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,

  13. Sohampal.101

    New Member

    Jan 28, 2018
    I made a 2 transistor fm transmitter but it seens like it doesn't work. The schematic and real image is attached, i used 2n222a transistors in place of 2n3904. 8.jpg

    Last edited by a moderator: Jan 28, 2018
  14. Ylli


    Nov 13, 2015
    I've never used that construction technique. But, my first question would be 'what kind of wire are you using to connect the various nodes? If looks like enameled magnet wire, and many of the junctions do not look like you removed the enamel. Second, lead lengths are rather long in the RF area. In particular the bypass cap in the base of Q2 has a lot of inductance (lead length) in series with it. Circuit wise, you really should have some capacitive bypassing between the positive supply rail and ground.

    For others, looks like this came from here: spy transmitter-1.aspx
    Last edited: Jan 28, 2018
    DickCappels likes this.
  15. atferrari

    AAC Fanatic!

    Jan 6, 2004
    Is the copper wire varnished?

    Please edit you OP and eliminate the repeated pictures. Downsize the remaining ones.
  16. Sitara


    May 2, 2014
    Assuming that the enamel has been scraped off the connection wire where it wraps around the nails, a dab of solder at those points will ensure that there are no loose connections (and that means one less thing to worry about). But the two main problems with the circuit are (1) the lack of a bypass capacitor between the positive connection (the wire marked +3 to 9 volts) and ground. A 10n cap should do the trick. (2) You have used a fixed ceramic cap for C5 instead of the 10 - 50pF trimmer specified in the schematic. The trimmer is needed to tune the transmitter to the frequency of your receiver, otherwise even if it is transmitting you will never know that it is. The music that you intermittently hear is probably due to the transmitter temporarily overloading the front-end RF amp of your cell-phone's receiver, making it into a mixer of sorts, which is picking up nearby radio stations transmitting music.
  17. Sensacell


    Jun 19, 2012
    This construction technique is not appropriate for 100 Mhz RF circuits.

    The length of wire between components becomes a significant inductance, each nail forms a capacitor between all other nearby nails, and ground.

    This picture shows a typical FM transmitter kit, parts are mounted close together to minimize the stray inductance, connection points are small to minimize stray capacitance.

  18. Audioguru


    Dec 20, 2007
    The circuit with the green and pink background has the audio transistor biased wrong for any battery between 3V and 9V. It has a 0.22uF capacitor cutting all audio frequencies above 100Hz at the collector of the audio transistor instead of using pre-emphasis to boost all high audio frequencies that FM transmitters are supposed to have.

    I re-designed a transmitter like the first one in this thread and built it compactly on stripboard. I used a low dropout voltage regulator and biased the audio transistor correctly. The voltage regulator also powers the RF oscillator so the radio frequency does not change as the battery voltage runs down. I added pre-emphasis so it sounds great. I added an RF amplifier transistor to isolate the oscillator from the antenna so the radio frequency does not change when something moves towards or away from the antenna.
    DickCappels likes this.