How FM transmitter works...

[Rajagopal87]

After going through a lot of fm transmitter theory and circuits in the net, Still I could not grasp how its working. So I need your help guys...

As far as I have understood , most of the circuits feature some form of oscillator with o/p cap coupled to antenna and the i/p signal coupled to the base. How the varying i/p applied to the base changes the freq of oscillation? If its an amplifier the i/p at base would control the amplitude but since here its an oscillator it ll not be the case. what are the consequences of the i/p signal at the base (of an oscillator) ? Please explain how it controls frequency and will it affect the amplitude? why?

 

[audioguru]

Yes, in a very simple FM transmitter circuit, the audio input modulates the amplitude of the oscillation, but since the amplitude changes also change the collector-emitter capacitance of the oscillator transistor then the frequency is also modulated, producing FM. The amount of AM is small and FM radios ignore it.

The frequency also changes as the battery voltage runs down.

Since the antenna is connected directly to the tuned circuit and to the collector of the oscillator transistor, then the frequency changes whenever anything gets near the antenna.

 

[stevez]

Audioguru's thoughts may be sufficient. If you need more information you might try an amateur radio handbook - ARRL publishes one annually. RSGB likely has some excellent publications that cover things like FM.

 

[bananasiong]

We have modulation and transmission subject in our college, but we have learned just the theory and some calculation, those block diagram and the waveforms. Never go to any circuit of FM and AM

 

[mdanh2002]

I noticed somewhere in this forum an FM transmitter using only one PNP transistor. But wonder how exactly only a single transistor can produce such a complicated formula to achive FM modulation? I meant, all the integrations and cosine thingy.

https://en.wikipedia.org/wiki/FM

 

[audioguru]

I noticed somewhere in this forum an FM transmitter using only one PNP transistor. But wonder how exactly only a single transistor can produce such a complicated formula to achive FM modulation?

A single NPN or PNP transistor can be an FM oscillator. If an antenna is connected to it then it becomes a very simple FM transmitter.

It makes FM easily:
1) The audio at the base makes the transistor conduct more and less, making AM.
2) When the transistor conducts more and less then its collector-emitter capacitance also changes more and less (look on a transistor's datasheet).
3) When the capacitance of the transistor changes then it changes the frequency of the oscillator, producing FM.

 

[bananasiong]

1) The audio at the base makes the transistor conduct more and less, making AM.
2) When the transistor conducts more and less then its collector-emitter capacitance also changes more and less (look on a transistor's datasheet).
3) When the capacitance of the transistor changes then it changes the frequency of the oscillator, producing FM.

Hi,
Is it as attached? That Cobo

Then for AM transmitter, whould there be any FM component in the modulated signal? Since the amplitude changes and the capacitance also changes.

Thanks

 

[audioguru]

The capacitance of the transistor is in parallel with the tuned circuit and the feedback capacitor. So the amount of change of total capacitance when the transistor is modulated is extremely small. The amount of FM is measurable at 100MHz for the FM broadcast band but not at 1MHz for the AM broadcast band.

 

[Rajagopal87]

Thank You..

Thank You Audioguru, I got it.
Thank you all.