Simple FM Modulator Design

Hello,

I want to build a system that can transmit in the FM band. I have already built my oscillators because I want to transmit at more than one frequency (not simultaneously). Now, what I want is a simple FM modulator that takes in the carrier from my oscillator and baseband audio signal.

I've seen numerous circuits on the net for FM transmitters but they all have in-cooperated oscillators (tank circuit) which is at the collector of an rf transistor and the baseband signal at the base. Seems clear enough but not what I want.

I wonder if someone can refer me to a simple laboratory circuit that may be of use to me, noting complex because where I live I find it toilsome to get components.

Thank you.

Your request is not clear.

In an FM system the carrier frequency is modulated but you say you have already built the oscillators. Normally, the modulation would be applied at the oscillator stage.

As you are transmitting at more than one frequency, there is a way of mixing one modulated carried with several fixed oscillators to produce different output frequencies. Is this what you are trying to do?

Brian.

There are two different ways to make an fm modulated signal:

1: make your oscillator tunable by placing a varactor diode in the frequency determining part of the oscillator. You can apply an AC coupled tuning voltage to that varactor diode and generate FM modulation directly.

2: have a fixed frequency oscillator, and apply some sort of modulator afterwards. For AM or PM modulation this is a simple thing. For FM modulation it is more complex. One way is to use an Armstrong Modulator: Armstrong phase modulator - Wikipedia, the free encyclopedia


Most people do method 1 above. They do that because they already have a DC coupled varactor diode in the oscillator circuit that they use to Phase Lock the oscillator to the exact frequency they want it to oscillate at. It is a simple thing to add an AC coupled connection to that same varactor to add FM modulation.

Well, my intention is to have a fixed frequency oscillator and get it modulated by the baseband in a separate circuit.

"One of the problems with the Armstrong method is that the frequency deviation—the amount of modulation—must be kept small to minimize distortion.[4] The maximum deviation is a fraction of 1 kilohertz, but FM broadcast requires 75 kilohertz deviation and a typical FM voice channel deviation is 5 kilohertz. To solve this problem, Armstrong multiplied the signal many times to a higher frequency to obtain the necessary deviation. For example, to generate an FM signal with 5 kilohertz deviation at 146.94 megahertz, the transmitter would generate a signal at 6.1225 megahertz with only 0.2 kilohertz deviation, and then multiply the signal 24 times."

What can anyone suggest? I want to transmit at 4 different frequencies at my will. That's the reason I have made the Colpitt oscillator with a simple switch that can vary the inductance of the tank circuit to the frequency I want. What I intend to do is take the output of the oscillator and pass it to a modulator and I'm done.
If someone has a better idea, then I'll savor a reply.

If you use an external box with a varacter (variable capacitance diode) in it across a tuned circuit, then you have built a phase modulator,i.e. the phase of the RF varies with your modulation frequency. This is similar to FM but the modulation index increases with increasing modulating frequencies (opposite to FM). It can be received by an FM receiver and can be compensated for by by using an audio filter. The problem was to get enough phase shift but as I am thinking of techniques used 45+ years ago, I suppose things are easier now. One trick is to use two varactor diodes that are in series for the audio and are anti series for the RF so as the RF voltage increases the bias on one it reduces it on the other else the RF gets distorted and harmonics are produced. use multi sections to get more phase mod.
Frank

@medwatt, the basic problem is that for FM you need to modulate the scillator frequency and that's something you normally do in the oscillator circuit itself, not afterwards.

You CAN make it AM because you can adjust the amplitude according to the baseband signal but shifting the frequency once you have created it is far more difficult if not impossible.

Brian.

The Armstrong FM modulator is the choice for you request, but is much complicated than using a simple varactor part of the resonant circuit.

See the original block diagram and schematic, from Armstrong patent file in 1936.

FM modulation of a colpitts osc is very using a varicap diode
see below a part of a common TX cct that I regularly work on


cheers
Dave


View attachment FM modulator.bmp