Stanford Research Systems DS345, Manual

Page: 36 / 139

Modulation and Sweeps
2-20
Start and Stop Frequencies
To enter the sweep start frequency press [START FREQ]. Set the stop fre-
quency by pressing [SHIFT][STOP F]. The start and stop frequencies may
have any values that are allowed for the displayed waveform. If the stop fre-
quency is greater than the start frequency the DS345 will sweep up, while if
the start frequency is larger the DS345 will sweep down.
Center Frequency and Span
To set the sweep center frequency press [SHIFT][SWP CF]. The center fre-
quency may be set to any value allowed for the displayed waveform. Set the
sweep span by pressing [SPAN]. The span value is restricted to sweep fre-
quencies greater than zero and less than or equal to the maximum allowed
frequency. If the span is positive the DS345 will sweep up, if it is negative the
DS345 will sweep down. The MODIFY keys may be used to change the
span: pressing [MODIFY UP] will double the span, while pressing [MODIFY
DOWN] divides the span in half. The MODIFY keys affect the span in oc-
taves–the size of the step is the value displayed by the step size. When the
center frequency is changed the span is held constant, while changing the
span holds the center frequency constant. If the center frequency or span is
changed such that the sweep frequencies are out of the allowed range an er-
ror will be displayed on the front panel.
SWEEP MARKERS
The DS345 has two sweep markers that may be used to indicate any two fre-
quencies in the sweep. The MARKER output is a TTL compatible signal that
goes high when the sweep frequency crosses the start marker frequency and
low when the sweep frequency crosses the stop marker frequency. In a trian-
gle sweep the markers are only active on the up sweep. The marker positions
may be set by entering the marker start and stop frequencies, or the center
frequency and span.
Marker Start and Stop
The marker start and stop frequencies are independent of each other and are
set by pressing [SHIFT][MRK START] and [SHIFT][MRK STOP] respectively.
The frequencies can be set to any value from 1
µ
Hz to 30.2 MHz. If the mark-
er start frequency is lower than the marker stop frequency the MARKER out-
put will initially be low, go high when the sweep crosses the start marker posi-
tion, and go low again when the sweep crosses the stop marker position. If
the marker start frequency is greater than the marker stop frequency the
MARKER output will be initially high, go low when the sweep crosses the
stop marker position, and go high again when the sweep crosses the start
marker position. If either of the marker positions are outside of the sweep
range the marker output will behave as if the sweep had crossed its position.
These cases are shown in the diagram below:
Mrk Start
Mrk Stop
Case 1: Marker Start Freq < Marker Stop Freq
Mrk Start
Mrk Stop
Case 2: Marker Start Freq > Marker Stop Freq
Mrk Start
Mrk Stop
Case 3: Marker Start Freq < Sweep Start Freq
Sweep Start Frequency Sweep Stop Frequency
Figure 1: Marker Output for differ-
ent relationships between the
marker start and stop frequencies.