Problem in FM transmitter

Discussion in 'Wireless & RF Design' started by prsarr, Jul 9, 2012.

  1. prsarr

    Thread Starter New Member

    Jul 8, 2012
    I have made an FM transmitter on a breadboard using a single transistor, BC547... Sometimes I can hear the music on my cell phone at a frequency of 100.35 MHz but mostly its all noisy... What can I do to remove the noise??? I am using a battery of 9V. Will a voltage regulator help in reducing noise?? Also plz tel any precautions to be taken when i build the circuit on a simple circuit booard... Plz help... Thanx
  2. TecknoTone

    New Member

    May 20, 2012
    Breadboards are not suitable for RF circuits as there is too much stray capacitance.
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011

    Yes, breadboard is not good and also: Where is the schematic and breadboard layout picture?
  4. vk6zgo

    Active Member

    Jul 21, 2012
    Noise free reception using FM relies on a number of things:-

    ( 1)Signal strength--Your BC547 will be really battling to produce much power at 100MHz,plus the FM receiver in your cell phone is not very sensitive.
    This means that you will lose noise immunity.

    (2) Sufficient frequency deviation-The FM Broadcast system standard is +- 75kHz max deviation.
    Anything much less than that causes the demodulated signal to be very low in amplitude,so that the lowest level passages of music will be eclipsed by circuit noise.
    You are unlikely to achieve a lot of deviation in such a simple circuit.

    (3) Frequency Instability---- Although the FM receiver is quite wideband,
    self-excited oscillators at 100MHz or so can easily drift out of the passband,& the receiver will need retuning.

    All these limitations mean your received audio will be very low level & noisy,& make it unlikely that the thing will ever be anything but an interesting experiment.
  5. DickCappels


    Aug 21, 2008
    Yes, by all means, post the circuit. Also, please describe the noise you are hearing and the audio signal you are feeding into the transmitter (over driving the transmitter can make the signal sound noisy on the FM receiver). Oh yeah...and get rid of the breadboard.
  6. radiohead

    Senior Member

    May 28, 2009
    It is possible make a static-free simple one-transistor transmitter on a breadboard with no problem. Can you post a schematic of the circuit you're using and a photo of your breadboarded circuit?

    Also, use a wide band receiver for best results.
  7. nisar.wazir

    New Member

    Sep 16, 2016
    i design this circuit but the problem i face is i see no current flow in this circuit
    what should i do now ?
  8. DickCappels


    Aug 21, 2008
    Please post a schematic of your circuit and if possible a photograph of it.
  9. Mahee141

    New Member

    Dec 14, 2017
    this circuit is not working properly so ,plz face difoult and solve my problem soon
  10. Ylli


    Nov 13, 2015
    Configuration looks reasonable. Double check that resistor you have in the emitter of the audio amp stage - 150K is way too high.

    With the antenna connected directly to the collector of the oscillator, the frequency is going to vary as you move the antenna around, or as you move around the antenna. Might want to connect the antenna to a tap on the coil near the +9 volt end to reduce loading.
  11. DickCappels


    Aug 21, 2008
    I think there is a problem with the emitter resistor in the first transistor. It says "100k" which when combined with a 10k collector resistor will get you a voltage gain of 1/10 or -20 db. That emitter resistor should probably be more like 4.7k.
    Last edited: Dec 16, 2017
  12. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Here is a two transistor FM spy transmitter on 3V,