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Thread: Time signal LF Receiver Teensy DCF77 with minimal hardware effort

  1. #26
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    Another issue: I wanted to try some cross correlation to try to detect weaker signal. I added an IntervalTimer to call my very small interrupt routine every 10ms so I could sample the current signal strength. The thing went kind of crazy. Parts of the display were messed up and I think maybe it affected the sound card(?)

    Is there some other interrupt driven timer I could use?

  2. #27
    Senior Member+ defragster's Avatar
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    Is 'current signal strength' an analogRead?

    See if the ADC library works with DMA to read it without interrupts?

    Or depending on how fast loop() cycles just start an elapsedMillis var and sample it every 10 ms

  3. #28
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    First Rule of Programming: It is probably your own fault.

    I was writing off the end of my sample array.
    Once I sorted that out everything was good with the display.
    IntervalTimer is exonerated.

  4. #29
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    My project was to convert this DCF77 receiver to WWVB at 60 kHz.
    I have that working now based on the amplitude modulation.

    The conversion really didn't touch the SDR components except to change the reception frequency.
    I added sampling of the amplitude at 10ms intervals and then cross correlation of the
    samples with prebuilt patterns of the three symbols: One, Zero, and Mark.
    I have a floating cross correlation window patterned after http://paulbourke.net/miscellaneous/correlate/

    Using the teensy 4, I went ahead and did this in floating point and it seems to be getting the
    job done without skipping any 10 ms beats.

    I would like to proceed to implement the BPSK phase demodulation which I think means more
    SDR related changes. I think what I need is carrier recovery and then phase recovery.
    But my SDR knowledge is shaky. Honestly, I could use a bit of tutoring or some helpful pointers.

    -- addition --
    I have gone back and found the previous suggestion to read this document: https://sdr.hu/static/msc-thesis.pdf
    So I will start there. It seems to be about the right information level for me.
    Last edited by ChrisHoward; 02-21-2020 at 12:05 PM.

  5. #30
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    Antenna note: I had some old wire from a CRT type television flyback transformer. I used a cardboard box approximately 2" by 14" by 12" and wrapped the wire around the perimeter of the box, about 30 turns (?) and secured with masking tape. Then I measured the inductance with my AudioVNA (https://groups.io/g/AVNA1 which is what brought me to teensy-land a few months ago). I then consulted an on-line LC circuit calculator and added capacitors in parallel with my loop to bring it into resonance at 60 kHz. The antenna is directional. I my case WWVB is directly west from me. So I orient the loop with the opening N/S, as if I were pointing west using a donut. I am approximately 850 miles from the WWVB transmitter and located in a small city, a fairly high noise area. I can receive the signal during hours of darkness but not when the sun comes up. The 600 Hz tone is clearly distinguishable in headphones plugged into the audio board. (But I noticed that having the headphones attached could also effect my signal reception)
    Last edited by ChrisHoward; 02-21-2020 at 01:15 PM.

  6. #31
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    I may be wrong, but if you increase the number of turns and reduce the capacitor, you may transform your antenna more into a magnetic receiver that is less sensitive to electric noise.
    Any trials with that?

  7. #32
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    No, once I got a working antenna I focused more on the software side.
    I did do a bit of online research on shielded loop antennas. But I decided to try the direct approach with the parts immediately at hand.

  8. #33
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    I feel like I'm on the edge of stupidity, looking over the fence into the land of understanding... but I can't seem to get there from here!

    I want to demodulate the phase encoded information on WWVB. I believe it qualifies as BPSK (Wikipedia confirms).
    To decode that, I think I need I and Q inputs into some various chunks of code that do carrier recovery and phase recovery.
    Am I talking sense so far?

    Is it possible to generate I and Q with the teensy 4.0 and audio board combination? I'm assuming the folks doing the teensy SDR systems would do that instead of having external I/Q front ends, if it were possible.

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