FM Transmitter help

Discussion in 'Wireless & RF Design' started by stevy123, Feb 4, 2008.

  1. stevy123

    Thread Starter Active Member

    Nov 19, 2007
    61
    0
    Hi All

    A quick question for all of you, im currently trying to build this circuit

    http://www.uoguelph.ca/~antoon/circ/fmt1.htm

    What way do i connect C4 in? It is a 3 pin variable capacitor? I take it 2 legs are the connections to the cap and the other is the wiper? Just not sure what way to connect it in? Any help would be greatly appricated.

    Also i have used 30amp fuse wire for my coil in l1, would this be ok?

    Regards
    Stephen
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You inductor will have a better response if the resistance is very low, so copper wire would be better. C4 is in parallel with the coil to make a parallel tank circuit for the best output to the antenna. It should be in circuit as a variable element, so only one leg and the wiper should connect around the coil.
     
  3. stevy123

    Thread Starter Active Member

    Nov 19, 2007
    61
    0
    Hi beenthere

    Thanks for your response, ill try that and give it ago, will also change my inductor to copper wire.
    Thank you.

    Stephen
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Hi Stephen,
    Somebody posted that circuit on another forum and asked why didn't it work.
    I calculated then simulated that its audio preamp transistor is saturated with a new 9V battery then it is cutoff when the battery runs down a little.
    So I built it and confirmed it.

    I also noticed that its frequency changed all over the place when something got near it or moved away since its tuned circuit is connected to its antenna.

    I also noticed that its frequency changed as its battery ran down.

    I also noticed that its transmitted sound had no treble frequencies and was very distorted.

    I also noticed that it had a short range.

    I fixed and modified it in 4 stages. Now it works extremely well.
    Its range is more than 2km to my very sensitive home stereo and car radio.
    It has an RF amplifier for good range and as isolation between its tuned oscillator and its antenna.
    It has a low-dropout voltage regulator for its oscillator and its mic preamp.
    It has negative feedback in its mic preamp for low distortion.
    It has pre-emphasis (treble boost) like FM radio stations have to match the de-emphasis in all FM radios.
    I made it on strip-board.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Has anyone contacted Tony Van Roon or Art Swan about this? They might want to upload a correction.
     
  6. stevy123

    Thread Starter Active Member

    Nov 19, 2007
    61
    0
    Hi

    with reference to the circuit diagram posted by Audioguru, is this variable capacitor a suitable choice?

    http://www.maplin.co.uk/module.aspx?ModuleNo=460&doy=13m2

    Also what sort of wire should the inductors be made from? I have 20swg copper wire would that be ok?

    Regards
    Stephen
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    And what is the power consumption of the modded circuit?
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The Philips 22pf trimmer capacitor will work at the higher frequencies in the FM broadcast band. I was lucky to find the tiny trimmer capacitors I used.

    I used 1mm enamelled wire from the crossover coil of an old speaker. 20swg insulated wire will be fine if you don't bend the coils.

    My FM transmitter's supply current is 53mA from a brand new 9V alkaline battery. Most of it is in the output transistor which I think operates in class-C for high power and high efficiency.

    The transmitted sound is very clear, wideband and with low distortion. It is too bad it is in mono.
     
  9. stevy123

    Thread Starter Active Member

    Nov 19, 2007
    61
    0
    thank you for your reply,

    i have purchased the 5pf-65pf capacitor, will this be suitable for the circuit or will it be at too high a frequency?

    Regards
    Stephen
     
  10. mik3ca

    Active Member

    Feb 11, 2007
    189
    0
    Standard frequency equation is: F = 1 / (2 * pi * sqr(L * C))
    where pi = pie (the magic number 3.14159...), L = value of inductor in henrys, C = value of capacitor in farads, and F = frequency in hertz.
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The 5pF to 65pF trimmer capacitor is perfect for an FM transmitter. I didn't know its minimum capacitance is nice and low.
    Its max capacitance is higher than mine so it will tune the entire FM broadcast band and below. Maybe to the audio band for North America's TV channels 5 and 6.

    An FM transmitter operates at VHF frequencies so a compact parts layout is needed on a pcb or tightly made on stripboard. It won't work on a breadboard.
     
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