Aliasing effects are common to all digital storage instru-
ments. Note that to avoid true aliasing the sampling rate
must be greater than twice the signal frequency (time/div
= 50/f) and to avoid visual aliasing the rate must be
greater than ten times the signal frequency (time/div =
10/f).
1d. The DSA524 incorporates a feature which automatically
sets the timebase to avoid aliasing effects and to provide
a sensible waveform period on the screen. Turn off the
sweep magnifier on the display oscilloscope. Press simul-
taneously the two keys marked AUTOSET within the
timebase section, note that the timebase is automatically
set to 20us/div.
1e. Try changing the generator frequency and using
AUTOSET, note how it sets the timebase to maintain
between 4 and 10 cycles of the waveform across the
screen. (AUTOSET can only operate when a trigger signal
of 50Hz or above is being generated.)
2. Roll Mode
For timebase speeds of 200ms/div and slower, the DSA
enters roll mode (indicated by an audible "beep" and
illumination of the ROLL lamp). In roll mode the waveform
data is written onto the screen continuously as it is being
digitised. This avoids the long delays that would occur if
the screen was only updated after digitising was
completed.
Set the timebase to 200ms/div and the generator to 2Hz,
note how the waveform appears to "roll" across the
screen. Try setting slower timebase speeds and lower
generator frequencies (DC coupling will be necessary for
lower frequencies). Set the timebase as slow as it will go,
note that below 500s/div the SEC and mSEC lamps illumi-
nate together to indicate minutes (speeds as low as 200
minutes/div can be set enabling waveform capture over
many hours).
3. Repeat Mode
The maximum sampling rate of the DSA524 is 20MS /s
(which is obtained at 5us/div). This is the maximum rate
that can be used to capture single event signals. However,
if the signal is repetitive, repeat mode can be used. This
builds the waveform up by repeatedly digitising it, and can
provide equivalent sampling rates up to 2GS /s. For time-
base speeds of 2us/div and faster the DSA enters repeat
mode (indicated by a "beep" and illumination of the RPT
lamp).
3a. Set the generator to 200kHz and set the timebase set to
2us/div (or press AUTOSET), note that the waveform is
being digitised at an equivalent rate of 50MS /s. Vary the
generator frequency slightly and note that the display
update rate is slower in repeat mode, and that the first
screen update after the signal is changed is invalid (this is
because the waveform is built up from multiple
digitisations).
3b. Ground the CH1 input and note that updating of the
screen ceases and a message "NO TRIGGER" appears on
the screen (this is because repeat mode requires a repeti-
tive trigger signal to be able to operate). Un-ground the
input and increase the generator frequency. Increase the
timebase speed and note that waveforms up to 35MHz
can be captured (this is limited by the input amplifier
bandwidth of the DSA) using timebase speeds of up to
50ns/div (2GS /s equivalent sampling rate). Note also that
the update rate is slowest at the fastest timebase speeds
when in repeat mode.
4. Turn on CH2 and Trace B. Note that the repeat mode
update rate is slower when both channels are turned on.
Set the timebase speed down to 20us/div noting that
when both channels are on, the ALT lamp illuminates for
speeds of 10us/div and faster, and that the CHOP lamp
illuminates for speeds of 20us/div and slower. CHOP
indicates that both input channels are digitised together
(on opposite cycles of the sampling clock) while ALT
indicates that the input channels are digitised alternately
(CH2 followed by CH1). Thus if digitising is un-triggered,
or if the input signals do not have a constant relationship
to the trigger signal, phase relationships between the
signals will not be maintained above 20us/div.
Triggering
(Start with the DSA reset and the generator set to SINE, 20kHz,
5V pk-pk.)
The trigger controls for the DSA524 are broadly similar to those
on a conventional oscilloscope. The exceptions are EVENTS DLY
and TIME DLY.
1. SOURCE The source of the trigger signal can be CH1,
CH2, or EXT (external via the BNC socket below the
trigger controls). Note that only AC coupling is available
via the external input.
2. MODE The mode switch has three positions, AUTO,
NORM, and LINE. AUTO mode is similar to Bright-Line
Auto on a conventional oscilloscope. Digitising is perfor-
med in synchronism with the trigger signal but, if the
trigger signal ceases or the trigger repetition rate falls
below 20Hz, digitising continues but without synchro-
nism. In NORM (normal) mode, digitising stops if the
trigger signal ceases. In LINE mode, digitising is
performed in synchronism with AC line (50 or 60 Hz).
Try removing the trigger signal by setting SOURCE to
external, note that digitising continues (without
synchronism) in AUTO mode, but ceases in NORM mode.
2.
LEVEL, SLOPE and COUPL (coupling) The function of
these controls should be self explanatory. To enable
variable trigger level, press the key marked ZERO(VAR
I (variable level is enabled when the lamp is off). Try
using all of the trigger controls, then return them to
their original settings.
PART TWO — MORE ADVANCED FUNCTIONS
The functions covered in the first part of the tutorial enable the
user to operate the DSA524 as a basic digital storage
oscilloscope. This second part covers the more advanced
features which would not be found on a basic DSO.
More about Triggering
(Start with the DSA reset and the generator set to SINE, 20kHz,
5V pk-pk.)
1. TIME DLY (time delay) With TIME DLY off, digitising
starts immediately following a trigger. Thus the left-hand
edge of the trace shows the waveform immediately after
the trigger point. TIME DLY shifts the point at which
digitising starts to allow the trace to show the waveform
either before the trigger (pre trigger delay) or after the
trigger (post trigger delay).
Set the generator to 5kHz; the trace should now show
about one cycle of the waveform starting at the trigger
point. Press the TIME DLY (minus) key, note that the
message "TIME DLY = +0000 DIV" is displayed. Press
the key again to decrement the number to —0001 (note
that the keys only increment or decrement when the
message is being displayed, on the first press they
merely cause the current delay value to be shown).
The trace should now start one division before the
trigger point. Try incrementing and decrementing the
delay using the + and — keys, note that the trace can
be positioned anywhere relative to the trigger point (in
steps of one division) and that the PRE and POST lamps
illuminate for negative and positive delays respectively.
The maximum pre trigger delay is —40 divs and the
maximum post trigger delay is + 9999 divs. Note that
the + and — keys autorepeat if held depressed, and that
the delay can be set back to zero at any time by
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