PLL FM Demodulator

Our local 99¢ has been selling FM radios using a Philips chip.

Bill

-- Fermez le Bush--about two years to go.

Try the LM566 or LM565

Ray

The 566 is a VCO, that puts out a square wave and a triangle wave, and sure won't go up to 10.7MHz.

The 565 is an analog PLL, but it's rated for about 500KHz, and will never make it up to 10.7MHz.

ANd of course, nobody has asked the obvious question, what's the deviation of this signal? Because it becomes harder to recover a useable signal when the ratio of signal frequency to deviation gets large. Which is why it's fine to use 10.7MHz for an FM broadcast band receiver IF, because the deviation is relatively high, but for narrow deviation a lot of receivers will bump it down to a lower IF to lower that ratio. A 5KHz deviation signal at 10.7MHz will cause very little fluctuation in most FM demodulators while the same signal down at 455KHz will cause a lot of fluctuation.

It's such a common practice that a lot of FM demodulators, of any kind, intended for narrow deviation use include an oscillator and mixer to drop the 10.7MHz signal down to 455KHz.

And of course we haven't seen any reasoning on why this has to be a PLL. Is there a real reason for choosing it, or is the poster simpling grasping at something without being aware of all that is possible?

For that matter, what's the reasoning for 10.7MHz IF?

If it's a narrow deviation signal, and assuming this isn't going in a mass production item (after all, someone doing that likely has a better background than is indicated by such a vague question) one might as well grab an old cordless telephone at a garage sale or rummage sale, and extract the IF strip from that. Most of those I've take apart use one of those older Motorola IF strip ICs that includes that mixer to drop down to 455KHz, and the phone comes complete with the crystal to drop from 10.7MHz to 455Khz, and the needed filters.

There are lots of solutions, but the original poster hasn't really described the problem, so the solutions can't really be offered.

Michael

You are correct or more correct than my first note. I have used the 565 to detect fm at 100khz-200khz ( see national ap note AN146. The 88 to 108mhz to

Ray