Understanding FM transmitter circuit

Hi,

I'm trying to understand how the following FM transmitter circuit works. I got it from the site
Wireless FM Transmitter. The site has some explanation on how the circuit works, however I'm not sure about a few things, including the electret mic & how the frequency modulation takes place.

**broken link removed**

The electret microphone has a current of 200uA which changes by +- 3 uA depending on sound waves. This sets the voltage across R1 to 2V and the voltage across the mic to 4 volts. As the sound hits the mic the current through R1 increases slightly reducing the voltage across the mic. Is that what is happening?

This changing voltage is passed on by the coupling cap, C1 to the base of the transistor, which is biased by R2 & R3 to approx 2V. The voltage across R4 with no signal on the mic will be Vb - 0.7 (drop across vbe), 1.3 volts. As the voltage at b changes R4 will change by the same amount. This change in voltage is seen at the base of the tank circuit. And the signals voltage is increased/decreased. Isn't this what happens in AM? As wouldn't the capacitance need to change in order to get Frequency modulation? And if it was amplitude modulation occuring in the FM spectrum, then how would a radio receiver be able to demodulate the signal?

At this point I'm not sure what is happening at the capacitor C3, what is that doing? Is it holding CE at a fixed voltage? And is it along with capacitor C2 considered a bypass capacitor? Or do bypass capacitors need to be connected to ground?

You almost figured it all out. You were right in your assumption about the mic. The Frequency is modulated by the tank circuit on the collector of Q1. It estaplishes the carier frequency and the mic modifies the freqency a little based on the audible frequency that the mic picks up. C3 is just a bypass capacitor. Since Q1 is configured as a collecter amplifier then the bypass capacitor helps to amplify the signal to carry it a greater distance.

This is not the best FM transmiter that you could hope to make, however it is a really simple transmiter that will work fine for your personal use. The best way to understand what is happening is to make the circuit yourself and gain an understanding through experience. Just don't try to make it on a bread board, this is a common mistake (one that I myself am guilty of). The wires on the breadboard will act as antenas and pick up stray RF, in additon to the capacitance between each strip messing with the calibration of the circuit.

I think that I have explained it fairly simply, if anyone else wants to add (or correct) anything that I've said feel free. (AG may be able to give a much more detailed explination)

Microphone output voltage coupled to the base of Q1 affects the junction capacitance and thus frequency modulation. C3 is used to maintain the oscillations generated in the tank circuit.

This circuit has has not much difference to an AM transmitter. This outputs both AM and FM, but the limiter stage of an FM receiver clips the upper portions of the IF signal -so only frequency deviations comes to the detector stage. In case of a PLL synthesizer, it is sensitive to only FM.

To minimize Amplitude Modulation, use a varicap diode.

jessfletcher said:

As the voltage at b changes R4 will change by the same amount. This change in voltage is seen at the base of the tank circuit. And the signals voltage is increased/decreased. Isn't this what happens in AM? As wouldn't the capacitance need to change in order to get Frequency modulation? And if it was amplitude modulation occuring in the FM spectrum, then how would a radio receiver be able to demodulate the signal?

Click to expand...

It does produce AM.

FM is also produced because the transistor's junction capacitance varies with the voltage.

As mentioned above, it's a pretty poor quality transmitter.

"The electret microphone has a current of 200uA which changes by +- 3 uA
depending on sound waves."

Not really. This type of mic has a built-in preamp. The 200uA is the current
required by the preamp.

The output of the mic is a varying voltage much greater than you are suggesting,
about 0.1 volt.

The Transistor Hartley Oscillator is in grounded base configuration.
Transistor Hartley Oscillator
see fig. C

Hope I'm correct

Jeeperz...you folks are HOT when it comes to helping people out with electronic stuff. There is an expert here for every application.

Really glad I joined this Forum.

C2 grounds the base of the transistor at radio frequencies because it operates as a common-base oscillator.
C3 provides positive feedback from collector to emitter so it can oscillate.

The oscillator is a modified Colpitts type, not a Hartley that uses a tapped coil.

The output of the electret mic is 10mV when you speak close to it (not 0.1V) and might be 0.1V when you scream at it.

Interestingly enough, I've never seen this circuit in any electronics books I've read.

The only place I've seen it is on the Internet.

Well I tell a lie, there's an example in The Art of Electronics 2nd Edition but it's an accidental oscillator, see attached.

Thanks audioguru, I always make mistakes.

You may like to try with a bit simple fm transmitter circuit and help yourself understand it better.