Equipment Required
1. A conventional real-time oscilloscope for the display, the
minimum specification: 5MHz bandwidth, single channel,
8x10 division graticule.
2. A wide frequency-range signal source, preferably a
function generator. (The tutorial assumes that a function
generator is being used.)
3. Three BNC to BNC connecting cables.
Setting-up the Equipment
1. Connect the rear panel BNC socket of the DSA marked
"composite" to the CH1 input of the oscilloscope.
2. Set the oscilloscope as follows:
Mode: Single channel (CH1)
CH1 input coupling: DC
Sensitivity: 100mV/div
Timebase: 50usecs/div
Trigger source: CH1 (not vertical mode)
Trigger coupling: AC
Trigger slope: Negative
Trigger Level: Negative (slightly)
Sweep delay, sweep
hold-off etc: Zero or minimum
3. Connect the main output of the function generator to the
CH1 input of the DSA. Connect the Sync or TTL output to
CH2.
First Steps
1 Switch on the DSA (the power switch is at the rear). The
oscilloscope should then show the message "CENTRE
ARROW TIP" (alternating with "THEN PRESS ESCAPE").
Adjust the trigger level if necessary.
2. Adjust the vertical and horizontal shift controls of the os-
cilloscope until the arrow tip is in the centre of the
graticule.
3. Set the function generator to SINE, 20kHz, 5V pk-pk.
4. Press the ESCAPE key. Then press SHIFT followed by
RESET. This ensures that all of the more complex func-
tions are cancelled, and that the DSA is put into a known
state as follows:
Both channels ON, 1V /div, AC coupled, 20usec/div AUTO
trigger from CH 1.
The display should now be showing a sine waveform and
a pulse waveform, each of around 5 divisions pk-pk ampli-
tude and around 2.5 divisions period.
What to Expect
The unit is now operating as a digital storage oscilloscope. Both
input signals are being repetitively digitised by an analog to
digital converter (ADC), and stored in a memory the contents of
which is being converted back into analog signals by a digital to
analog converter (DAC) and then displayed on the screen via a
display multiplexer. This process takes a little time, introducing a
slight delay between a change occurring at the input and that
change appearing on the display.
The display multiplexer mixes together the two traces, the trigger
signal and, when needed, text. At the far left of the screen the
valid section of the trace is preceded by a very narrow negative
pulse followed by a horizontal line at the trace zero level (this can
be removed by using a two cable connection — see Appendix A).
Following the valid section, at the far right of the screen, the
trace is shifted up to the top of the display area.
When the display is correctly centred, the valid section of the
traces starts 0.12 divisions to the left of the graticule and ends
0.12 divisions to the right of it. If the timebase of the display
oscilloscope is not perfectly calibrated, the valid section may be
slightly longer or shorter than this.
Each trace is made up of 1024 samples across 10.24 horizontal
divisions of the screen (100 samples per division). Each sample
can be at any of 256 levels within 8.53 vertical divisions (30
levels per division).
Because the display is made up of discrete steps, waveforms do
not look quite the same as they do with an analog oscilloscope.
The minimum level change between samples is one thirtieth of
a division, and even where the input signal level is constant the
digitised level may vary slightly. This variation is caused by
noise on the signal, noise in the input amplifiers, or digitising
noise in the ADC (typically ± )bit) and results in the digitised
waveform looking slightly "ragged" when compared with an
analog oscilloscope. Noise on the waveform of an analog
oscilloscope is averaged out by the eye.
The Display Oscilloscope
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.
Front Panel Controls
The front panel of the DSA524 is completely "cold", that is to
say that there are no direct links between the front panel and
the circuitry. All of the controls, including the rotary ones, are
monitored by a microprocessor which then performs the con-
trol. All of the switches are of a momentary non-latching type
whose status is indicated either by lamps under the panel or,
where appropriate, by text on the display. This system enables
complete digital programming of the front panel when required.
The keys within the numeric keypad section (right-hand side of
unit) have up to three functions. Their normal function is
printed in black above each key. After the SHIFT key 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) auto-
matically set the RUN/HOLD function to HOLD. If digitising
ceases unexpectedly check the condition of RUN /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).
Getting out of Trouble
If at any stage you lose track of the operation of the DSA, it can
be reset to a known state by pressing SHIFT followed by RESET
(as in First Steps). Each section of the tutorial starts with the
DSA in the "reset" state and the generator set to SINE, 20kHz,
5V pk-pk. Thus it is easy to restart the tutorial at the beginning
of any section.
PART ONE — BASIC FUNCTIONS
The functions covered in this first part of the tutorial are the
ones necessary to operate the DSA524 as a basic digital storage
adaptor.
Controlling Digitising
(Start with the DSA reset and the generator set to SINE, 20kHz,
5V pk-pk.)
The digitising of the input signals is controlled by the two keys
at the top of the TRIG section marked RUN /HOLD and SINGLE.
1. When RUN/HOLD is set to RUN, digitising is repeated
continuously. Try changing the amplitude or frequency
of the generator and note how the display is updated
about three times each second.
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