[time-nuts] WWVB BPSK Receiver Project?

Fuqua, Bill L wlfuqu00 at uky.edu
Thu Mar 15 09:33:45 UTC 2012


You are correct, however, I suppose you are using a loop antenna with a relatively high Q.
The "antenna gain" is related to the Q when you have an antenna with a diameter much less than
a wavelength.
  With a Q of 100 you would have a bandwidth of .6 kHz, If you go to say 20.3333kHz you would not
need that high of a Q. 
  Now, why do you need 1200 Hz bandwidth? Is it sending over 1kbaud data rate?  I have not 
looked at the details. Just recall the data rate was 1 bit/second for time. 
   1200Hz at 60 kHz would represent a very low Q antenna only 5 or so. 
  If you use CMOS switches you will still get aliasing at odd harmonics. So you would still need
a front end filter.
  If you want a closer sampling frequency just make a simple frequency multiplier and you still
can use the sound card output.
  The real point is where does SDR begin. As I said with CMOS switches you are effectively multiplying
or mixing the incomming signal with square waves which have odd harmonics and you still get aliasing. 


73
Bill wa4lav
PS Just retired Friday. Maybe I will have some time to catch up with these discussions.

> Many A/D converter systems use a sample and hold before the A/D converter.
> If you do the same before your sound card (your A/D converter) and drive the S&H with an audio output from your sound
> card, say at 6.1 kHz you would get a 1 kHz signal into your sound card to process. You can call it under sampling
> aliasing or whatever.

Yes, this would work, but instantaneous sampling would tend to alias
in many harmonics, requiring good prefiltering at RF (if you can call
60 kHz RF).  Just as easy would be a mixer from CMOS switches, driven
say at 50 kHz to get 10 kHz into the sound card.

The WWVB signal apparently has a double-sided bandwidth of about 1200
Hz (not clear from the paper if that means 3 dB bandwidth or something
else).  To get all of the signal something like 2 or 3 kHz might be
safest, requiring an IF of several kHz at least.

Cheers,
Peter





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