RE: LF: Sound Cards

["Talbot Andrew" <ACTALBOT@mail.dstl.gov.uk>]

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

-----Original Message-----
From: DL4YHF@aol.com [mailto:DL4YHF@aol.com]
Sent: 2002 July 23 19:36
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Sound Cards

Related topic: I bought some a low-cost PIC with 10-bit ADC (PIC16F870) recently, planning to connect it to the serial interface as replacement for my laptop's audio device (which only has a lousy *mono* MIC input). Has anyone on the group ever tried to use this PIC for weak-signal work and check the ADC's performance, compared to a soundcard ?



Regards,
Wolf  DL4YHF

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