Stanford Research Systems DS345, Manual

Page: 34 / 139

Modulation and Sweeps
2-18
BURST MODULATION
Introduction
The DS345 generates tone bursts of any of its periodic output functions. The
frequency of the output function is limited to 1 MHz for sine and square
waves, 100 kHz for triangles and ramps, and no limits for ARBs. When a trig-
ger signal is received the DS345 initiates a burst starting at a specific point
(phase) in the output waveform, outputs the exact number of programmed
waveform cycles, and then stops. The rear-panel TRIGGER OUTPUT gener-
ates a TTL compatible signal that goes high when the burst is triggered and
low when the burst is complete. This signal may be used to synchronize ex-
ternal equipment to the burst. The SYNC output is not active during tone
bursts.
Burst Count
The number of complete cycles in a burst is set by pressing [SHIFT][BRST
CNT]. The number may be set from 1 to 30000 cycles. The maximum time
for a complete burst is 500s, the time for a burst is easily computed from the
following formulas:
Burst Time
=
Burst Count
Frequency
for sine, square, triangle, ramp
Burst Time
=
Burst Count
×
# Waveform Points
Sampling Frequency
for ARB
Waveform Starting Point
The point in the waveform at which the burst starts (the phase) may be ad-
justed for sine, square, triangle, and ramp waves. For ARBs the burst always
starts on the first waveform point. Changing the PHASE changes the point at
which the burst starts. 0.000 degrees phase corresponds to the positive zero
crossing of the function, and values up to 359.999 degrees increment
through the waveform. PHASE values larger than 360 degrees are set to
modulo 360 degrees.
Triggering a Burst
Burst modulation is a triggered function, and therefore a signal needs to ini-
tiate the burst. The trigger generator can initiate a burst from the front panel
[TRIG] key, the internal rate generator, the external trigger input, or the pow-
er line frequency. Setting the trigger generator is detailed in the TRIGGER
GENERATOR section (2-22). The TRIG'D LED flashes green each time a
burst is triggered. If the DS345 is triggered before the previous burst is com-
plete the TRIG'D LED flashes red, indicating a trigger error. At high trigger
rates a combination of triggers (green) and trigger errors (red) can make the
TRIG'D LED appear orange. Once a burst is triggered the DS345 will ignore
all other triggers until the burst is complete.