Daqarta
Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
Software for Windows  The following is from the Daqarta Help system:

# FM +/- Deviation (Depth)

Macro: FMdev

This control is normally configured for frequency deviation in Hz. This is the maximum frequency difference, positive or negative, from the initial main wave frequency when the modulator is at its positive or negative peak.

Note: Entry of a negative value gives the equivalent of inverted (180 degree) modulator phase.

Daqarta allows continuous through-zero modulation, so you can set the deviation higher than the main (carrier) Frequency, allowing negative output frequency.

To get a good feel for this, set a slow FM Modulator Frequency (say, 1 hertz or less) so you can follow the action on the Spectrum display. When the output frequency moves down through zero, the effect is to "reflect" off of zero by reversing the polarity of the waveform and the direction of the frequency motion, so the frequency rises again. When the modulator reaches its negative peak and starts back up, the output frequency starts back down to zero, where it again reverses back to normal.

When used with Play Waves, Deviation is shown as a Rate fraction and the units change from Hz to X, just as the main Frequency control does. For example, if Rate is 1.000 and Deviation is +/- 1.000, then in one modulator cycle the actual rate varies from normal (1.000) up to twice normal (2.000), down through normal to zero (stopped) and back up to normal.

When Deviation is greater than Rate the modulator can drive the effective rate negative, which results in playing that portion of the file in the reverse direction.

Note that FM Deviation does not use the Frequency Step that the Main Frequency and Modulator Frequency (and other Generator frequencies) use. You can enter any value directly or scroll by +/-1 Hz steps.

Macro Notes:

L.1.FMdev=500 sets Left Stream 1 FM Deviation to 500 Hz.

L.1.FMdev=>1 increments FM Deviation by 1 Hz, while L.1.FMdev=>-1 decrements by 1 Hz. Only +/-1 Hz steps are accepted; Frequency Step is ignored. GO: