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: [email protected]
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----- Original Message ----- From: "Kasper Pedersen"
<[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
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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