# Determine and calculate fm frequency

#### lemonjuice

Joined Nov 26, 2016
5

Hi. I find this circuit for my transmitter for 97MHz. I'm trying to calculate the fm frequency using f=1/2(pi)(sqrt(LC)). However, I am confused since i'm not confirmed with which tank circuit should i use L1C4 or L2C7. Can someone tell me? Thanks.

#### Ramussons

Joined May 3, 2013
871
TR2 is the Oscillator and the frequency is determined by L1C4.
TR3 is a power amplifier where L2C7 forms a network to feed the Antenna.

#### lemonjuice

Joined Nov 26, 2016
5
TR2 is the Oscillator and the frequency is determined by L1C4.
TR3 is a power amplifier where L2C7 forms a network to feed the Antenna.
Then, I just focus on TR3 to calculate the FM frequency? It shows that the f= 41MHz, but i can receive the output at 97MHz.

#### Ramussons

Joined May 3, 2013
871
There will be stray capacitances and inductances; you cannot get the exact frequency by calculation.
That aside, what shows "f=41 MHz" ? 4 turns of wire - on what? What Diameter over what length?

You may be getting harmonic of the oscillator in the FM band, and you mayalso get it around 145 MHz, 194 MHZ ..... and so on.

#### lemonjuice

Joined Nov 26, 2016
5
I try to calculate the frequency sent by the antenna using f = 1/(2pi√LC), (L=1uH and C=15pF) and that's why I get 41MHz.
For L1, I make it 4 turns, 3mm for diameter and 6mm for length.
Is harmonic the reason that I can still receive the output even at 97MHz?

#### Magician

Joined Aug 18, 2014
13
I think, you are misinterpreting L2C7 as a tank, but actually it's not. C7 is not connected in parallel to L2, it's in series with antenna.