# How do I calculate the frequency of an FM transmitter circuit?

Below is an FM transmitter circuit I found on the web that I would like to build from scratch:

How I would calculate the carrier signal frequency so that I know where to tune my FM radio?

If I want this to transmit my own signal, instead of the microphone signal, could I just replace the microphone with the output of a TRS connector?

• You want to build it and you don't even know the frequency? Anyway, if you want to broadcast in the FM band (88MHz - 108MHZ for Belgium), you'll need a license and a much better schematic. Crystal stable frequency to start with. Jun 7, 2012 at 14:44
• i want to build a remote control that sends a radio wave to an other circuit that is supposed to pick up on this wave, so i need to know the frequency to adjust the reciver Jun 7, 2012 at 14:55
• L1/C2 form a resonant circuit, but since L1 doesn't have a value it doesn't have a resonance frequency either. Jun 7, 2012 at 15:02
• Are you constrained by the receiver? If you are trying to achieve just a transmitter/receiver RF pair, then 434MHz or similar OOK is much, much easier to do. Jun 7, 2012 at 15:27
• To be clear, this will not realistically work as a remote control. Additionally the 433 MHz units mentioned are tricky to work with, being crude receives which require software filtering. Cleanest results with be achieved with digital packet radio ICs have an SPI or similar interface to a packet buffer, eg, the nRF24L01+ clone on 2.4 GHz or various comparable parts in the 300-900 MHz range from a variety of RF semiconductor vendors. All of these offer a variation of FSK which is far more robust than on-off keying. Oct 22, 2020 at 10:38

$$F = \frac{1}{2\pi\sqrt{LC}}$$

L and C2 are components oscillator

but this is a very awful bad circuit.

1 transistor as oscillator an as modulator is not the way to do it it isn't as stable as an regular modular transmitter

for just a sine out you wont use this circuit you could look on the Internet for

• Colpitts oscillator
• Hartley oscillator
• clapp oscillator
• Meissner / armstrong oscillator
• Wien bridge oscillator
• ...

or some VFO (Variable ferquency oscillator)

• PLL
• DDS
• ...

or when is must be very stable a XTAL oscillator

For just data send and receive google for 433MHZ serial data
433 psk data
433 spi
...

• the only thing i want it to do is to send a single wave (that why i asked the question about the microphone) to an other circuit a few meters away... this circuit will fail to do that as well? and just to clarify, in the formula you wrote, L is the number of turns in the coil and C is C2? Jun 7, 2012 at 15:10
• L is the value in Henry of the coil , C is the value in Farad. there are a lot fo websites where you can calculate the henry's of a self wound coil. the prefix p stands for pico (Farad) and is equal to *10^-12 Jun 7, 2012 at 15:24
• This circuit will drift badly with temperature and proximity of metal or people. Jun 7, 2012 at 15:27
• by the schenatics i got from google it looks more compilicated... but it was also very unclear and i'm not sure that the circuits were what i'm looking for, do you happen to know any schamatics of a simple 433MHz transmitter and receiver? Jun 7, 2012 at 16:14
• i took a little project that involves a remote control, and its fun to build as much of it as i can alone... Jun 7, 2012 at 16:31

It's not exactly clear what you mean by "transmit a single wave, not whatever the microphone outputs". Is that OOK (On-Off Keying)?

It's not worth the trouble and the money to start building a transmitter yourself (unless the purpose is in the building, not the use). Digikey lists 433MHz transmitters for around 4 dollar:

This one can send data at up to 3kHz and has a range of 50m. Check the Digikey site (or another distributor) if your requirements are different.