FM transmitter oscillator question

[shehry]

Hi,

I was trying to design an FM transmitter and there is one thing that is puzzling me somewhat.

I know that I will have to build an oscillator running at a certain frequency. I want to make a BJT based hartley oscillator. I will have to give an audio signal as an input to the RF oscillator. Meaning that the oscillator will change its frequency. However, the tank circuit in the hartley would remain at more or less the same resonating frequency: 1/(2*pi*sqrt(LC)). So why:

1. would the oscillating frequency of the oscillator change in the first place.

2. if it would change then there ought to be a loss in efficiency for input frequencies that make the oscillator shift too far from its resonant frequency. Am I right in assuming that this loss would be negligible since we are RF frequency would be much larger than the audio frequency?.

Also, I have been trying to find some site which would explain hartley and colpitts oscillators in detail but I havent been able to...could somebody help in this regard.

thanks...

 

[stevez]

A simple form of FM transmitter includes a varactor diode in parallel with the capacitor in the LC circuit that controls the frequency. The audio is applied across the varactor to vary it's capacitance - modulating the frequency. The amount of frequency variation is of concern and ought to match the receiving equipment.

Lots of amateur radio publications discuss this in detail.

 

[Leftyretro]

The other thing to keep in mine is that most freq or phase modulated oscillators work at a much lower frequency then the final transmitter output frequency. Frequency multiplying stages will not only increase the frequency but the deviation amount. The tuned circuits you mention will have a Q value such that there can be frequency variation without suffering much gain loss.

Lefty

 

[audioguru]

A cheap and simple Colpitts oscillator FM transmitter uses the audio to modulate its amplitude (AM). When its amplitude is modulated then the capacitance of the transistor is also modulated because its capacitance changes with voltage like a varactor diode, and the capacitance change changes the frequency of the oscillator (FM). FM radios ignore the AM.

The bandwidth of the tuned circuit is much wider than the maximum deviation (full volume) of an FM transmitter which is only 75kHz. So the efficiency doesn't change as the frequency shifts the tiny amount.

Look in Google for FM Transmitter , Hartley Oscillator and Colpitts Oscillator.

Most FM transmitter circuits on the web are child's toys. Their frequency drifts all over the place if something or someone gets near it and as its battery voltage runs down. Most don't have pre-emphasis which is the treble boost used by FM stations, so when received on an FM radio the treble is cut to sound like an AM radio or a stereo with its treble tone control turned all the way down.
Many have such low power that they barely reach across a street.

 

[mneary]

Many have such low power that they barely reach across a street.

Considering their audio performance, this is a Good thing.

 

[Hero999]

The bandwidth of the tuned circuit is much wider than the maximum deviation (full volume) of an FM transmitter which is only 75kHz.

That's not quite true, an FM signal can vary by 75kHz from either side of the center frequency. However interestingly FM actually has an infinite bandwidth but in practice it's much smaller.

**broken link removed**
where fΔ is the peak deviation of the instantaneous frequency f(t) from the center carrier frequency fc (assuming xm(t) is in the range ±1) and fm is the highest modulating frequency of xm(t).
Ref: https://en.wikipedia.org/wiki/Frequency_modulation#Carson.27s_rule