If the XR2206 is dead, I need an alternative discrete circuit.

**speakerguy79** · 16th July 2008 04:33 PM

OK, I am looking for an FM modulator circuit (or IC if it exists) that will have a carrier frequency between ~30kHz and a few hundred kHz.

In the current plan for this device in which it will be used, I want 0-20kHz (audio signal) mapped to a ~1KHz variation in the carrier frequency, so that the audio signal modulates the 30kHz carrier wave from 29 to 31kHz. These numbers are ballpark, but within 10% or so of the final values (the tolerance on the key part this circuit will be feeding is poor).

Is there a discrete, low distortion (<1%) FM modulator circuit or IC that will do this? After reading this you probably know why I am in love with the XR2206

In an alternatively designed device I could use AM modulation with the same carrier frequency but a much wider modulation range (1-99% AM modulation).

Any help is appreciated!!!

**ericgibbs** · 16th July 2008 04:38 PM

hi spkrguy

Anything in this link, that will do.?

http://www.discovercircuits.com/M/modulators.htm

Must the carrier be sinusoidal or will square do.?

If square OK, look at the CMOS 555 [ Vc pin]

**speakerguy79** · 16th July 2008 05:04 PM

Square wave is fine for now (for FM modulation of course

). It is feeding an H-bridge driver and four N-channel FET's that are driving a resonant transformer (high Q resonance @ 30Khz). I want to shift the output away and towards the output to get an amplitude modulated signal on the secondary. Any harmonics will be way above resonance and contribute little to the output (hopefully).

**ericgibbs** · 16th July 2008 05:07 PM

Originally Posted by speakerguy79

Square wave is fine for now (for FM modulation of course

). It is feeding an H-bridge driver and four N-channel FET's that are driving a resonant transformer (high Q resonance @ 30Khz). I want to shift the output away and towards the output to get an amplitude modulated signal on the secondary. Any harmonics will be way above resonance and contribute little to the output (hopefully).

Hi,
Have you looked at the 555 for modulation, pin 5..?

**speakerguy79** · 16th July 2008 05:18 PM

I'm also looking at the LM331 to see if it would work. It's precision is really high.

**Nigel Goodwin** · 16th July 2008 07:11 PM

Originally Posted by speakerguy79

OK, I am looking for an FM modulator circuit (or IC if it exists) that will have a carrier frequency between ~30kHz and a few hundred kHz.

In the current plan for this device in which it will be used, I want 0-20kHz (audio signal) mapped to a ~1KHz variation in the carrier frequency, so that the audio signal modulates the 30kHz carrier wave from 29 to 31kHz. These numbers are ballpark, but within 10% or so of the final values (the tolerance on the key part this circuit will be feeding is poor).

Doesn't the modulation frequency (0-20KHz) have to be far less than the carrier frequency, you're not going to get 0-20KHz modulation on a 30KHz carrier with only 1KHz deviation.

**JimB** · 16th July 2008 10:26 PM

A 20khz audio signal can FM modulate a 30khz carrier with a deviation of 1khz.
In theory.
However in practice, there will be spectrum components at 10, 30 and 50 khz which in this case the high Q 30khz resonant transformer will filter out.
Result - no modulation.

If you AM modulate the 30khz carrier with 20khz audio, there will be spectrum components at 10, 30 and 50 khz, and again the high Q 30khz will attenuate them and filter them out.

Again result is no modulation.

JimB

**speakerguy79** · 17th July 2008 04:25 PM

If I can get 1KHz or even 100Hz out of this instead of 20kHz, I'll be happy. I'm using COTS parts for this right now. Custom stuff will allow for higher frequencies.

**Ubergeek63** · 17th July 2008 04:33 PM

Well since it is cheap, easy, and very few people know about it...

if you use a X7R cap in a schmitt trigger relaxation oscillator and feed your modulation signal into it instead of connecting it to ground you will get an FM modulator!

This works because as you change the DC bias on the cap you change it's capacitance. The oscillator responds by running at a different frequency. Not particularly accurate, but it is good practice for those who might otherwise get confused by circuit function.

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