
and 9 of the Mixer socket using the test probe. The probe center goes on pin 9.
A
faulty level may also be the result of a faulty output from the A19 LO Divider Assembly.
The sweep can be halted by pressing the SPCL key. Use manual tune to manually set the
from
above 50 to below 1.25 MHz in the manner described in step 3 above.
The test probe will cause a low-frequency
1.25
MHz.
above.
it
from
is
to
sweep sequentially across bands 4 through
of
the
LO
Amplifier, use the
its
socket with a screwdriver blade.
0
to
1400
MHz signal. Connect
to
below
40
MHz. The sweep will occur over
reached, the sweep will stop, jump up slightly in frequency,
NOTE
rolloff
to about
a
LO
should be between +7
+4
dBm at
LO
its
input to
3.
The
8.
The LO should
LO
@)
The low-frequency bands can
oscilloscope should have a
wave with an amplitude
rolloff
due to the probe.
Input
and
Mixer Check
1.
Set the signal generator to 18 MHz CW
ac coupled oscilloscope. Switch the input impedance
input in 500 using a tee.
2.
Fine adjust the signal generator’s level
3.
Reconnect the signal generator’s output to A17J2 (RF IN). Reconnect the oscilloscope to A17J1
(IF
OUT).
4.
Press the blue key, then press INSTR
the instrument. Key in
a sine wave with an amplitude
fuzziness on the waveform is normal;
annunciator (A17DS1) should be
18
of
and press MHz to set the LO to 19.5 MHz. The waveform should be
of
70 to 110 mVpp and a period of approximately 670 ns. A slight
off.
also
be viewed on an oscilloscope.
500
termination.
approximately 2 Vpp excluding ringing and the
at
0
for
an oscilloscope display of
PRESET
it
is the partially filtered sum frequency. The 455 kHz
The
signal should be a square
dBm. Connect
of
the oscilloscope to 500
(the AUTOMATIC OPERATION key) to preset
its
RF output to the input of an
800
The
or
mVpp.
terminate the
IF
8F-28 Service Sheet
5
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