Stanford Research Systems DS345, Manual

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Modulation and Sweeps
2-19
FREQUENCY SWEEPS
Introduction
The DS345 can frequency sweep its function output for sine, square, triangle,
and ramp waves. The sweeps may be up or down in frequency, and may be
linear or log in nature. The frequency changes during the sweep are phase
continuous and the sweep time may be set between 0.001 and 1000 sec-
onds. The DS345 has an analog SWEEP output that may be used to drive an
x-y recorder or oscilloscope, a TTL BLANK/LIFT output that can lift a chart
recorder pen during the sweep retrace, and a TTL MARKER output that may
be set to make transitions at two programmable frequencies during the
sweep.
Sweep Type
Pressing the MODULATION TYPE UP/DOWN ARROW keys sets the sweep
to either a linear or log sweep. The output frequency in a linear sweep chang-
es linearly during the sweep time. The output frequency in a log sweep
changes exponentially during the sweep time, spending an equal amount of
time in each decade of frequency. For example, in a sweep from 1 kHz to
100 kHz, the sweep will spend half the time in the 1 kHz to 10 kHz range and
half the time in the 10 kHz to 100 kHz range). It should be noted that these
are digital sweeps, and that the sweep is actually composed of 1500 to 3000
discrete frequency points, depending on the sweep rate.
Sweep Waveform
The DS345's sweep waveform may be set to single, triangle, or ramp using
the MODULATION WAVEFORM UP/DOWN arrow keys. With the SINGLE
setting the DS345's output frequency will be the sweep start frequency until a
trigger is received. The output will then sweep to the stop frequency, reset to
the start frequency and wait for another trigger (see the TRIGGER GENERA-
TOR section for setting the trigger source). If the waveform is set to a RAMP
the DS345 will sweep from the start to the stop frequency, jump back to the
start frequency, and repeat continuously. If the waveform is a TRIANGLE the
DS345 will sweep from the start to the stop frequency, sweep back from the
stop frequency to the start frequency, and repeat continuously.
Sweep RATE/Time
The duration of the sweep is set by [RATE], and the value is entered or modi-
fied with the keypad. The sweep rate may be set over the range of 0.001 Hz
to1 kHz. The sweep rate is the inverse of the sweep time, a 0.001 Hz rate is
equal to a 1000s sweep time, and a1 kHz rate is equal to a 1 ms sweep time.
For a TRIANGLE sweep the sweep time is the total time to sweep up and
down.
SWEEP FREQUENCIES
The DS345 may sweep over any portion of its frequency range:1
µ
Hz to
30.2 MHz for sine and square waves, and 1
µ
Hz to 100 kHz for triangle and
ramp waves. There are no restrictions on minimum or maximum sweep span.
The DS345's sweep range may be set by entering either the start and stop
frequencies, or the center frequency and span. The relationships between
the frequencies are:
Center Frequency = (Stop Fre Start Frequency) / 2
Span = Stop Frequency - Start Frequency
Start Frequency = Center Frequency - Span/2
Stop Frequency = Center Fre Span/2