[time-nuts] DSP WWVB Receiver Idea
time-nuts at kasperkp.dk
Wed Apr 22 06:25:05 UTC 2009
Brooke Clarke wrote:
> On the PICLIST there has been a discussion about the CMAX WWVB front
> ends and noise. Olin mentioned that you could use a dsPIC to look at
> the I and Q signals resulting from mixing the WWVB signal with a
> carrier at 60 kHz. His example case was to use a cheap crystal (+ or
> - 3 Hz) and so use a 10 Hz low pass filter on the I and Q signals
> prior to squaring and adding them.
I've built such a thing ( http://n1.taur.dk/dcf/ ). The zero-if I/Q
approach has a few things that make it less ideal than it sounds.
There's the 1/f noise, discovering and compensating for DC offset on
each of the channels requires that you remove the input, and it might
not be a nice divider from 10MHz.
If you choose a small arbitrary offset you can solve these problems in
software, only the filters in hardware need to be wider. Having the
first filters wide, I found, was a good thing: In the very early morning
I get a lot of sferics, and my steep filter rang like a bell with every
crackle. A low-Q front end allowed throwing those samples away.
Since that was done I have added a narrow bandwidth phase integrator
(2mHz) in software, and it will happily pull out ~10ns rms phase with a
+60dB carrier 1Hz from center. It even stayed locked when the antenna
amplifier broke and output 5Vp-p instead.
The real advantage of the I/Q method is that the bandpass filter becomes
two lowpass, and two lowpass is easier than a similar width bandpass
with enough precision and phase stability to be centered around 60kHz
(and if you use crystal resonators in the front end you can't track
anything else, and you get a problem with suppressing sferics).
You might not be able to get continuous reception no matter how hard you
try; I've seen inversions where the carrier just slowly fades and comes
back inverted with no apparent phase jumps (it looks like extremely slow
If I did it today I'd try phk's approach first. Preferably with a
somewhat tuned antenna to keep harmonics from PAL horizontal retrace
from clipping the converter. The one above was built with what was
available in the junkbox at the time.
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