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FM transmitter (mod4)

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, Oct 26, 2006.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Reduce the length of the antenna to reduce the amount of overload. Then increase it for good range after you have tuned the radio to the real signal, not to the overload signal frequency. The radio is not overloaded if it tunes in the transmitter at only a single spot on its dial.

    Maybe your transmitter is at the image frequency of your radio. It is 10.7MHz above or below the real frequency and will have poor reception.

    A trimmer capacitor is very hard to tune. It is adjusted only one time. The tuning capacitor and knob from an FM radio would be better.
     
  2. bananasiong

    bananasiong New Member

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    I think I got what do you mean, let me try.
    Reduce the length of the antenna is to reduce the overloading, so that the transmitter tune to the right frequency of the receiver (maybe the other side round). After getting the right frequency, the length of the antenna can be increased to increase the transmission distance, right?
    Why is it 10.7MHz?

    I'm using 33pF trimmer capacitor, can I know what is the frequency range?

    But this type of capacitor cannot be got easily.

    Thanks
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Yes.

    The IF amplifier of an FM radio operates at 10.7MHz. The local oscillator is 10.7MHz higher or lower and the image frequency is the opposite because a super-heterodyne radio mixes the sum and difference frequencies of the input signal with the local oscillator.

    It is only part of the frequency determining parts. The inductance of the coil, the inductance of the wiring, the capacitance of the oscillator transistor and its voltage, the capacitance of the capacitor from its collector to its emitter and stray capacitance also determine the frequency.
     
  4. dave

    Dave New Member

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  5. Hero999

    Hero999 Banned

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    What power supply are you using is it a battery or a powerbirck?

    Powerbricks can be noisy and transmit lots of mains hum. Try adding a 1000:mu:F to 4700:mu:F capacitor across the output or use a regulated supply.

    You might be using the wrong sized pre-emphasis capacitor, try changing it.
     
  6. bananasiong

    bananasiong New Member

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    I'm using a power supply at 9 volts.
    I'm using a 7805, a 100:mu:F and a 1:mu:F at both input and output. Should I remove the 100:mu:F at the output and replace with the higher that you mentioned?
    That is C4 right? I'm uisng a 150nF. The recommended is either 150nF or 100nF.

    Thanks.
     
  7. bananasiong

    bananasiong New Member

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    Hi,
    The transmitter is working with my superhet radio :D. I've changed the coil of the radio to be the same as the transmitter. The transmission is more than 10 feet (I'm using power supply, can't go further).
    But it stoped working suddently, no matter how I tuned the receiver, I can receive some radio channel only but not from the transmitter. I tried to measure the antenna of the transmitter with oscilloscope, nothing is there anymore (prevously there is sinewave with high frequency) and, the Q3 becomes hot. The collector voltage of Q3 is around 8 volts.
    How come? This circuit cannot last long? I supply 9 volts. Is the Q3 already burned?

    Thanks
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I guess the RF oscillator stopped. Q3 is biased on by R8 and will be conducting up to about 40mA into the collector coil and will dissipate up to about 365mW and be fairly hot. Its max heat dissipation rating is 625mW so it should not be burned.

    Q3 in my transmitter becomes warm while it is working.
     
  9. bananasiong

    bananasiong New Member

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    Do you mean L1 and C6? Maybe my trimmer capacitor is spoilt? How to check whether it is spoilt or not?

    Or maybe the coil is not connected well?
    I have to check soldering.

    [/quote]
    I used power supply, it is quite hot after the power is turned on.

    I've check the voltage at the mic and the collector of Q1 when the transmitter is not working, same as previously.

    Thanks
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I am guessing that you don't have RF output from the transmitter because the oscillator stopped. Then Q3 would get hot.

    I don't know why the oscillator stopped.
     
  11. bananasiong

    bananasiong New Member

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    The oscillator is form by L1 and C6 right? I'll check the connection, maybe either of them is not soldered properly.

    Thanks :)
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Every part connected to Q2 is part of the oscillator.
     
  13. Hero999

    Hero999 Banned

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    Did any of the componants get hot?

    Have you measured the acutal output voltage from your power supply?

    It might be higher than 9V, is this a propper regulated supply or a cheap and nasty wall plug?
     
  14. bananasiong

    bananasiong New Member

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    Okay, I'll check them. Thanks.

    Yes, Q3 is hot, L2 is a little bit warm, audioguru has explained, the oscillation stops.
    Yes, it is 9 volts. The experiment is done in my college, I think the power supply is okay as well as the wall plug.

    **If I make my own 9 volts power supply, is there anyway to make it stable output without any noisy? I'll post my schematic soon.

    Thanks
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You can make a power supply with a 7809 regulator that won't be noisy.
     
  16. bananasiong

    bananasiong New Member

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    I should step down to 12-0-12 right?
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A 12-0-12 transformer will make a peak voltage of 17V and the two rectifiers will drop it to about 16.3V. The filter capacitor's ripple will drop it to about 15.3VDC. It will work fine since the 7809 needs a minimum voltage of 12VDC.

    A 12V transformer without a center tap will make a voltage of 14.6VDC which is also fine. It uses a bridge rectifier (4 diodes).
     
  18. bananasiong

    bananasiong New Member

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    I think Ill choose the bridge rectifier. The '0' is not being used right? Use only +12 volts and -12 volts.


    I still can't find what's wrong with my transmitter circuit. I've changed the trimmer capacitor of the oscillator, and I've taken out some resistors and measured the resistance, nothing is wrong. Reallllly sad :( how come it was working at the 1st time..
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A 12VAC transformer uses 4 rectifier diodes to make 14.6VDC.
    A 12-0-12 transformer has its center-tap connected to the circuit's ground and uses only 2 rectifier diodes to make 15.3VDC. If you don't use the "0" wire then its output is zero or 32.2VDC with a lot of ripple.
     
  20. Hero999

    Hero999 Banned

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    Are you using the origional circuit?

    It sounds like you're having problems with thermal runaway. Replace Q3, it's now ruined. Add a small resistor between 10:eek:hm: and 100:eek:hm: to the emitter of Q3.
     
  21. bananasiong

    bananasiong New Member

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    Ya, I mean where should be the '0' connected to for a bridge rectifier?
    [​IMG]

    Not really. I have some replacement:
    low drop out voltage regulator -->7805
    160k (R2) -->180k
    30k (R3) --> 33k
    30pF (C12) --> 22pF
    I'm using a 150nF for C4.
    I'm using a power supply unit to supply 9 volts.
    I've changed Q3, it is still getting hot (without any resistor at the emitter). But I've measured the collector of the oscillator transistor (Q2) with probe, there is no high frequency sinewave like previously.
    I haven't tried to add the resistor yet.

    Thanks
     
    Last edited: Nov 17, 2006

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