Ametek 7230 Instruction Manual page 61

Analog. In this case a TTL reference at a frequency no greater than 3.0 kHz
should be applied to the rear panel TRIG IN connector.
External analog
In this setting the reference channel is configured to accept an analog reference
source applied to the front panel REF IN input connector
Virtual
This activates the virtual reference mode, and the Main Controls page is
reconfigured to support the additional controls that are required (figure 5-22)
Figure 5-22, Virtual Reference Controls
The virtual reference mode is very similar to the single reference mode with
internal reference, and is the simplest of the three additional modes of operation.
NOTE: This mode is only suitable for signals at frequencies between 100 Hz
and the upper frequency limit of the instrument (120 or 250 kHz).
The controls operate as follows:-
Start Freq
This control defines the start frequency from which the instrument will begin to
try to find the applied signal. It should be set to a value close to, but less than,
the expected signal frequency.
Stop Freq
This control defines the stop frequency at which the instrument abandons its
search for the signal, and should be set to a value greater than the expected signal
frequency.
Step Size
This control defines the frequency step size used during the automatic search.
Chapter 5, WEB CONTROL PANEL OPERATION
5-19