Electronics > RF, Microwave, Ham Radio

Decoding WWVB phase modulation

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For a while WWVB has had a BPSK-modulated signal that is supposedly more robust against fading and interference. What kind of discrete circuit would be used to demodulate it? For a exercise, I've wanted to make my own receiver that can output the baseband signal without using SDR.

I don't know for sure how to analog detect the phase signal in the WWVB signal, I'm much more of a digital guy, but a PLL would do the job.  The PLL would run at 60 kHz to match the carrier.  When the phase shifted the change in the output of the phase detector would shift.  If the tracking is fast enough the control signal would be a spike on every phase shift, either negative or positive. 

The phase shift of the signal is on top of the AM signal.  So when the AM is low the phase is harder to detect.  I suppose you might want to detect the low amplitude and disable the PLL tracking.  But if you can detect when the amplitude is low you don't need the phase detection. 

Or maybe it would be easier to do this the way I would do it in digital.  Have a fairly stable and accurate 60 kHz reference signal to beat against the incoming signal in quadrature and take the ratio.  Hmmm... that already sounds difficult in analog.  Any interest in doing this digitally?  Anyway, the quadrature ratio is the phase difference which will show a slow variation from the difference in frequency of the reference and the WWVB signal.  It will also show a sharp change at every phase shift.  Hmmm... the ratio actually won't change with a 180° change in phase due to the symmetry of the tangent.  But the sign of both portions of the quadrature signal will. 

I used to know how to do this, but I seem to have forgotten the details and a quick google search didn't help. 

I do recall that when the government produced this new format a company worked with them to design a chip which I guess they expected to sell to all the clock makers.  This company has a patent on the method of demodulating the time code signal.  I don't know if they priced the chip too high or what, but I've never seen a standard wall clock that made any claims about demodulating the phase and I'm sure if they were using the chip, they would be crowing about it. The patent still has over 10 years to go I believe.

There is a company that made WWVB timecode receivers that plugged into the Tektronix TM500 system power modules. Those receivers no longer work correctly because the modulation scheme is no longer used. I see the receivers for sale on Ebay from time to time but they are useless. Efratom I think was the company that made them. Blue colored single space plugin. They were sweet back in their day!!!!

Those little plugin receivers were made by spectracom. They also had the WWVB rack mount models which I currently see on Ebay. I assume those are also not useful today since they had the same guts. Perhaps WWVB has some new modulation standard but I thought with all the GPS disciplined stuff today that WWVB has no compensation modulation scheme anymore????

I see I am totally ass backwards on this, It is the new PM signal which has made the old Spectracom stuff useless. Sorry about that!! When the phase shifts the old Spectracom and others see this as a frequency/phase discrepancy which is out of tolerance and the error/unlock light comes on as the unit tries to re-establish phase coherency. Ouch, my bad, I just remember millions of dollars of WWVB disciplined oscillators becoming worthless. Cheers mates!!


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