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A very interesting chip, which acts as a digital variable capacitor, which can serve as a fundamental phase shift circuit.
Wish I i had the knowhow to design !

The PC64904 looks a bit limited for many of our applications. The minimum
frequency covered in the datasheet is 100 MHz and the capacitance range is
only 1.1 to 5.1 pF. One plus is that it's specified for use at high signal
levels up to 38 dBm. It might be useful in a tuned front end for VHF and
UHF. Information here:


Another variable capacitor chip that looks more useful in HF application is
the PCD2400M. It has a range of 1.7 to 194 pF and the spec sheet covers use
at 10 and 50 MHz. It's not designed for high signal levels (the absolute
maximum input is 3.3V) so it might not be so useful in a front end.
Information here:

I'm thinking of giving the PCD2400M a try in a variable bandwidth crystal
filter. With suitable signal limiting in front it looks like it has the
potential to work well. But it's hard to know without actually putting a
few of them on the bench to see what happens; there might be some gotcha
that makes it less useful.

Way back in the 1980s we designed tune-able RF filters using varicap diodes driven by a DAC to control the frequency response.

Hi Shirley,

I had exactly the same thought and also wondered at the 3.3V limit which, if I calculate correctly is about 14-15 dBm in a 50 ohm system. The impedance and the voltage could be much higher in a crystal filter. Two possible mitigations occur to me. First start out with a few poles of a passive filter that is as wide as the widest bandwidth and follow it with a variable filter to reduce the bandwidth even further then the passive filter would bear the brunt of strong off-frequency signals. The other idea is to use as high a frequency as possible, 20 MHz or higher if you can find fundamental crystals higher. At the higher frequencies the crystal impedances are lower so the voltages will be as well.

73,
Jim WA2EUJ