On Mon, 25 Jan 2016, Andy Talbot wrote:
Having true baseband as an intermediate step is useful for other bands - in
particular 24GHz where several types of off the shelf surplus equipment
already has an image cancelling mixer, just needing I/Q baseband drive.
You can do this easily in software too in a similar way. Many sound cards
can be dc-coupled (or at least the coupling caps can be increased, so
there is only a narrow dip near 0Hz) - i'm not sure about the 1/f and
other noises near 0Hz, but that should not be too bad. Actually i'm not
sure that you would need to modify anything for ssb, voice should be
understandable with a few 10s of Hz gap around 1.5kHz, and digital
transmissions can just avoid the few Hz around 1.5kHz.
The software is simple: in gnuradio use a frequency xlating fir filter
with a 1.5kHz frequency and float->complex conversion, feed this into a
complex-> float block and the outputs into a stereo sound card sink. Very
simple to draw in gnuradio-companion, you don't even have to do any
programming (and while you're at it, you can also do a receiver which
would be equally simple). I suspect this could also be trivially done in
Spectrum Lab.
Alberto's DSPic solution to generating I/Q baseband signals from voice band
looks promising, so long as it has analogue output and not just SPI drive
for a DDS
Yes, this is great, because you don't need a PC for it. Running gnuradio
on a raspberry pi would also work (and be easier to modify to your needs),
but the dsppic solution probably has less latency.
Still for LF/MF this seems to be too complex, upconverting in software to
around 20kHz and then to 136/472kHz and LC filtering seems much simpler,
and can be debugged just with an audio generator.
Example for 2200m band: a cheap 3.686MHz crystal divided by 32 gives
115.1875kHz. To transmit on 135.7-137.8kHz you need to generate
20.5125-22.6125kHz with the sound card (no problem with a cheap 48kHz
sound card). The mirror frequency is around 94kHz, so it is easy to filter
out from the 137kHz output.
Example for 630m band: use a 455kHz crystal (or a piezoceramic if you
don't mind worse frequency stability). To transmit on 472-479kHz you need
to generate 17-24kHz (cheap sound card). The worse case mirror frequency
would be at 17kHz, trying to filter 472kHz and reject 432kHz, still doable
but with a few more LC circuits (see how much an old AM radio uses for
this selectivity).
VY 73
Jacek / SQ5BPF
|
