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Archive for November 17th, 2017

WWVB Reception: 60 kHz Tuning Fork Resonator Filter

Some early morning data from the WWVB preamp with the 60 kHz tuning fork resonator filter in full effect (clicky for more dots):

WWVB - xtal filter - waterfall 5 fps RBW 109.9 Hz Res 0.02 s - gqrx window - 20171116_103542

WWVB – xtal filter – waterfall 5 fps RBW 109.9 Hz Res 0.02 s – gqrx window – 20171116_103542

The dotted line comes from WWVB’s 1 Hz PWM (-ish) modulation: yeah, it works!

The filter cuts out the extraneous RF around the WWVB signal, as compared with a previous waterfall and some truly ugly hash:

WWVB - 24 hr reception AGC - 2017-01-16 to 17 - cropped

WWVB – 24 hr reception AGC – 2017-01-16 to 17 – cropped

Well, not quite all the hash. Enabling the SDR’s hardware AGC and zooming out a bit reveals some strong birdies:

WWVB - xtal filter - waterfall - hardware AGC - 2017-11-16 0612 EST

WWVB – xtal filter – waterfall – hardware AGC – 2017-11-16 0612 EST

The big spike over on the left at 125.000 MHz comes from the Ham-It-Up local oscillator. A series of harmonics starting suspiciously close to 125.032768 kHz produces the one at 125.066 MHz, just to the right of the WWVB signal, which leads me to suspect a rogue RTC in the attic.

There is, in fact, a free running “Test Signal Source” on the Ham-It-Up board:

Ham-It-Up Test Signal source - schematic

Ham-It-Up Test Signal source – schematic

Although I have nary a clue about that bad boy’s frequency, measuring it and cutting the inverter’s power trace / grounding the cap may be in order.

The SDR’s AGC contributes about 30 dB of gain, compresses the hottest signals at -25 dB, and raises those harmonics out of the grass, so it’s not an unalloyed benefit. Manually cranking on 10 dB seems better:

WWVB - xtal filter - waterfall - 10 dB hardware preamp - 2017-11-16 0630 EST

WWVB – xtal filter – waterfall – 10 dB hardware preamp – 2017-11-16 0630 EST

The bump in the middle shows the WWVB preamp’s 2 kHz bandwidth around the 60 kHz filter output, so the receiver isn’t horribly compressed. The carrier rises 30 dB over that lump, in reasonable agreement with the manual measurements over a much narrower bandwidth:

60 kHz Preamp - Bandwidth - 1 Hz steps

60 kHz Preamp – Bandwidth – 1 Hz steps

With all that in mind, a bit of careful tweaking produces a nice picture:

WWVB - xtal filter - waterfall - 10 dB hardware preamp - 2017-11-16 0713 EST

WWVB – xtal filter – waterfall – 10 dB hardware preamp – 2017-11-16 0713 EST

I love it when a plan comes together …

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