Dear LF Group,
I don't know about the SPM-12, but I have the SPM-6 and SPM-19, both of
which use a 10kHz final IF, available at a socket on the rear. The little
SPM3 has a different arrangement with 650kHz and 1.5kHz IFs. The filters
use pot cores and discrete crystals, so there are no easily identified
filter cans inside. The -6 and -19 both have demodulated SSB outputs, the
-19 has an internal speaker, while the -6 has sockets on the rear panel. As
previously mentioned, all these units can be interconnected via the
auxilliary sockets to the corresponding PS- series generator, to allow
level meter and generator to be tuned simultaneously via the level meter
tuning knob. Both units measure signal levels extremely accurately over
very wide ranges, and with the generator can also measure gain and loss
very accurately too, so the combination is excellent for investigating
filters, bridge measurements and so on. However, they are not really
intended to be receivers, so the front ends have no preselection at all,
the demodulated outputs are very inflexible (eg. fixed 2kHz pitch and 3kHz
bandwidth, intended for "frequency division multiplex" SSB voice), and lack
things like variable gain controls and AGC. It is possible to use them as
receivers quite successfully, but you will have to add your own
preselection, and an external demodulator is probably best.
I used the SPM-19 with a homebrew external demodulator quite effectively
last winter, to receive VA3LK and others. I have also used DL4YHF's
Spectrum Lab software to demodulate the 10kHz IF output. This is very
flexible - the software can mix the 10kHz down to 650Hz or 800Hz or
whatever you like. The latest version allows you to implement audio filters
in software of any bandwidth and center frequency within reason too. You
can listen on the sound card output and watch the waterfall display
simultaneously. All you really need is a lead to connect IF output to sound
card mic input, although you will have to experiment with the gain levels
of both the level meter and the PC's "volume control" software to get a
sensible gain distribution, and also to mute unwanted audio paths to the
soundcard output. If you try using Spectrum Lab in this way and find you
have problems such as no audio or hearing input and output simultaneously,
it usually will actually be "volume control" settings causing them.
Cheers, Jim Moritz
73 de M0BMU
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