Stanford Research Systems DS345, Manual

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Modulation and Sweeps
2-24
DS345 AS A PULSE GENERATOR
Introduction
The DS345 can be easily used as a pulse generator. Pulse widths down to
500 ns with rise/fall times of 30 ns and repetition rates up to 10 kHz internally
and 500 kHz externally triggered are possible. You can even do bursts of
groups of pulses (each pulse has to be the same width and separated by 1
pulse width).
Procedure
S tart with a square wave as the main waveform, selecting the square wave
frequency (1 MHz max. for this purpose) so that half of a period is the width
of the pulse you want. Then choose burst modulation, with a burst count of 1
and turn on sweep mode. Use the phase control to adjust the phase of the
square wave within the burst so that only a positive going or negative going
half cycle is visible (generally a phase shift of 180 degrees for a positive
pulse). Note that by varying the phase you can also delay a pulse by up to
one half a period of the frequency with respect to an external trigger. Finally,
use the offset control to adjust the baseline of the pulse to be 0 Volts. You will
have to start with a square wave amplitude of at most one half of the maxi-
mum DS345 peak to peak amplitude. This gives a maximum pulse amplitude
of 10 Volts into high impedance or 5 Volts into 50 Ohms.
After setting this up, changing the burst rate will change the pulse repetition
rate, and changing the square wave frequency will change the pulse width.
To do groups of pulses, simply increase the burst count to the number of
pulses you want in the burst. Use the trigger input connector and set the trig-
ger control to external (positive or negative) for externally triggered bursts. If
the source of the trigger has a 10 MHz clock input, connecting this to the
DS345 clock output will reduce pulse to pulse jitter.
Arbitrary Waveform
T he other way to create pulses is using arbitrary waveforms, which can be
done from the front panel using vector entry mode, or through the AWC soft-
ware. This technique is a little more complicated and does not allow the same
ease of changing pulse width or repetition rates, but pulse widths down to 50
ns and externally triggered repetition rates up to 2 MHz are possible.See oth-
er sections of this manual for instructions for arbitrary waveform generation
and AWC software.