A DSP Receiver by Rob Frohne, KL7NA

SSB, ISB, AM, DSB and CW can all be demodulated in DSP using only linear op-
erations.



Two channels of finite impulse response (FIR) filtering are employed for this. These are the I and the Q channel filters (see Figures below). FIR filters were
chosen because of their simplicity. Any band-limited linear operation can be ap-
proximated by selecting the proper FIR filter coefficients, h(M). The more coeffi-
cients used, the better the approximation.




A FIR filter is really just a tapped delay line. The coefficients are the weights of
each of the delayed signals that are summed at the output. This makes programming particularly easy; there is only one program, but there are eight sets of coefficients, two for each demodulation type. To design your own custom frequency response, you need not understand the details of the program—you need only understand the basics of FIR filter design.








Block diagram of the program used for all modes: SSB, CW, AM, DSB and
ISB. The signals x(nT) and y(nT) represent the sampled in-phase and quadrature
modulations, respectively. With the SSB, CW, ISB demodulator, the I and Q channels have identical gain characteristics, but the Q channel’s phase lags the I channel’s by 90°. The outputs for USB and LSB (or one of the outputs for AM and DSB) are sent to the two output channels of the stereo codec.






FIR filter represented as a tapped delay line. There are M filter coefficients,
h(•), and the delays are each 1/fs second, which is the time between samples.



After x(t) and y(t) are filtered, their sum yields the one sideband, and their difference yields the other sideband. Each sideband is sent to a channel of D/A on the stereo codec.


AM and DSB Demodulation

The I channel is simply passed straight through. The Q channel is unused. Actu-
ally, its gain is set at zero so it won’t affect anything when the outputs of the two channels are summed. See Figure 4. To accomplish this, the I filter coefficients are set to {1,0,0,0,0,...}, and those of the Q filter to {0,0,0,0,0,...}.


CW, ISB and SSB Demodulation


For CW, ISB and SSB, the I and Q channel filters have identical-magnitude re-
sponses; the phase of the Q channel’s filter response is delayed by 90° with respect to the I channel.


Source : ARRL