SCA.7
From the above rather long-winded description, you can see that the
is a lot of other junk floating around on the composite signal, namely
all the stereo stuff below 53 KHz. To make the SCA demodulator work
and be free of interference from the main channel, it would be well
put a little selectivity ahead of the SCA demodulation if you are using
the PLL trick.  I haven't experimented with this, but I would think a
simple high-pass filter to attenuate everything below 53 KHz would do
fairly well.  Of course with the VLF receiver you have all kinds of
selectivity, so there should be no problem.

I hope this helps you understand what is going on with SCA.  If you
followed all the details, it should be clear that there is no black
magic going on, but that once the composite signal has been received, a
VLF FM receiver (a rather rare beastie) is logically what is needed
recover the SCA signal.  That's why it happens to be possible to string
together such a seemingly unlikely pair of receivers and get SCA
broadcasts.

                                  -John Sangster, W3IK
                              jhs at MITRE-Bedford, MA

CONTINUED IN SCA.8