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| To: | [email protected] |
|---|---|
| Subject: | RE: LF: Sound Cards |
| From: | "Talbot Andrew" <[email protected]> |
| Date: | Wed, 24 Jul 2002 08:47:18 +0100 |
| Reply-to: | [email protected] |
| Sender: | <[email protected]> |
|
I
built a custom LF receiver last year, filtering to 180Hz bandwidth, then
converting the signal to a 1kHz tone which was sampled by a microchip MCP3204 12
bit A/D.at 4kHz. Presumably, being made by the same company the A/D core
is the same as that internal to their PIC processors (10 bit vs. 12 bit
notwithstanding). Then I and Q baseband signals at 1kHz were
generated by taking four successive samples in groups (labelled
S1, S2, S3 and S4) and calculating I = S1 + S2 - S3 -
S4 and Q = S1 - S2 - S3 + S4. This is
effectively the same as multiplying the 1kHz audio by two 90 degree shifted
square waves at 1 kHz. Provided the input is band limited to
the range 750 - 1250 Hz, no mixer products or alliases ought to
appear.
After
writing some waterfall software to read the I/Q samples (at effectively 13 - 14
bit resolution) I was never too happy with the result. Although a dynamic
range of 50dB was achieved, there were spurious lines present, which
appeared to be related to 1kHz and subharmonics so I can only assume these were
artifacts of the A/D converter itself. Better results
were achievable with the RA1792 plus 56002 EVM sampling at 8kHz (better
than most soundcards) and no sprog lines were present with the antenna
disconnected, so I put the LF receiver on the shelf for a 'later' follow
up
WIth a
10 bit converter you will be limited to at most 60dB dynamic range which poor
soundcards can match and many better, but you will have the very considerable
advantage of being able to control the sample rate properly.
Furthermore you can probably control stray pick up much better as the audio
circuits can be separated from the PC, linked with a properly filterd data link
(RS232, USB or whatever) With decimation and post processing this dynamic
range will of course be increased. Try it and see, but don't expect too
much. I've had a serial port 8 bit 16C71 PIC based converter going
for some years (before the 56002EVM) , and it did a reasonable job of digging
signals out of noise in its day, but couldn't match later 16 bit
performance.
Andy G4JNT
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