200MHz Filter with 200kHz bandwidth

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Annirak

Joined: 26 Jul 2009
Posts: 13

13 Oct 2009 16:22 200MHz Filter with 200kHz bandwidth

I need to design a 200MHz filter with 200kHz bandwidth and a very steep rolloff. In particular, I need to expressly reject everything at 200kHz intervals from the 200MHz center.

Active filters are totally fine, and I'm already using a number of BF998 dual gate mosfets, so using more of those is fine by me. The amplitude of the signal where I'm filtering should be 1mV to 100mV.

I need to build this as a compact element. I'm fine with using some RG-174 as an element in the design, but I'd like to keep it to quarter-wave lengths, as coiling up half a meter of coax inside my filter will add quite a lot to the bulk.

Originally, I had planned to use a crystal or ceramic filter, but I soon discovered that they don't go this high. SAW filters, conversely, don't go this low.

Is my best bet going to be a pair of notch filters and a high-Q bandpass filter?

Phytech

Joined: 30 Apr 2008
Posts: 89
Helped: 9
Location: USA

13 Oct 2009 16:26 200MHz Filter with 200kHz bandwidth

Crystal filter works up to 300-400 MHz (3rd over tone, 5th over tone). So I guess you could try crystal filter.

Annirak

Joined: 26 Jul 2009
Posts: 13

13 Oct 2009 23:08 Re: 200MHz Filter with 200kHz bandwidth

Phytech wrote:

Crystal filter works up to 300-400 MHz (3rd over tone, 5th over tone). So I guess you could try crystal filter.

In order to use a crystal in an overtone mode in a ladder filter, do I simply connect an LC tank, resonating at the 5th overtone, between one pin of the crystal and GND?

e.g. in this image, would I replace C1 and C2 with LC tanks at the 5th overtone?
200MHz Filter with 200kHz bandwidth

Thanks,
Annirak

Added after 5 hours 15 minutes:

One additional question: Is it possible to use a ceramic filter in an overtone mode? If so, how do I go about implementing that? Do I simply need a bandpass filter either before or after the resonator?

Thanks,
Annirak

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Phytech

Joined: 30 Apr 2008
Posts: 89
Helped: 9
Location: USA

15 Oct 2009 18:09 Re: 200MHz Filter with 200kHz bandwidth

I am not sure whether the circuit you draw will work for this bandwidth (very wide bandwidth for crystal filters). You may have to look on the design of crystal filters with two arms in each section.

For the ceramic filter overtone, can you get very high unloaded Q? Say a few thousands? My understanding is only crystal filter offers the required unloaded quality factor for your application.

Annirak wrote:

Phytech wrote:

Crystal filter works up to 300-400 MHz (3rd over tone, 5th over tone). So I guess you could try crystal filter.