3156B User Manual
Racal Instrument © 2005
Getting Started 1-19
The 3156B requires an enable command to start frequency
modulation. The source of the enable command is selectable from
software, front panel trigger input, or from a backplane TTLTrg(n) line.
When FM function is selected but an enable command has not been
received yet, the 3156B outputs continuous carrier frequency.
Arbitrary FM
The Arbitrary FM function is very similar to the standard FM function
except the modulating waveform is user programmable. The arbitrary
waveform to modulate the carrier is generated using the FM composer
panel from WaveCAD. It also can be generated by the user using
other utilities. The arbitrary FM waveform has an array size of 20000
frequency points. If you look at the following FM composer example,
you’ll see that the vertical scale is made of frequency points, The
change in frequency will follow the curve shown in the FM composer
panel.
The frequency of the arbitrary FM wave is computed in the same way
as a standard arbitrary waveform.
Frequency = Sample Clock / Number of waveform points
In the example below, the frequency of the waveform will change from
900kHz to 1.1MHz. The sample clock for the modulating waveform
can be programmed from 1S/s to 5MS/s while the carrier waveform
frequency can be selected from 1Hz to 25MHz. Note that the CW
wave is always sine waveform however, the modulating waveform can
take any shape defined by the FM composer panel.
The 3156B requires an enable command to start arbitrary frequency
modulation. The source of the enable command is selectable from
software, front panel trigger input, or from a backplane TTLTrg(n) line.
When FM function is selected but an enable command has not been
received yet, the 3156B outputs continuous carrier frequency.
