# FM Transmitter circuit

#### AlexDani

Joined Apr 3, 2021
14
Hello guys, i have a question about this circuit, how do you choose the values for the capacitors or the values for the resistors? Or these values are the basic ones?

#### ericgibbs

Joined Jan 29, 2010
18,012
hi Alex,
As this homework, give your suggestions, we can then help.
E

#### AlexDani

Joined Apr 3, 2021
14
Hey, i dont really know what suggestions to give, i found this schematic on the internet and i kinda understood how it works but i dont understand how the guy who made the schematic chose the values for resistors or capacitors, i mean how he calculated the values and why he chose this particular values.

#### LesJones

Joined Jan 8, 2017
4,013
Try calculating the frequency range that the resonant circuit will cover. I think this will reduce your confidence in the design.

Les.

#### AlexDani

Joined Apr 3, 2021
14
I did that, but i was curious about the resistors and the capacitors, i thought it may be important to know how to choose them.

#### LesJones

Joined Jan 8, 2017
4,013
As it is homework we don't give you the answer to the question. The idea of homework is to see if you have understood what you have been taught. Does your calculation show that it is capable of transmitting on the required frequency ? (Which you have not specified.)

Les.

#### AlexDani

Joined Apr 3, 2021
14
For a value of VC1 of about 1pF and the value of the inductor of 1H the frequency is 159.15KHz, and the range is about 20 to 30m. So, this homework was not mendatory to do, i did it becouse i liked it. And the questions was more for me becouse i was curios about how to determine the value of the resistors and the capacitors.

#### AlbertHall

Joined Jun 4, 2014
12,268
Given the large number of turns required to male a 1H inductor how do think its self-capacitance would compare to the 1pF tuning tuning capacitor (or even 10-100pF)?

#### BobTPH

Joined Jun 5, 2013
7,541
What is the actual homework question?

Bob

#### Audioguru again

Joined Oct 21, 2019
6,155
Since the circuit you found is completely wrong then why not find many other FM transmitter circuits that work because they use the same proper parts values?

This circuit has a tuned circuit and very high value if C3 for a very low radio frequency and it uses an inductor the size of a car. Do you know the radio frequencies of the FM broadcast band? Look it up.

#### AlexDani

Joined Apr 3, 2021
14
Given the large number of turns required to male a 1H inductor how do think its self-capacitance would compare to the 1pF tuning tuning capacitor (or even 10-100pF)?
I...don't know, i just found the schematic and i thought it was good.

#### bertus

Joined Apr 5, 2008
22,217
Hello,

The coil is VERY large. Usualy the value for the coil is in the μH range.

Bertus

#### AlexDani

Joined Apr 3, 2021
14
What is the actual homework question?

Bob
Well, it wasn't really a homework because it was not mandatory to do it, it was more like an idea to go and research on your own and to see how a fm transmitter works.

#### AlexDani

Joined Apr 3, 2021
14
Hello,

The coil is VERY large. Usualy the value for the coil is in the μH range.

Bertus
I see, i will try to find another schematic, thank you.

#### AlexDani

Joined Apr 3, 2021
14
Since the circuit you found is completely wrong then why not find many other FM transmitter circuits that work because they use the same proper parts values?

This circuit has a tuned circuit and very high value if C3 for a very low radio frequency and it uses an inductor the size of a car. Do you know the radio frequencies of the FM broadcast band? Look it up.
I will, and i will try to find another circuit, thank you.

#### AlexDani

Joined Apr 3, 2021
14

#### Audioguru again

Joined Oct 21, 2019
6,155
The new circuit you found has the value of R2 10 times too low. Use 47k then it will work better because then its parts values are the same as hundreds of FM transmitter circuits in Google Images.
Its coil value is 100 thousand times smaller than your first circuit and its positive feedback capacitor C2 is 213 times smaller.
But it will sound muffled with no audio high frequencies because it is missing the pre-emphasis produced by all FM radio stations to match the de-emphasis in all FM radios.

Pre-emphasis boosts high audio frequencies that are transmitted then de-emphasis is used in the receiver to reduce the high audio frequencies down to normal and reduce noise.

#### MrAl

Joined Jun 17, 2014
10,599

View attachment 237424
Hi,

As you may know now the values for the inductor and parallel capacitor are determined by the resonate frequency of a parallel L and C and a parallel L and C is also often called a "tank" circuit. The value of L determines the sharpness of the tank circuit. The circuit should be sharp enough to tune to one frequency without too much gain in adjacent frequencies (in the regular FM band) but not too sharp or else the circuit will be too hard to tune and any drift will loose the station completely. VC1 is made variable so you can tune to the station of your choice within some limits determined by the value of the inductor and the tuning range of VC1.
R1 and R2 bias the transistor so that it can work as a nonlinear amplifier (or just think of it as an amplifier).
The value of C2 works in conjunction with the transistor you might find more about this on the web and possibly using an AC small signal model for the transistor. It also provides positive feedback.
C1 filters the input so the circuit does not feed back RF into the input.
C3 filters the DC buss to help keep the battery voltage constant and keep RF off of the DC buss.

I made several FM transmitters in the past just for the fun of it really. The one i used the most though was designed using TTL logic (standard 7400 series TTL) and a TTL crystal oscillator. I used it to transmit my favorite music to my car while i drove to work. It got up to about 2miles range before it would drop out.
At first you would not think using standard TTL logic with a frequency limit of maybe 20MHz would be possible since the FM band range is something like 80MHz to 100MHz. However, a square wave contains odd harmonic content that goes well up into the 100's of MHz so getting 90MHz isnt hard at all. Just a filter to help narrow down the frequency of interest and an antenna. The crystal oscillator keeps the frequency very constant unlike LC tank circuits which drift like crazy. One thing i cant remember though is how i got the frequency to modulate. I may have biased the oscillator with 5v plus some audio which adds and subtracts a little from the DC buss although i may have biased the crystal itself with some audio which would cause sum and difference frequencies to appear. This was done more than 40 years ago The 7400 series ruled the day back in the early 1970's.

Last edited:

#### Audioguru again

Joined Oct 21, 2019
6,155
These simple FM transmitters use the audio to amplitude-modulate the transistor which also amplitude-modulates the capacitance in the transistor. The varying capacitance of the transistor varies the oscillator frequency producing FM. FM radios ignore the AM that is also transmitted.