How I can extract audio from a Directional Coupler for FM?

[tebron]

directional coupler audio

Hi,
I am designing a directional coupler for FM Band, by 5KW power rf for old radio stations without remote control.
I have to measure forward power and reflected power and to read the TX modulation to activate an alarm if the modulation fails.
Because I have very much signal, How can I draw the audio?
Is it possible to utilize quadrature demodulator.
I have made already the interface circuit utilizing a uADC812 and directional coupler but, I have a little difficulty with Audio circuit.
I hope in your help.
Best Regard

 

[biff44]

Re: How I can extract audio from a Directional Coupler for F

What is a uADC812, and analog to digital converter chip?

You need to sample the modulated rf signal, probably with something like a 50 dB directional coupler, and demodulate it. You can demodulate it a number of ways, but an LC frequency discriminator with schottky diodes works pretty well--it will output an analog voltage proportional to frequency deviation. If the analog voltage pk-pk value gets too big, you are overmodulating. If the analog voltage pk-pk value gets too small you are not modulating enough.

You can get fancier and lowpass filter the analog demodulated voltage, and DC rectify it, and then the DC value will roughly correspond to the FM deviation.

BTW, 5 KW is a lot, watch out for arcing in the directional coupler design.