Softrock as IF subsystem
- I've completed installing a 9mHz softrock into my old Ten Tec Argosy,
sampling the IF just after the post-filter amplifier. The softrock is
mounted in place of the useless Noise Blanker board, and power
switched from the "NB" front panel switch. I replaced a triple
phono-jack "accessory" strip on the back panel with three small stereo
jacks, for IF Out to the soundcard, Audio In from the soundcard, and
paddles (for a built-in keyer). A switching transistor across the
"enable" pins on the softrock is toggled from the "Receive" line in
the Argosy, to take care of muting during transmit.
I also built a little FET audio switch to allow the NB-switch voltage
to also select the input source for the audio chain. That way, during
SDR useage, I still get sidetone and audio output at the phone jack
and internal speaker, and with the NB switch "off", the radio performs
The IF passband was very obvious on the spectrum display even without
an antenna, as a sharply-defined area of increased noise a few kHz
wide. I set Rocky's tuning for the midpoint of the passband and left
it there, doing all subsequent tuning with the main tuning dial of the
Argosy. I had to use a second receiver to find the correct RIT offset
to put my transmitted signal back onto the received frequency for
proper transceiver operation (this offset comes from Rocky tuning, the
softrock clock frequency, and the transceiver CW offset).
I finished it in time to try it out in the CQWW DX test this weekend,
and it was an interesting experience (although mostly I used my old
Drake twins). It worked rather well in the context of a transceiver,
and tight dsp-derived filtering was very helpful to hear the DX
stations through the loud US callers. The displays, especially the
waterfall mode, were particularly useful when using a narrow passband,
to stay aware of loud nearby (but inaudible in the passband) signals.
Switching and processing delays were not as problematic as I expected,
in terms of supporting full break-in. If I couldn't hear between
high-speed dots, at least I could certainly hear during longer gaps,
and it was definately better than the VOX-derived "QSK" provided by
I used Rocky, and found an item for a "wishlist": an easier way to
rapidly adjust the passband width. Easy-to-mouse large buttons or a
large slider (not near the tuning area) would be nice, or better yet,
a keyboard alternative (perhaps some configurable keys?).
I'll put more mileage on it in the coming week or two, but I'm quite
pleased with it so far.