Determine and calculate fm frequency

Discussion in 'Wireless & RF Design' started by lemonjuice, Nov 26, 2016.

  1. lemonjuice

    Thread Starter New Member

    Nov 26, 2016
    5
    0
    transmitter circuit.jpg
    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.
     
  2. Ramussons

    Well-Known Member

    May 3, 2013
    747
    128
    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.
     
  3. lemonjuice

    Thread Starter New Member

    Nov 26, 2016
    5
    0
    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.
     
  4. Ramussons

    Well-Known Member

    May 3, 2013
    747
    128
    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.
     
  5. lemonjuice

    Thread Starter New Member

    Nov 26, 2016
    5
    0
    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?
     
  6. Magician

    New Member

    Aug 18, 2014
    13
    6
    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.
     
    lemonjuice likes this.
Loading...