[time-nuts] DSP WWVB Receiver Idea

Max Robinson max at maxsmusicplace.com
Thu Apr 23 21:01:08 UTC 2009


This message sent me on a Google search to find what I had missed about 
WWVB.  The terms I and Q signals sends me into phase modulation space.  The 
only reference I found on this is a 45 degree phase shift at 10 minutes 
after the hour and a return 5 minutes later.  Is there something else going 
on with the phase of the WWVB carrier that I haven't heard about?

Regards.

Max.  K 4 O D S.

Email: max at maxsmusicplace.com

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----- Original Message ----- 
From: "Kasper Pedersen" <time-nuts at kasperkp.dk>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Wednesday, April 22, 2009 1:25 AM
Subject: Re: [time-nuts] DSP WWVB Receiver Idea


> 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 
> bpsk).
>
> 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.
>
> /Kasper Pedersen
>
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