[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.
http://www.prc68.com/I/117A.shtml
Have Fun,
Brooke Clarke
http://www.prc68.com
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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