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Varactor-tunable RF filters with a wide range

These rather interesting papers and thesis show a design of varactor-tuned RF bandpass filter that spans a wider range than any previous one. I hope folk here will find it interesting.

30 MHz constant bandwidth in the frequency range from 380 MHz to 920 MHz

- nice!

"The filter is thermally stable. It has 1 dB compression point of 10 dBm and third-order intercept point of 21 dBm."

The IP2 figure is relatively low, probably because only single diode used, not two in anti-phase. The mixing products should be outside the pass-band, but still...

I've included the direct link to the PDFs on sci-hub:

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Same author's thesis (in Polish - Google translate works though):

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The varactors he used:

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/ Clifford Heath

Reply to
Clifford Heath
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Clifford Heath wrote: ==================

** Errr - not really.

Average 70kHz drift per degree C.

...... Phil

Reply to
Phil Allison

That's not too bad at upwards of 400MHz, it's under 0.02%/degree? Also pretty easy to calibrate out using a lookup table too.

Reply to
Clifford Heath

Clifford Heath wrote: =====================

** Yawnnnnnnn.....
** Thing is, its ONLY an IF filter.

A bit of temp drift hardly matters, long as the BW is consistent.

The same drift in local oscillator would be fatal, in many apps.

FYI

I used to build broadcast FM tuners with varicaps. Drift was a serious issue.

..... Phil

Reply to
Phil Allison

Go away and break some more toasters Phil. We're talking MHz, not KHz here. If you lack the imagination to see what use such a tunable filter might be, you should just keep quiet.

There are many general-coverage receivers and handheld transceivers that have bugger-all input filtering, so their receive response is full of images due to LO harmonics interacting with mixer non-linearities and intermodulation in the front-end amplifiers. No amount of digital cleverness can get rid of these false signals once they're in the signal bandwidth.

This kind of filter is almost trivially simple but would dramatically improve the performance of these radios.

I'm surprised Gerhard hasn't responded yet...?

Clifford Heath

Cue dummy-spit from PA. Don't bother PA, nobody cares, however angry you get.

Reply to
Clifford Heath

Thanks for the pointer.. Neat stuff, considering how expensive tunable filter ICs are.

I have a sack of varicaps and an application in mind, so I might have to give this topology a shot. Phase stability near midband is critical, but I don't care about behavior near the passband edges.

-- john, KE5FX

Reply to
John Miles, KE5FX

It seems to be related to the capacitance variation with frequency that's used in interdigitated filters and capacitors, etc. This means that the capacitance variation changes the coupling between the two central inductors. A friend is currently trying to figure out how to simulate these, since the effects are basically immune to analysis.

Reply to
Clifford Heath

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