Tunable Bandpass Filter [Design]

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From: Guy Eschemann on 16 Nov 2009 15:10

I need to design an analog channel selection filter (tunable bandpass
filter) for a communications application. The middle frequency is in
the range 2..7 MHz, and the required bandwidth is 600 kHz. How do I
get started with this?
Many thanks,
Guy.

From: Joel Koltner on 16 Nov 2009 15:41

"Guy Eschemann" <guy.eschemann(a)gmail.com> wrote in message
news:b33af39e-d6b1-44ae-9649-97987de8514c(a)x31g2000yqx.googlegroups.com...
>I need to design an analog channel selection filter (tunable bandpass
> filter) for a communications application. The middle frequency is in
> the range 2..7 MHz, and the required bandwidth is 600 kHz. How do I
> get started with this?

Couple of questions:

-- Is the entire analog channle also 2-7MHz? Or wider?
-- What power levels are you dealing with?

A few approaches are:

-- Mix your signal with an LO of, say, 19.4-14.4MHz such that the band center
of interest is at 21.4MHz (use a low pass filter so that you don't pick up the
image frequencies above 7MHz). Use a cheap off-the-shelf 21.4MHz IF filter
(probably ceramic) to get your 600kHz passband (this is a Q of
21.4MHz/600kHz=36 -- easy peasy). Mix again with the same LO to put your
center band back where it came from. (High power levels -- much above, say,
0dBm -- start creating intermods and compression problems from the mixers.)
-- Build yourself a bank of switched capacitor and inductors that get switched
in and out as appropriate to "build" a bandpass filter wherever you need it.
(Use PIN diodes or MMIC switches for the switching.) If you need very fine
control you'll end up using a varactor diode (or perhaps a DC bias on an
inductor) to set the exact center frequency. (High power levels here push
your varactor or inductors far enough outside of their linear ranges that get
start getting frequency responses that are functions of power levels as well
as intermods.)
-- Same as above, but use relays for switching inductors and capacitors in and
out and motorized variable capacitors (or slug-tuned inductors) if you need
fine tuning. (Higher power levels are attainable, but you end up consuming a
lot of physical space and tuning is slow.)

If the filter is simple enough, you *might just* be able to get away these
days with an FPGA-based "all digital" implementation: Feed your signal to an
ADC, have the FPGA run a FIR or IIR filter, and spit it back out to a DAC. As
with most things "DSP," there are a lot of upsides, although your signals are
at a high enough frequency you'll probably consume a fair amount of power
running all the multipliers in your FPGA, and it isn't going to be the
"bargain basement price" series of FPGAs that'll have enough horsepower to
pull it off.

---Joel

From: Jan Panteltje on 16 Nov 2009 15:44

On a sunny day (Mon, 16 Nov 2009 12:10:17 -0800 (PST)) it happened Guy
Eschemann <guy.eschemann(a)gmail.com> wrote in
<b33af39e-d6b1-44ae-9649-97987de8514c(a)x31g2000yqx.googlegroups.com>:

>I need to design an analog channel selection filter (tunable bandpass
>filter) for a communications application. The middle frequency is in
>the range 2..7 MHz, and the required bandwidth is 600 kHz. How do I
>get started with this?
>Many thanks,
>Guy.

You could mix up to some higher frequency, use a fixed filter at that frequency,
and then mix down again.
And tune the local oscillator.

From: Jon Slaughter on 16 Nov 2009 16:06

Guy Eschemann wrote:
> I need to design an analog channel selection filter (tunable bandpass
> filter) for a communications application. The middle frequency is in
> the range 2..7 MHz, and the required bandwidth is 600 kHz. How do I
> get started with this?
> Many thanks,
> Guy.

It would help to know how sharp you need the filtering.

From: christofire on 16 Nov 2009 16:12

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