Digital Modulation On The Com3010 - Ramsey Electronics COM3010 User Manual

6.10 Digital Modulation on the COM3010

When connecting the COM3010 an external broad-band modulation analyzer, the analyzer
will typically see a larger deviation than expected. It may be as much as 5%. This is due to the
very fast frequency changes used to "emulate" FM by digitally switching frequencies in steps.
The upper line is the desired modulating waveform at 1 kHz, and the lower line is what is
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translates to a 5 kHz low pass filter in the audio demodulated before any further processing.
The radios typically have more audio filtering to allow signals in the voice band of 300Hz to
3 kHz, and signals below 300 Hz are interpreted for squelch control signals like CTS and DCS.
This means that the artifacts of the digital signals are filtered off for us by the radios
themselves! The 10 kHz, 20 kHz etc. are well above the audio bandwidth that the radios can
receive.
Modulation analyzers, however, have extremely wide reception bandwidths of over 1 MHz.
They see the audio signals the COM3010 is creating, as well as the digitizing artifacts. The
modulation analyzer will record a wider than expected modulation unless the proper low pass
filter is selected on the analyzer. Typically, the 3 kHz filter switch should be IN to get proper
readings of the modulation waveform.
needed digitally to re-create this same signal. As can be
seen the upper line is continuous and smooth while the
lower is in steps at regular intervals. This rate of level change
is called the sample rate. The small steps with sharp edges
create a high frequency component in the waveform at
multiples of the sampling frequency. If the sampling
frequency is 10 kHz you will have some spurs on the signal
10 kHz away from the carrier, as well as 20 kHz, 30 kHz...
determined by the modulating waveform.
The same rules of thumb which apply to digital to analog
converters apply to digital modulation. Typically the highest
frequency signal we can re-create is at ½ the sampling
frequency, or the Nyquist frequency. It is good practice to
stay below 1/3 the sampling frequency so the sampling
components can be filtered out with a simple low-pass filter.
Typically the radios that will be tested with the COM3010
are narrow-band. This means the total bandwidth they are
able to receive is +/- 5 kHz, depending on the model. This
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