What Distinguishes AM from FM Transmitter Circuit Designs?

[Razzledazle]

I have been studying multiple AM and FM transmitter schematics trying to distinguish the differences in the circuit that create either am AM (amplitude modulated) or and FM (frequency modulated) RF signal. I have hit a road block as I am unable to see any significant differences between the two sets of schematics that would create the different types of modulation.

FM example http://www.electronics-project-design.com/images/3VFMTxSch.GIF

AM ecample http://www.diy-electronic-projects.com/projects/82/bigs/amtx.gif

My over all goal is to create an FM AV transmitter for FPV flying of a model aircraft, but first I have to understand the basics of the FM transmitter circuit.

 

[skeptic2]

I agree, those schematics are remarkably similar. I suspect that they both produce FM and that the AM receiver can receive it by slope detection. Slope detection is the process whereby an AM receiver can receive FM by slightly mis-tuning the frequency so that the frequency modulation is centered on the slope of the AM bandwidth. As the frequency is modulated, it slides up and down the slope producing an amplitude modulated signal which can be detected by an AM detector.

You can amplitude modulate a carrier by modulating the voltage to the final stage. This takes an extra stage because if you try modulating the voltage of the oscillator, you run the risk of killing the oscillation.

 

[waht]

The oscillator stage is pretty ugly and it will be drifting in frequency. The output is not filtered and so it will be producing a lot of garbage noise. The oscillator bias is being pulled right on the spot by the input stage which will cause AM modulation, as well as FM modulation, because capacitance of the transistor will vary with respect to input, and hence the LC frequency.

 

[Razzledazle]

Waht, After doing a little more research what you said made sense, about the input to the transistor causing FM modulation because changes in the transistors internal capacitance.

I am still having a hard time understanding what you are trying to explain here.

The oscillator bias is being pulled right on the spot by the input stage which will cause AM modulation.

Waht and skeptic2 thank you for your help so far.

Here is a link to an explanation of the FM schematic that I posted.
http://www.electronicecircuits.com/electronic-circuits/3v-fm-transmitter/

 

[sophiecentaur]

In the interests of frequency stability, why not use a 'proper' oscillator, based on a crystal. This can be 'pulled' with a varactor diode for a good, linear frequency modulation.
Then, for AM, why not, again, use a good oscillator followed by a good amplitude modulator / output stage?
There are loads of project circuits which are much easier to understand than the two that you have considered- largely because they work on the lines of the functional block diagram of FM and AM systems. They will involve a few more components but you would have a better chance of understanding what's going on than in a circuit that 'does everything' with just one transistor in the output.
Someone should mention that your local regulations may consider your project illegal. Perhaps you should make sure. Those circuits are not likely to behave in a very neighbourly way, compared with a better engineered system.

 

[vk6kro]

This diagram shows an oscillator with a few parts added to provide FM.

[PLAIN]http://dl.dropbox.com/u/4222062/FM.PNG

Except for the 3 components at the left of the diagram, it is a fairly standard oscillator which could be used from 3 MHz to 15 MHz. This frequency is set mainly by the coil inductance and the capacitance of the capacitors across it.

The varicap (varactor) diode varies its capacitance according to the voltage on it. If this is audio, the capacitance changes cause the oscillator frequency to change with the audio.

Varicap diodes do not produce a linear capacitance change with voltage, so this circuit will produce distorted output if the audio input voltage is too high. However it will sound quite OK for small audio levels.

The varicap diode must be reverse biased, so the audio shown would have to have a positive DC component.