Electronics > RF, Microwave, Ham Radio

Decoding WWVB phase modulation

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--- Quote from: seamusdemora on January 31, 2023, 11:21:02 pm ---
--- Quote from: gnuarm on January 31, 2023, 04:14:21 pm ---
I have mind designed a direct conversion WWVB receiver for some time now.  I would sample with a 1 bit ADC at 240 kHz.  The input signal can be mixed with an LO that consists of the pattern 0, 1, 0, -1 without multiplies.  Every other sample can be used as the I and Q signals, giving phase.  The amplitude can be approximated by adding the I and Q signals. 

For this to work, the sampling rate has to be pretty close to 240 kHz.  A VCO can be used by initially syncing it to a reference like a 32,768 kHz crystal (I was trying to keep the design minimal power).  240 kHz crystals exist, but are hard to find.  The phase samples should be averaged over 50 ms periods to extract the signal phase.  An offset to the VCO can be measured and subtracted out.  The VCO should be adjusted to minimize the phase drift.  I was going to use a set of binary valued capacitors to trim the VCO.  Or, if you can still find them, a varactor would do the job (tuning diode).  I think LEDs might make adequate tuning diodes.  The DC value on the diode can be adjusted using a 1 bit DAC (sigma delta). 


I should still do this.  It would make for a radio controlled clock that could be powered by scavenged energy.

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I agree 100%... you should still definitely do this!  :-+
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At the moment, I have a sizable contract in negotiation.  If I get that, I'll be pretty busy for at least 6 months, maybe 9 months.

--- Quote ---Some notes:

Note I (FWIW): I'm not particularly interested in the scavenged power aspect; batteries work fine for my intended usage.

Note II: I found this repo on GitHub wherein the author has some working code to implement a SDR using the ADC on a Raspberry Pi Pico:
This seems a good start to me... Pico chips are available for US$1.00; an entire board for US$4.00; at 500 ksps max, the onboard ADC easily covers your proposed sampling rate.
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I'm a minimalist at heart, so I have a lot of interest in finding ways to do this using as little as possible.  The synchronous sampling at 4x the carrier frequency automatically down converts to DC.  The trick is to keep the sample rate stable and synchronous.

--- Quote ---Note III (Question): RE Varactors - They still seem to be available from several sources, and through distribution. Also, would it make any sense to use a PLL to set the bias on the varactor so that the antenna stays tuned at precisely 60kHz?

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I don't think it is important to tune the antenna in real time.  You can tune it on the bench, and it should remain pretty stable.  My idea is to tune the local VCO for the LO to be mixed with the incoming signal.  That will drift and needs to be updated in real time. If you run from a crystal, you still get some tens or even hundreds of ppm error, unless you go to lengths to stabilize them.  I can't remember off the top of my head how stable it needs to be, but maybe 200 ppm is not a problem.  I didn't want to use a fast crystal oscillator because it sucks down power.  32.768 kHz can be pretty light. That can be compared to 240 kHz at 128 Hz. 


--- Quote from: seamusdemora on January 31, 2023, 11:45:53 pm ---
--- Quote from: edavid on January 31, 2023, 11:34:48 pm ---Millions of cheap WWVB clocks show that you don't need active tuning of the antenna.

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But two of them I've owned suggest you need something more... They work for a while, and then they don't, they work in one location, but not another. My expectations were that the signal was only available in the late PM or early AM, but even that was wishful thinking in my experience. If not a tuned antenna, what's your opinion on how best to improve reception?

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Tuned antenna, yes.  Just not real time adjustment of the tuning.  Unless your set up see extremes of temperatures, you won't have issues with the antenna tuning.  It's sharp, but not that sharp.

I know ham sets can have issues with tuned "magnetic" loops, where the bandwidth of the loop is more narrow than the voice signal being received!  Still, that's 4 kHz and you only need some tens of Hz.  The signal changes on 100 ms boundaries, with a minimum pulse spacing of 200 ms.  So 40 or 50 Hz bandwidth should be plenty.  I think you'll have a hard time getting your loop stick or even a large loop that narrow.  So if it drifts, you will still be in the tuning range.

No one is challenging Nyquist.  I use 4x because that can be worked to give me I and Q to get phase (essentially Fs = 2 x BW for two signals).  The sampling at 2x trick only works if you are doing direct conversion, because it automatically down converts to DC.

In theory, you can demodulate the phase at a lower SNR than you can distinguish the amplitude changes.  That's why they added the phase modulation.  Then someone they were working with patented a receiver design.  No one (or very few) has produced a chip to demodulate the phase.  If you consider the market for most of these clocks, if the chip costs $0.50 more, it won't get used.  It's a bit like the Beta vs. VHS issue, Beta was better, but not demonstrably so.  How would you market the clock with the slightly better receiver? 

@jmw (post#40)
Attached are images of what my analog signal looked like.  That was easily cleaned up easily with a PIC12F683/12F1840.

Unfortunately, I "finished" that project in 2014, and it is now buried in other papers.  Where in the US are you?
Edit: Corrected year

Central San Francisco, so there are probably lots of noise sources nearby at all times. I also have one of the Everset ES100 kits, and it is often able to get the time in the middle of the day, so the BPSK signal is definitely more robust than the AM signal.


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