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Does the "shape" of an FM bandpass filter matter?

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Since I'm not an analog EE expert, I'd like to ask anyone here who might be...

I'm designing a bandpass filter that goes in front of a tank circuit for decoding an FM signal. Considering there are tons of filter designs to choose from, each with their own unique frequency response shape, I'm wondering:

Given an FM signal input, does the "shape" of a bandpass filter's frequency response have any significant affect on the decoded signal?

I'm talking about the band pass area specifically. Does it need to be flat? Or can it have up to a +/- 1.5 db ripple and still work effectively for the tank circuit to lock onto the signal? What if it has a "dip" in the middle of the bandpass (like two camel humps)?

My educated guess is that as long as the lower and upper cutoff frequencies are within spec for the desired FM signal, and as long as whatever ripple is "inside" the bandpass is no more than +/- 1.5 db, the tank circuit will lock onto the carrier and the decoded signal will be just fine. Am I missing anything here?

My educated guess is that what is important is reasonably smooth group delay, otherwise you will get distortion.

I guess it depends on the actual demodulator you are using, but the flatness of the passband should not matter too much. I would think 3db flatness would be OK.

Also you obviously need to meet the stop band requirements to prevent interference.

So I would guess that you don't want designs like the Tchebychev filter with lousy delay characteristics.  Bessel has flat group delay, but is lousy in the stop band. So the best compromise may well be the humble Butterworth filter which doesn't have a flat delay, but it is pretty smooth at least.

If there are any RF designers, it would be great to hear what they say.


I think you have the wrong end of the stick with the "tank circuit".

A "tank"circuit is Radio people's slang for a resonant LC circuit.
It normally refers to the circuit which is the plate load circuit for a vacuum tube amplifier.
A resonant LC circuit doesn't "lock" onto anything,it just resonates,that's all!

So what is your "decoder" circuit,a PLL,a quadrature detector,a ratio detector,Foster Seeley discriminator,or what?
If you are feeding a resonant LC circuit with a bandpass filter,you will find that the resonant circuit  will be the dominant influence in determining the bandwidth & shape of the resulting passband.



--- Quote from: vk6zgo on October 20, 2011, 09:17:17 am ---I think you have the wrong end of the stick with the "tank circuit".

--- End quote ---
Forgive me for not being clear.

The chip I'm using has three stages to it. The first stage is an amp for an antenna. The second stage is a limiter. The third stage is an FM demux.

The BPF goes between stages 1 & 2. The tank goes between stages 2 & 3.

Although the tank does have a resonant f, it's recommended that a BPF be used to help isolate the carrier f that you wish to demux. (The tank's Q is understandably low. Otherwise it would be difficult to, what I've probably inaccurately called, "lock on" to the carrier, or more precisely, feed the FM demux the proper resonant feedback in order to properly demodulate the FM signal.)

You're not getting very helpful replies because you've not given enough information.

What IC are you using? Can you post the schematic or a datasheet?


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