[time-nuts] DSP WWVB Receiver Idea

Brooke Clarke brooke at pacific.net
Thu Apr 23 23:40:04 UTC 2009

Hi Max:

That's what I was talking about.

The HP 117 is really a frequency comparator and it displays that phase shift. 
The magnitude of the shift is a sanity check on the plot scale factor.

Have Fun,

Brooke Clarke

Max Robinson wrote:
> 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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