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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 real-
time 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 auto-
ranging 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 simultan-
eously 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 non-
volatile i.e. their contents are not destroyed when the power is
turned off.
Note that if either of the digitising memories is storing a wave-
form 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 oscillo-
scope:
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 syn-
chronism 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).
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