Stanford Research Systems DS345, Manual

Page: 39 / 139

Modulation and Sweeps
2-23
ARBITRARY MODULATION PATTERNS
Introduction
In addition to the usual sine, square, triangle, and ramp waveforms the
DS345's AM, FM, and PM functions can modulate the output waveform with
an arbitrary modulation pattern. The arbitrary modulation pattern can only be
set using a computer interface and the AMOD? query command. The com-
puter downloads a list of amplitude percentages, frequencies, or phase shifts
to the DS345. The DS345 then modulates the waveform using these values.
To use arbitrary modulation, the modulation type must be set to AM, FM, or
PM, and an arbitrary pattern must then be sent to the DS345. If no pattern
has been loaded the DS345 will display the message "arb corrupt". [SWEEP
ON/OFF] enables the arbitrary modulation. Switching to a different modula-
tion type or waveform after a pattern has been downloaded to the DS345 will
erase the donwloaded pattern.
Modulation Rate
Pressing [RATE] sets the modulation rate. The value displayed when the
modulation waveform is set to ARB is different than the usual modulation
rate. The value displayed is the value of the
modulation rate divider
(MRD)
, which can be set between 1 and 2 23 -1 (8,388,607). This value sets
the
time
the DS345 spends at each point in the arbitrary modulation wave-
form. The time at each point is given by:
Type of Arb Modulation Time
AM 150ns * MRD
FM 1
µ
s * MRD
PM 250ns * MRD
Waveform List
The ARB waveform is created by downloading a list of values via the comput-
er interface. For ARB AM the values are percentages of the programmed am-
plitude. The waveform may have up to 10,000 AM points. For ARB FM the
values are the frequencies to be output, and must be a valid frequency for
the selected function. The waveform may have up to 1500 FM points. For
ARB PM the values are phase
shifts
(relative to the current phase) in the
range ±180°. The waveform may contain up to 4000 PM points.
Downloading
The waveform list may be downloaded to the DS345 via the RS232 or the
GPIB interface. The data format is discussed in the PROGRAMMING section
under the AMOD? command. The PROGRAM EXAMPLES section (pgs. 3-5)
provides examples of generating and downloading waveform data.