I want to make a bandpass filter at 6 MHz with Q>1000. It seems the only known application of quartzes on the market is oscillators.
Are there ready-made filters? Otherwise which circuit do I have to associate with the quartz? Or are there other solutions?
In message , Jean-Pierre Coulon writes
Xtal filters have been commercially available for decades , or you can make your own -e.g
Brian
A 6 MHz quartz will have an equivalent circuit as shown in
You can make any filter profile you like, using crystals, as long as the bandwidth is not ridiculously wide (more than a few kHz). The necessary factors are: source/load impedance versus crystal (characteristic) impedance; and coupling between crystal(s).
From the other replies, it sounds like you want a repeated pole filter, or something more linear-phase?
Obviously, you can always get a single pole pair from one crystal.
The standard network is a series chain of crystals. Coupling is controlled with a capacitor to ground, at each node between crystals. Crystal frequency is trimmed with a capacitor in series with each crystal.
Beware spurs: crystals resonate at frequencies above their cut frequency, with various bandwidths, impedances and couplings. A quality filter system still needs modestly high-Q LC filters to help attenuate these.
Tim
Try searching on "single-crystal filter". You may get something.
Basically, when it's close to resonance, the crystal looks like a several- henry inductor, in series with a several-fempto-farad cap, all in parallel with a few-pF cap. So build on that.
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