# trying to understand how this FM transmitter works

#### tomshong

Joined Oct 6, 2011
36
Hi experts,

Given my very theoretical education on wireless systems, I am used to seeing transmitters with mixers, filters, PAs, etc.

So I came across this simple FM transmitter online, and I am trying to understand how it works.

http://transistor-man.com/max2606 rfAN1869.pdf

So there's one MAX2606 IC, a VCO. I suspect it's a LC tank variety because of the L1. But I don't see how the audio input is up converted to FM band by the VCO. And I don't understand how the output from the VCO gets 'frequency modulated' before hitting the output antenna.

I appreciate any pointers.

Tom

#### Dodgydave

Joined Jun 22, 2012
9,956

#### vk6zgo

Joined Jul 21, 2012
677
Hi experts,

Given my very theoretical education on wireless systems, I am used to seeing transmitters with mixers, filters, PAs, etc.

So I came across this simple FM transmitter online, and I am trying to understand how it works.

http://transistor-man.com/max2606 rfAN1869.pdf

So there's one MAX2606 IC, a VCO. I suspect it's a LC tank variety because of the L1. But I don't see how the audio input is up converted to FM band by the VCO. And I don't understand how the output from the VCO gets 'frequency modulated' before hitting the output antenna.

I appreciate any pointers.

Tom
It is,as you note,a VCO which means it is a Voltage Controlled Oscillator.

If you apply a variable DC voltage to pin 3,the oscillator's frequency may be varied.
This sort of device is normally used in Phase Locked Loop circuits.where the output from the Phase/Frequency
comparator is a DC voltage.

In the circuit shown,the L & R stereo channels are added together,& used to vary the instantaneous value of the
DC voltage on pin 3.
The oscillator frequency varies with this variation.

#### tomshong

Joined Oct 6, 2011
36
Thanks.

How does the left and right channel 'add' together by R3 and R4? and how it add so that it can be picked up as left and right channel at the receiver end?

It looks like the R1 is providing the variable DC voltage to 'tune' the VCO to the FM band, but how does the audio data gets passed on?

where/how did the frequency modulation occur?

what's the limitation for this topology? I am curious if this so simple then why don't all transmitters build similar to this? By just add a few more filters and maybe add a PA to boost the range?

Tom

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#### nsaspook

Joined Aug 27, 2009
8,393
Thanks.

what's the limitation for this topology? I am curious if this so simple then why don't all transmitters build similar to this? By just add a few more filters and maybe add a PA to boost the range?

Tom
The FCC has very strict limits on carrier frequency and deviation that's impossible for this circuit to obtain. For a part 15 device it's fine. http://en.wikipedia.org/wiki/Title_47_CFR_Part_15

This is an example of a professional FM modulator. http://www.starkelectronic.com/btc15-7.htm

I use one for a in-house mp3 to FM stereo broadcast from a music server.

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#### vk6zgo

Joined Jul 21, 2012
677
Thanks.

How does the left and right channel 'add' together by R3 and R4? and how it add so that it can be picked up as left and right channel at the receiver end?
It doesn't,it is mono only.
Mono is L+R.Two audio signals can be combined with a circuit like R3 & R4 in this unit.

It looks like the R1 is providing the variable DC voltage to 'tune' the VCO to the FM band, but how does the audio data gets passed on?
As I said in the previous posting,the DC voltage is varied by the audio.
When the audio goes positive,it adds to the DC voltage,when it goes negative,it subtracts.

where/how did the frequency modulation occur?
The internal oscillator in the chip is tuned by a varicap,which is a device,the capacitance of which can be varied by an applied DC voltage.

what's the limitation for this topology? I am curious if this so simple then why don't all transmitters build similar to this? By just add a few more filters and maybe add a PA to boost the range?

Tom
This circuit is basically a toy.
As nsaspook points out real FM transmitters have to be designed to very much stricter specifications.

#### lawncegary

Joined Sep 27, 2012
6
It looks like the R1 is providing the variable DC voltage to 'tune' the VCO to the FM band