Thurlby DSA524 Operating Manual
Thurlby DSA524
digital storage adaptor
OPERATING MANUAL AN D TUTORIAL
V2.0
1
INTRODUCTION
This manual is divided into four sections as follows:
Section 1 is the main reference section which contains a detailed
description of the operation of each function.
Section 2 is a tutorial for new users (or for existing users needing
examples of how functions operate).
The Appendices provide additional detail on some of the more
complex functions and on some of the theoretical aspects of digital
storage.
The Specifications give detailed technical specifications for the
instrument, plus service and warranty details.
N.B. The DSA524 can use either an oscilloscope or a personal
computer (IBM compatible) as the display device. This manual only
covers use with an oscilloscope. To use a PC as the display device
the "DS-PC Link" software package must be purchased, an additional
manual is supplied with this software.
FIRMWARE LEVEL
From time to time small revisions may be made to the
firmware of the DSA524 (the firmware is the ROM based
program code for the unit).
The firmware level is displayed briefly whenever the unit is
first switched on (and when SET 'SCOPE is used) as DSA524
X.XX, where X.XX is the firmware level.
This issue of the operating manual applies to units with a
firmware level of 2.71 and above.
IT IS VERY STRONGLY RECOMMENDED THAT NEW
USERS START BY FOLLOWING THE TUTORIAL.
INDEX
Title Section Page
REFERENCE SECTION
Connecting an Oscilloscope..................................... R1 ...............2
Setting up the Display .......................................... R2 ...............2
Front Panel Controls ............................................. R3 ...............2
Escape and Reset ................................................ R4 ...............2
Controlling Digitising ............................................R5 ...............2
Timebase Setting ................................................. R6 ...............2
CH1 and CH2 ....................................................... R7 ...............3
Digitising Memories .............................................. R8 ...............3
Trigger ................................................................ R9 ...............3
Trace A and Trace B ............................................. R10 ..............4
Memory Search (Comp, Scan, Mag) ........................ R11 .............5
Indexed Waveform Memories ................................. R12 ..............5
Display Update Rate .............................................. R13 .............5
Roll Mode Operation ............................................. R14 .............6
Repeat Mode Operation ......................................... R15 .............6
Cursor Measurement ............................................. R16 .............6
Program Mode ...................................................... R17 .............6
Sine Interpolation ................................................R18 .............7
Average ...............................................................R19 .............7
AxB (Trace Multiply) ............................................. R20 .............7
Line or Dots Display .............................................R21 .............7
Plot ..................................................................... R22 .............7
Print ...................................................................R23 .............8
Remote Control and Data Transfer ......................... R24 .............8
RS-423 Interface ................................................... R25 ............8
GP-IB (IEEE-488) Interface ................................... R26 ............8
Directly Acting Functions ...................................... R27 ............8
USERS TUTORIAL
Index to Tutorial .......................................................................9
APPENDICES
The Oscilloscope Display ........................................ Appendix A 16
Aliasing ............................................................... Appendix B 16
Bandwidth and Interpolation .................................. Appendix C 16
Analog Plotter Function ......................................... Appendix D 17
Connecting the Serial Interface .............................. Appendix E 18
Software Commands ............................................. Appendix F 18
SPECIFICATIONS
DSA524 Technical Specifications ............................ S1 ................20
Electrical Isolation ................................................. S2 ...............22
General Safety Considerations ............................... S3 ...............22
Fuse Replacement ................................................ S4 ...............22
Maintenance and Repair .......................................S5 ...............22
Updating the Firmware ......................................... S6 ...............22
Guarantee ........................................................... S7 ...............22
ADDENDA AND ERRATA .........................................................22
2
REFERENCE SECTION
R1. CONNECTING AN OSCILLOSCOPE
Any conventional oscilloscope with an 8x10 division graticule can
be used with the DSA524. Only one input channel is needed and
a bandwidth of 5MHz is sufficient. The oscilloscope can be
connected using a single BNC to BNC cable. Alternatively two
cables can be used (see Appendix A).
Connect the rear panel socket marked "composite" to the vertical
input of the oscilloscope. If a two cable connection is preferred,
connect the rear panel socket marked "trigger" to the external
trigger input of the oscilloscope.
Set up the oscilloscope controls as follows:-
Vertical Input: DC coupling
100mV per division
Timebase: 50usec per division
No Sweep Hold-off
Trigger: Source — as input channel (single
cable connection)
— or external (two cable
connection)
Mode — normal (not bright-line auto)
Coupling — AC
Slope — negative
Level — negative
R2. SETTING UP THE OSCILLOSCOPE DISPLAY
Switch on the DSA524 (the power switch is at the rear) and wait
while the unit performs a brief selftest and initialisation. The
oscilloscope should then show a message as follows:"CENTRE ARROW TIP" alternating with "THEN PRESS ESCAPE"
Adjust the oscilloscope's horizontal and vertical position controls
until the tip of the arrow is exactly in the centre of the screen.
Two horizontal lines should be visible, one at three divisions
above the graticule centre and one at three divisions below.
Pressing the ESCAPE key on the DSA524 (top right-hand corner)
will remove this set-up display and start normal operation.
Once the display oscilloscope has been set up, no further use
need be made of its controls except for occasional re-centering of
the trace to correct for any warm-up drift. To re-centre the trace
press the two keys marked "SET 'SCOPE" (on the lower right of
the DSA) and adjust the oscilloscope's position controls to centre
the arrow tip. If problems occur when setting up the display read
Appendix A.
The oscilloscope setup
display.
R3. FRONT PANEL CONTROLS
All of the controls, including the rotary ones, are monitored by a
microprocessor which then performs the control. All of the
switches are of a momentary non-latching "key" type whose
status is indicated either by lamps under the panel or, where
appropriate, by text on the display. Each rotary control is enabled
or disabled by the key to its right. The control is enabled when
the lamp above the key is off, and is disabled when the lamp is
on.
The keys within the numeric keypad section (right-hand side of
unit) have up to three functions. The normal function is printed
in black above each key. After the SHIFT control is pressed the
function becomes as printed in green above the key. When a
function ending in (NN) is pressed (e.g. SAVE(NN)) the key will
enter the number printed alongside it.
Note that when SHIFT or a function ending in (NN) is pressed,
the lamp marked ENTER will flash to indicate that one or more
further key presses is required. Any function that causes the
ENTER lamp to flash can be cancelled by pressing that function
key again.
Note that some of the functions (including SET 'SCOPE)
automatically set the RUN/HOLD function to HOLD. If digitising
ceases unexpectedly check to see if you have unintentionally
set RUN/HOLD to HOLD.
All of the front panel settings are retained when the power is
turned off (except for RUN/HOLD which is always set to HOLD
at power-on).
R4. ESCAPE AND RESET
The ESCAPE key is used to terminate some of the functions of
the DSA such as "SET PLOT" and "SET AVG". At other times it
has no effect except after SHIFT has been pressed.
Pressing SHIFT followed by ESCAPE gives the RESET function.
RESET places the DSA into a known state. It cancels all of the
more complex functions and sets the DSA into a state which
makes it easy to display the input signals.
The main effect of RESET is to set up the unit as follows:-
RUN /HOLD RUN.
CH1 and CH2 Coupling: AC, Volts/div: 1V, Offset: ZERO,
On/off: ON.
TRIGGER Run/hold: RUN, Source: CH1, Mode: AUTO,
Level: ZERO, Slope: POS, Coupling: AC.
TRACES A Trace A source: CH1, Trace B source: CH2,
and B Gain variable: CAL, Position: HOME.
TIMEBASE Time/div: 20usec.
ALL OTHER
FUNCTIONS OFF.
After pressing RESET the DSA will be set such that the CH1
signal is displayed on trace A and the CH2 signal on trace B.
The display will be updated about three times per second. The
input sensitivity will be set at 1V per division. The timebase will
be set to 20usec/div. The input sensitivity and timebase speed
will need to be adjusted to suit the signal.
R5. CONTROLLING DIGITISING (DATA ACQUISITION)
The digitising of the input signals is controlled by the two keys
at the top of the TRIG section marked RUN/HOLD and SINGLE.
When RUN/HOLD is set to RUN, the DSA is automatically rearmed following each acquisition. Thus digitising is repeated
continuously provided that triggering is active (see Section R9
Trigger — Mode).
When RUN/HOLD is set to HOLD, digitising is stopped and the
contents of the digitising memories is frozen. When set to
HOLD, the DSA can be armed for a single acquisition using the
SINGLE key.
Digitising can be performed on both input signals
simultaneously, or on either signal individually. Each input
channel (CH1 and CH2) is enabled or disabled using the key
marked ON/OFF at the top of each channel section. When one
of the input channels is turned off, the digitising memory
associated with that channel is frozen and digitising takes place
on the other channel alone.
R6. SETTING THE TIMEBASE
The timebase of the DSA524 is somewhat different from that of
a conventional real-time oscilloscope. The time per division
keys, instead of setting a sweep speed, actually control the
3
sampling rate of the digitiser. The shorter the time per division
the higher the digitising rate. After it has been stored, the
digitised data is displayed as 1024 samples across the screen
(100 samples per graticule division with 12 samples before and
12 samples after the graticule). Thus the time per division is
equal to 100 divided by the sampling rate.
The timebase should be set in much the same way as with a realtime oscilloscope but, unlike a real-time oscilloscope, the effect of
setting the timebase speed to too slow a rate can cause aliasing
effects on the display (see Appendix B). For this reason if the
signal frequency is unknown a fast timebase speed should be
selected initially.
Alternatively, if the signal is repetitive, the AUTOSET facility can
be used. It is fully described at the end of this section.
As well as setting the digitising rate, the time per division keys
also set the digitising mode. There are three modes:
NORMAL MODE: 100msec to 5usec per division (1KS/s to 20MS /s
sample rates) suitable for single or repetitive events. The screen
is updated after each digitising is completed.
ROLL MODE: 200msec to 200 minutes per division (500S/s to
8.3mS/s sample rate). Suitable for single or repetitive events. The
screen is updated continuously. As digitising takes place, new
data is written onto the right-hand side of the screen and old
data disappears from the left-hand side. Thus the waveform
appears to "roll" across the screen similarly to a strip chart
recorder. (See Section R14. Roll Mode Operation.)
REPEAT MODE: 2usec to 50nsec per division (50MS/s to 2GS /s
equivalent sample rate). Suitable for repetitive events only. The
screen is updated more slowly than in normal mode. The signal
must be repetitive and must be generating a trigger signal. Auto
free-run (trigger mode = AUTO) is not available in repeat mode,
neither is LINE trigger. (See Section R15. Repeat Mode
Operation.)
The time per division keys will autorepeat if held. When a change
in timebase speed causes a mode change a "beep" will be heard.
ROLL and REPEAT (RPT) modes are indicated by front panel
lamps.
When CH1 and CH2 are ON simultaneously the DSA524 uses a
single high speed digitiser to digitise both signals. For timebase
speeds of 20usecs per division and slower, both signals are
digitised together using a very high speed chopping technique.
For timebase speeds of 10usecs per division and faster the
signals are digitised separately on alternate trigger events. The
CHOP and ALT lamps show which mode is being used.
AUTOSET: The autoset function can be used to automatically set
the timebase speed so as to avoid aliasing effects. The input
signal must be repetitive and be generating a trigger signal
whose repetition rate is between 50Hz and 5MHz.
To initiate autoset press the two keys in the timebase section
marked AUTOSET simultaneously. If the trigger rate is between
50Hz and 1MHz the timebase will be reset to a position that gives
between 4 and 10 trigger events across the screen. The AUTO
lamp will illuminate for 3 seconds and the display will show the
message "AUTOSET SUCCESSFUL" for 3 seconds.
If the trigger rate was below 50Hz the timebase setting will
remain unchanged, the AUTO lamp will not illuminate and the
display will show the message "TRIGGER RATE TOO LOW" for 3
seconds.
If the trigger rate was above 1MHz the timebase will reset to
1usecs/div, the AUTO lamp will illuminate for 3 seconds, the
display will show the message "TRIGGER RATE ABOVE 1MHz"
and the DSA will "beep" to indicate that the timebase speed may
need to be set faster.
R7. CH1 AND CH2
The two input channels are identical. The input sensitivity is
variable between 10V and 2mV per division. The maximum
bandwidth is 35MHz down to 20mV/div, 20MHz at 10mV/div,
10MHz at 5mV/div and 5MHz at 2mV/div. The volts per division
keys autorepeat if held. The input coupling can be AC ( 3db =
5Hz) or DC.
When both keys are pressed simultaneously the unit enters an
AUTORANGING mode whereby the sensitivity is automatically
set to maintain a peak to peak signal amplitude between 2 and
4 divisions. The AUTO lamp is then illuminated. To leave autoranging mode press either key. Autoranging does not operate in
"roll" or "repeat" timebase modes.
The digitiser has an input dynamic range equivalent to ± 4.25
screen divisions. To maintain the input signal within this range
an OFFSET control is provided which has a range of ±8
divisions. OFFSET can be set to zero by pressing the
ZERO/(VAR) key. Press it again to return to variable offset.
If the signal goes outside the dynamic range of the digitiser
while digitising is taking place the AUTO/OVLD lamp flashes to
show that a signal overload is occurring. Signal values which
exceed the dynamic range will be digitised as either + 4.25 divs
or - 4.25 divs. Note that the lamp will continue to flash when
the waveform is "held" even if the signal overload is removed.
The input amplifiers have an automatic system for removing
offset errors. If the GND and AC/DC keys are pressed simultaneously an "autocalibration" takes place which re-checks and
removes offset errors from every volts/div position. AUTOCAL
should be used when the unit has warmed up, i.e. after about
20 minutes.
Each channel has an ON/OFF key. The operation of these keys
is explained in the next section (Digitising Memories).
R8. DIGITISING MEMORIES
Each channel has its own digitising memory of 4K words. When
both channels are turned ON (using the channel ON/OFF keys),
both memories are updated whenever digitising is performed.
When one channel is turned OFF, the contents of its digitising
memory is frozen and only the memory for the other channel is
updated.
The contents of the digitising memories is unaffected by the
settings of the trace controls. The digitising memories are nonvolatile i.e. their contents are not destroyed when the power is
turned off.
Note that if either of the digitising memories is storing a waveform which gave rise to an overload condition, the OVLD lamp
will continue to flash until the memory is overwritten with a
waveform which is within the dynamic range of the digitiser.
R9. TRIGGER
The trigger circuitry of the DSA is broadly similar to that of a
real-time oscilloscope. The trigger signal defines the exact
moment at which digitising takes place. Control of the trigger
circuit defines which part of a waveform will be stored.
Many functions are similar to those of a conventional oscilloscope:
SOURCE: Can be from CH1, CH2 or EXT (external
trigger BNC socket).
MODE: AUTO — similar to "Bright Line Auto", the
digitiser free runs if there is no trigger or if the
trigger repetition rate is below 20Hz. If the rate is
above 20Hz digitising takes place in synchronism.
NORM (Normal) — digitising is always in synchronism with the trigger signal. If there is no
trigger signal digitising stops.
LINE — digitising is in synchronism with AC line
rate (50 or 60Hz).
N.B. The trigger modes operate differently when
the timebase is set to "roll" mode (see Section
R14. Roll Mode Operation).
Neither Auto nor Line are available when the
timebase is set to "repeat" mode (see Section
R15. Repeat Mode Operation).
4
LEVEL: Variable over ±4 divisions. Press ZERO/(VAR)
key to set zero level triggering, press again to
return to variable.
SLOPE: Can be POS (positive edge triggered) or NEG
(negative edge triggered).
COUPL: Can be AC, DC or HFREJ (high frequency
reject). With TV and video signals, use AC for Line
sync and HFREJ for Frame sync.
N.B. The lamp marked TRIG, to the right of the SINGLE key,
indicates the presence of a trigger signal.
Other functions are specific to the digital storage function:
RUN/HOLD: RUN — enables continuous digitising with
synchronism defined by the trigger mode control.
The previous contents of both digitising memories
are overwritten unless one has already been frozen
using the channel ON/OFF key.
HOLD — disables digitising. The contents of both
digitising memories are frozen.
SINGLE: Only operates when RUN/HOLD is set to
HOLD. Enables a single update of the digitising
memory with synchronism defined by the trigger
mode control.
EVENTS Sets a number of trigger events (0 to 15) which
DELAY: must occur before a trigger is sent to the digitiser.
This allows delayed triggering on complex single-event
waveforms, and can be used to avoid mis-triggering of complex
repetitive waveforms (similarly to Sweep Hold-off on a real-time
oscilloscope). Pressing either key shows the existing value on the
display for three seconds. Repeated pressing increments or
decrements the number, the keys auto-repeat if held. To rapidly
turn events delay off (i.e. to 00) press both keys simultaneously.
Note that when using AUTO trigger mode, events
delay will increase the minimum frequency at
which it can be used from 20Hz to (1 + 20Hz,
where n is the number of events.
TIME DELAY: Sets the time delay between the trigger event and
the start of the digitising process. The delay is
measured in screen divisions and can be positive
(POST TRIGGER DELAY) or negative (PRE
TRIGGER DELAY).
POST — this acts similarly to sweep delay on a
real-time oscilloscope. By adding time delay and
then selecting a faster timebase speed, a section of
a waveform occurring long after the trigger event
can be examined. For "normal" mode timebase
speeds (100msec/div to 5usec/div) the number of
divisions of post-trigger delay is automatically
increased or decreased to maintain a constant time
delay when the timebase speed is changed (within
a limit of 9,999 divisions maximum).
PRE — there is no equivalent to pre-trigger on a
real-time oscilloscope. Pre-trigger allows the part of
the waveform which occurred before the trigger to
be stored and observed. The maximum pre-trigger
delay is —40 divisions.
Pressing either key shows the existing value of
delay on the screen for three seconds. Repeated
pressing increments or decrements the number,
the keys autorepeat if held. To rapidly turn time
delay off (i.e. to 0000) press both keys
simultaneously.
N.B. Neither pre nor post trigger delay is available
in "repeat" mode. In "roll" mode only post trigger
delay is available. See Section R14. Roll Mode
Operation for an explanation.
A waveform burst (4
cycles) captured with
no trigger time delay.
The same waveform
burst captured with 4
divisions of post
trigger delay.
The same waveform
burst captured with 9
divisions of pre
trigger delay.
R10. TRACE A AND TRACE B
The DSA524 can display two traces simultaneously, the traces
are called A and B. Each trace has its own 1K word memory
which can be loaded with data either from the corresponding
digitising memory (CH1 for trace A or CH2 for trace B) or from
an indexed waveform memory.
The trace controls operate on the waveform after it has been
digitised but before it has been 'displayed. Therefore they can
be used to modify waveforms recalled from a waveform store
as well as waveforms taken from the digitising memories of the
input channels.
POSITION: Enables the vertical position of the trace on the
display to be adjusted. Pressing the HOME/(VAR)
key toggles between fixed position (HOME) and
variable position.
GAIN Enable intermediate sensitivity levels to be set.
VARIABLE: Pressing the CAL/( UNCAL) key toggles
between calibrated gain (X1) and variable gain
(X1 to X0.2).
CH1 Pressing this key causes the trace to display the
(or
CH2): contents of the digitising memory for the
corresponding input channel.
RCL(NN): (Recall indexed waveform memory). Pressing
this key followed by a two digit number on the keypad
causes the trace to display the contents of the
corresponding waveform memory. It also sets RUN
/HOLD to HOLD.
OFF: Pressing the CH1 (or CH2) and RCL(NN) keys
simultaneously turns the trace off (the CH key should be
pressed first, otherwise RUN/HOLD will be set to HOLD).
5
INV(Invert): Applies to Trace B only. Causes the waveform
to be displayed inverted.
ADD B: Applies to Trace A only. Displays the sum of
what is presently on Trace B and what was
previously on Trace A.
SAVE(NN): Pressing this key followed by a two digit number
causes the waveform presently displayed on that
trace to be stored in the corresponding waveform
memory. It also sets RUN/HOLD to HOLD.
R11. MEMORY SEARCH (COMP/SCAN/MAG)
Although the size of each trace memory is 1K words (equivalent
to 10.24 divisions across the screen) the size of each digitising
memory is 4K words (equivalent to 40.96 divisions). Normally
each trace memory is filled with the first 1K words from the corresponding digitising memory. (Note that in "roll" mode it is the
last 996 words — see Section R14. Roll Mode Operation.)
To enable all of the digitising memory to be viewed, a SEARCH
key is provided next to the timebase section. Pressing SEARCH
cycles through three states as detailed below. The actual mode is
indicated both by a corresponding front panel lamp and by a
diagram on the display which remains there for three seconds
after the mode has been changed.
COMP: Compresses all 4K words of the digitising
memory into the 1K words of the trace memory by
transferring every fourth word.
SCAN: Enables any 1K section of the digitising
memory to be transferred to the trace memory via
a moveable window. The window is moved using
the two keys marked SCAN/MAG. The position of
the window is shown graphically on the display for
three seconds whenever either key is pressed.
Repeated pressing of either key moves the window
left or right in one division (100 word) steps. The
keys autorepeat if held.
MAG: Digitally magnifies any 100 word section of the
trace memory by ten so that it fills the display. The
section is selected via a moveable window which is
moved using the two keys marked SCAN/MAG. The
position of the window is shown graphically on the
display for three seconds whenever either key is
pressed. Repeated pressing of the key moves the
window left or right in half division (50 word)
steps. The keys autorepeat if held.
A captured waveform
displayed using Compress mode.
The same waveform
displayed using Scan
mode.
The same waveform
displayed using
Magnify mode.
To leave SEARCH mode and set the display mode back to
normal, press the two SCAN/MAG keys simultaneously.
If one of the traces is displaying a waveform recalled from an
indexed waveform memory when SEARCH is selected, the recalled waveform will remain unaffected.
R12. INDEXED WAVEFORM MEMORIES
The DSA524 has sixteen 1K word memories for temporary or
permanent storage of waveforms. Each memory is identified by
a two digit number from 01 through to 16. The memory is supported by trickle charged Ni-Cad batteries which will remain
charged for several months should the unit be left unpowered.
To store a waveform in an indexed memory, obtain the required
waveform on the display and press the SAVE(NN) key for the
appropriate trace. This will cause the ENTER lamp above the
numeric keypad to flash. Enter the two digits corresponding to
the required memory position, the display will show "MEMORY
NN = TRACE A (or B) for 3 secs.
Note that the waveform is stored exactly as it appears on the
display except for its vertical position which is stored as it would
be if the trace position control was at HOME.
To recall a waveform from an indexed memory, press the
RCLINN) key for the trace on which the waveform is to be displayed. This will cause the ENTER lamp above the numeric keypad to flash. Enter the two digits corresponding to the required
memory, the display will show "TRACE A (or B) = MEMORY NN"
for 3 secs.
When a waveform recalled from an indexed memory is being
displayed, the MEM lamp on the corresponding trace will be illuminated. The memory number from which it was recalled can
be checked by pressing the RCLINN) key twice.
Any trace recalled from a waveform store is temporarily suppressed when in "roll" mode RUN or SINGLE.
R13. DISPLAY UPDATE RATE
In "normal" mode (timebase speeds between 100msec and
5usec per division) the display update rate can be varied. This
is done with the key marked RATE (within the timebase
section).
The default update rate (NORM) re-starts the digitising process
about 300msecs after the completion of the previous one. This
gives a display update rate of about 3 per second for higher
timebase rates, slowing down as the timebase speed gets into
the tens of milliseconds (at 100msec per division digitising takes
4 seconds).
Pressing the RATE key once enters SLOW update mode. This
increases the delay from 300msec to 3secs giving more time for
each digitising of the waveform to be observed carefully. Pressing the key again returns to normal update rate.
Holding the RATE key depressed for two seconds enters FAST
update mode. This reduces the delay from 300msec to 20msec
giving a virtually instant update which mimics a real-time
oscilloscope. Pressing the key again returns to normal update
rate.
Entering FAST update mode reduces the acquisition memory
size to 1K words per channel and disables all of the trace
control functions except for trace position. Search mode
(COMP/ SCAN/MAG) and Average (AVG) are also disabled.
6
When RUN/HOLD is set to HOLD, the remaining 3K words per
channel of the digitising memory are set to zeroes and the
disabled functions are restored. Pre-trigger time delay is limited
to 10 divisions in FAST mode.
In "roll" and "repeat" modes the RATE key is disabled.
R14. ROLL MODE OPERATION
In "roll" mode new data is written onto the right-hand side of the
display as digitising takes place. Thus the waveform appears to
roll across the screen. The operation of the DSA524 in "roll"
mode (timebase speeds of 200msecs and below) is different from
its operation in other modes in several respects:
a) Triggering and Display Update Rate
Because the waveform data is written continuously onto the display, it is not necessary to continually stop and restart the digitising process. Consequently when RUN /HOLD is set to RUN,
digitising takes place continuously and triggering is disabled. The
RATE key is disabled.
When RUN/HOLD is set to HOLD, triggering is enabled for
operation via the SINGLE key. Pressing SINGLE commences the
"roll" and the trigger event stops it, thus the waveform data
captured is entirely pre-trigger data. Consequently if a trigger
occurs immediately, no data will be captured unless some posttrigger time delay has been set. When the trigger mode is set to
AUTO, triggering occurs immediately. When the trigger mode is
set to NORM, triggering occurs synchronously with the trigger
signal.
Because in "roll" mode the trigger stops the digitising process
instead of starting it, all the data in the digitising memory is pretrigger data unless some post-trigger time delay has been set.
Pre-trigger time delay is therefore not needed and is disabled.
b) Display Window and Search mode
Because the waveform data is written continuously to the screen
as digitising takes place, the display shows the last part of the 4K
digitising memory instead of the first part. Because the left hand
edge of the display window is set in steps of 100 words, only 996
words are displayed and the remaining 28 words are set to zero.
SEARCH can only be selected in HOLD, and the scan window is
initially set fully to the right instead of fully to the left.
c) Trace Controls
When in roll mode, operation of any of the trace controls (e.g.
gain variable) will effect only the waveform data on the display
which was captured after the control was changed. However,
when the digitising process is stopped, the new trace control
values will be applied to the whole of the contents of the
digitising memory including waveform data captured before the
change was made.
R15. REPEAT MODE OPERATION
In normal mode the DSA524 can digitise a waveform at up to
20MS /s digitising rate, equivalent to 5usecs per division. If the
waveform is repetitive (as opposed to being a single event) it can
digitise the waveform repetitively, gradually building up the data
in the digitising memory. This mode of operation, known as
"repeat" mode, is used for timebase speeds of 2usecs and above
and provides equivalent digitising rates of up to 2GS /s (50nsecs
per div.). The operation of the DSA524 in repeat mode is
different from its operation in normal mode in several respects:
a) Triggering
Repeat mode requires a synchronously triggered waveform,
AUTO and LINE trigger modes are therefore disabled. Time delay
is not available (neither pre nor post trigger). The trigger event is
not captured, the first section of the waveform captured starts
350 nanoseconds after the trigger event.
b) Display Update Rate
Repeat mode involves complex manipulation of data which can
take several seconds per digitisation. The display update rate
varies between about one per second for a single channel at
2usecs/div to one every 10 seconds for both channels at
50nsecs/div. The Rate key is disabled in repeat mode.
R16. CURSOR MEASUREMENT
Moveable cursors allow accurate measurements of voltage and
time to be made on either Trace A or Trace B via an on-screen
digital readout.
To select cursor measurement, press the key marked ON/OFF
within the numeric keypad. Cursors will appear on Trace A, and
Trace B will be suppressed. Pressing the key marked A/ B
selects Trace B instead of Trace A. To turn cursor measurement
off press the ON/OFF key again.
Cursor measurement.
The cursors are flashing horizontal lines which each terminate
at a point on the waveform. The reference cursor is on the left
hand side of the screen, the delta cursor is on the right hand
side. The measurement is made between the two lines. The
readout shows the voltage and time difference between the end
of the reference cursor line and the start of the delta cursor
line. The cursors are moved using the arrow keys marked REF
CURS and ∆ CURS respectively.
R17. PROGRAM MODE
The DSA524 can operate as a fully programmable instrument. It
can "learn" individual or sequential front panel settings, store
them in non-volatile memory, and "replay" them on demand.
Up to 50 settings can be stored.
Each front panel setting includes the state of every key and
rotary control and the source of each trace waveform. Thus if a
setting is "learned" which includes a trace recalled from a waveform store, that same store will be recalled again when the setting is "re-played". Thus reference waveforms can be recalled
automatically within PROGRAM mode.
To "learn" one or more settings press the LEARN key, this turns
on learn mode. The display will show the message "NEXT
POSITION = NN" for 3 seconds where NN is a number between
01 and 50 corresponding to the position set when the unit was
last used.
To store a front panel setting in this position, ensure that the
front panel is set as required and press the SET/NEXT key. The
display will show the message "STORED IN NN" followed by
"NEXT POSITION = NN + 1". To store another front panel setting, reset the front panel as required and press the SET/NEXT
key again. To store the setting in any other memory position,
press the SET(NN) key followed by a two digit number between
01 and 50.
The next memory position can be checked at any time by
pressing the LEARN key again.
To "replay" one or more settings press the REPLAY key, this
turns on replay mode. The display will show "NEXT POSITION =
NN" as for learn mode. To replay the front panel setting corresponding to this position press the SET/NEXT key. The display
will show "REPLAYED FROM NN" followed by "NEXT POSITION
= NN + 1". Press the SET/NEXT key again to replay this next
position. To replay any other position press the SETINN) key
followed by a two digit number between 01 and 50.
The next memory position can be checked at any time by
pressing the REPLAY key again.
Whenever the front panel is set using the replay mode all the
rotary controls become inoperative. To make any rotary control
operative again press the key directly to its right. (Press once if
the lamp above the key is on, or twice if it is off.)
To exit either learn or replay modes press the LEARN and
REPLAY keys simultaneously.
7
R18. SINE INTERPOLATION
When the display mode is set to MAG (magnify) the number of
true samples shown across the screen is reduced to 102. Although 1,024 samples are displayed, 9 samples out of each block
of 10 are mathematically calculated using linear interpolation.
Linear interpolation provides a good reconstruction of the original
waveform provided that either the number of samples per cycle
of the waveform is reasonably high (i.e. ten or more) or, if the
number of samples is low, that the original waveform is made up
mainly of linear elements (e.g. pulse, sawtooth, triangle
waveforms).
When the waveform is basically sinusoidal and the number of
samples per cycle is low, linear interpolation will not provide a
satisfactory reconstruction and sine interpolation should be used
instead. Sine interpolation uses a mathematical curve fitting algorithm and can provide a near perfect reconstruction of a sine
wave from only four samples per cycle. See Appendix C for a
detailed discussion of interpolation.
Pressing the SINE INTERPOLATE key toggles the function on or
off as indicated by the lamp above the key. Interpolation is only
active when the SEARCH mode is set to MAG. Sine interpolation
slows down the display update rate considerably.
R19. AVERAGE
The RUN AVERAGE function allows a number of digitisations of
the waveform to be summation averaged before being displayed.
This improves the signal to noise ratio of a noisy signal and can
often enable a signal to be observed that would otherwise have
been masked by large amounts of HF or LF noise. The average
function reduces the size of the digitising memories to 1K words
per channel.
The number of waveform digitisations which are averaged can be
Set to any number between 2 and 256. To change the number of
readings averaged press SHIFT (key 8) followed by SET AVG (key
4). The current averaging value is shown on the display and can
be incremented or decremented using the REF CURS keys (keys 2
and 3). When the required number is displayed, press ESCAPE.
Pressing the RUN AVERAGE key enables averaging, pressing the
key again disables it again, the status is indicated by the lamp
above the key. Acquisitions continue to be controlled by the
RUN/HOLD and SINGLE keys.
When the display update rate is set to NORM the display is
updated only whenever the total number of averages has been
completed. When the update rate is set to SLOW the display is
updated continuously as averaging takes place. The display
shows the number of averages currently accumulated.
Note that averaging slows down the display update rate. This is
particularly so at fast timebase speeds in "repeat" mode and slow
timebase speeds in normal mode.
R20. A= AXB (TRACE MULTIPLY)
This function provides a display of the Trace A waveform multiplied by the Trace B waveform. It is particularly useful for obtaining power waveforms.
Press SHIFT (key 8) followed by AxB (key 2). The two waveforms
will be multiplied together and displayed on Trace A (Trace B
continues to be displayed). To return to a normal display press
ESCAPE.
The multiplied waveform is scaled using units of one screen
division with the centre of the screen being zero. The traces are
multiplied exactly as they appear on the oscilloscope display including offsets introduced by the trace position controls. Thus at
a point where the position of Trace A is +2 divs and of Trace B is
—1.5 divs, the value of the multiplied waveform will be — 3 divs.
It should be noted that if either trace has values greater than ± 1
divs then overflows could occur on the multiplied waveform. The
displayed waveform is limited to ± 4.25 divisions and consequently —3 divs multiplied by —2.5 divs will be displayed as +
4.25 divs instead of + 7.5 divs.
The AxB function cannot operate while digitising is taking place.
Pressing the AxB key will set RUN/HOLD to HOLD. The function
multiplies the traces exactly as they appear at the moment that
the AxB key is pressed. When the function is enabled all of the
trace controls are inoperative with the exception of the Trace A
position control which can be used to adjust the position of the
multiplied waveform.
R21. LINE OR DOTS DISPLAY
In line display mode the 1,024 individual points of the display
are joined with angled lines to simulate one continuous line.
This gives a display which is easier to view and less subject to
visual aliasing than one made up of dots (see Appendix B).
Sometimes, however, it is desirable to see the exact points on
the waveform where samples have been taken, along with their
levels. Pressing SHIFT (key 8) followed by DOTS (key 7) causes
the points to be joined by right-angled lines which form a staircase. Since the horizontal lines are brighter than the vertical
ones, this gives an impression of dots and makes it easier to
see each sample point and its associated level.
Also the line display may reduce the amplitude of waveforms
where successive samples can have very large vertical
separations. Such a waveform will also appear brighter if a dots
display is used.
To return to a line display press LINE (key 7). The unit defaults
to a line display after RESET.
R22. PLOT
The waveforms shown on the oscilloscope display can be sent
either to a digital X-Y plotter (HP-GL compatible) using the RS423 interface; or to an analog X-Y plotter or Y-T chart recorder
using the analog plotter interface.
The plot function can be directed either to the RS-423 interface
for use with a digital plotter, or to the analog plotter interface.
The DSA is factory initialised for use with a digital plotter. To
change to use with an analog plotter key in "function 26" (press
FUNCTION NN, followed by 2 followed by 6). To return to use
with a digital plotter key in "function 27".
a) Digital Plotter
The digital plotter must be compatible with HP-GL plotting commands and must be fitted with a serial interface for connection
to the RS-423 interface socket of the DSA. For details of making
connections and of setting the baud rate see Section R25
’RS-423 Interface’. Alternatively, if the DSA has the IEEE-488
interface option fitted, a plotter with an IEEE-488 interface may
be used.
To commence plotting press PLOT (key 6), the time and voltage
per division and a graticule will be plotted followed by Trace A
followed by Trace B. If the plotter is a multi-pen plotter the text
and the graticule will be plotted using pen 1, Trace A will be
plotted using pen 2, and Trace B will be plotted using pen 3.
The entire plot fits onto an A4 sized sheet. To abandon plotting
at any stage press ESCAPE.
b) Analog Plotter
The analog plotter interface is mounted on the rear panel and
comprises five 4mm sockets marked X, YA, GND, YB, and PL
(pen lift).
The waveforms for Trace A and Trace B are sent simultaneously
to their separate output sockets along with an X axis signal for
use with X-Y plotters. The 10.24 divisions of the display are
plotted using a level of 100mV/div (for both Y and X) at a rate
selectable as 1, 2, 5 or 10 seconds per division. The pen-lift
signal is normally "low to lift" but can be inverted.
To set the plot rate, press SHIFT (key 8) followed by SET PLOT
(key 5)., The oscilloscope display will show the present plot
rate. Press one of the REF CURS keys (key 2 or 3) to select a
new value, then press ESCAPE.
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