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