BNC 645 - Manual (Page 42)

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internal modulating waveform is sine wave. For internal source, one may
also choose from square with 50% duty cycle, URamp (Up or Positive
Ramp with 100% symmetry), DRamp (Down or Negative Ramp with 0%
symmetry), triangle (ramp with 50% symmetry), noise and arbitrary
waveforms. If an arbitrary waveform is chosen, the number of points in
the waveform is limited to 4k points with an automatic decimation
process. The default frequency for internal modulating waveform is
100Hz, and the acceptable range is from 2 mHz to 20 kHz.
When external source is selected, the carrier waveform is modulated
with an external waveform present at the Modulation In connector on the
rear panel.
Modulation Depth
The modulation depth controls the magnitude of the amplitude variation
of the modulation output. It is defined as the following formula:
ModulationDepth = (MaxAmplitude – MinAmplitude) / Amplitude x 100%
where Amplitude is the specified amplitude of the carrier waveform;
MaxAmplitude and MinAmplitude are the maximum and minimum
amplitudes of the resulting modulation waveform. Model 645 is so
designed that when Modulation Depth is 0%, the amplitude of the
modulation waveform stays at half of the specified amplitude of the
carrier waveform. As a result, MinAmplitude may be negative when
ModulationDepth is larger than 100%. The default Modulation Depth is
100% and the allowable range is from 0% to 120%.
When external source is selected as the source of modulating waveform,
the modulation is controlled by the specified modulation depth and the
±5v signal levels of the external waveform. When the external waveform
is at +5v, the amplitude of the output will reach MaxAmplitude. When
the external waveform is at -5v, the amplitude of the output will reach
MinAmplitude.
Note that the modulation output of Model 645 will not exceed ±5V peak