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Monthly Science: 60 kHz Preamp Resonator Bandwidth

Putting a small capacitor in series with the tuning fork resonator pulls the series resonant frequency upward and reduces the amplitude:

60 kHz Quartz TF Resonator - CX variations

60 kHz Quartz TF Resonator – CX variations

So something around 10 pF, net of stray capacitance and suchlike, should suffice. Plunk a small twiddlecap on the preamp board and tune for best picture:

LF Crystal Tester - resonator protection

LF Crystal Tester – resonator protection

Using the DDS generator as a manual signal source with 1.0 Hz step size shows the resonator tightens up the preamp’s response quite nicely:

60 kHz Preamp - Bandwidth - 1 Hz steps

60 kHz Preamp – Bandwidth – 1 Hz steps

I’m not convinced the preamp will have filter skirts that low farther away from the peak, but it’ll do for a start.

Zoom in on the peak with 0.1 Hz steps:

60 kHz Preamp - Bandwidth - 100 mHz steps

60 kHz Preamp – Bandwidth – 100 mHz steps

The bandwidth looks like 0.6 Hz, centered just slightly above 60.000 kHz, which should be fine for a first pass.

I’m tickled: all the hardware & firmware fell neatly into place to make those graphs possible!

Next step: install it in the attic and see whether the filter cuts back the RF clutter enough to stabilize the SDR’s AGC gain.

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  1. WWVB Reception: 60 kHz Tuning Fork Resonator Filter | The Smell of Molten Projects in the Morning

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