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FM transmitter (mod4)

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

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You 'know' wrong then, a condensor mike requires power, usually 48V - the little inserts are 'electret condensor' mikes, and have a built-in FET amplifier to buffer the condensor element.

    Essentially the electret mike has a permanent charge stored on the active element, this removes the requirement for an external supply - but adds the requirement for a buffer, which then needs a supply itself.

    Check Wikipedia! http://en.wikipedia.org/wiki/Electret
     
  2. mr.gone

    mr.gone New Member

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    My my my. This is gonna be a fun place...lol. What is piezo-electric material? Does it require electricity to produce electricity? You go to Wikipedia for electronic advice?....LOL That's too much...LOL. That one takes the cake!!!! A condensor mike requires no power! Period! The teacher has spoken.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Sorry, what has 'piezo-electric' got to do with condensor mikes?, piezo mikes are a completely different beast, and you are quite right that piezo mikes don't require any power.

    Those who can do, those who can't teach! :D :D :D

    And even teach absolute rubbish!
     
  4. dave

    Dave New Member

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  5. mr.gone

    mr.gone New Member

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    Because condensor mikes have piezo-electric crystals in them.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Nope. Read about the electret mic and about the condensor mic in Google.
    A condensor mic needs a 48V power supply to allow the capacitive voltage divider action of the diaphragm. An electret mic has the 48V built-in on its electret material. An electret mic doesn't have an amplifier, it has a FET transistor impedance converter which doesn't have much voltage gain.

    No they don't! Besides, we are using an electret mic, not a condensor mic and not a piezo crystal mic.

    The RF oscillator's transistor's collector/emitter changes its capacitance a tiny amout with the modulation. A hyper-abrupt varactor diode has a wide capacitance change and is usually used to tune the entire band of frequencies.

    There are many FM transmitter circuits on the web. Most use just the transistor's small changing capacitance to produce the FM. Some use ordinary diodes or transistor junctions as varactors.
     

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  7. mr.gone

    mr.gone New Member

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    Whatever. I'm not gonna go on and on about a condensor mike.

    Well first off. The same varactor diode that is used in a PLL to shift the frquency is also the chosen modulation device in most applications today. Even though theoretically it is PM (Phase Modulation) it does the same thing and is detected the same way. Now if you consider the circuit, and to be honest with you, I have not seen the circuit. Just considering what you said above, what do you think you are doing by applying audio to the collector of the transistor? You are applying an Audio voltage in reverse bias to a P/N junction. A transistor is just two diodes connected back-to-back. So, when you apply this audio voltage to the collector of the transistor, you are causing the depletion layer of one of the diode junctions in the transistor to change in size by the amount of reverse bias applied to it. This causes the P and N material to act as plates of a capacitor which will vary with audio frequency and amplitude. Personally I find this to be very trashy design and I would never do this myself.
     
    Last edited: Nov 13, 2006
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My RF oscillator is just a Colpitts circuit. It is Amplitude Modulated a little by the audio signal and its changing collector/emitter voltage changes its capacitance (as shown on the datasheet) which changes its frequency, creating FM. An FM radio's detector rejects the AM.

    My FM transmitter circuit that is talked about in this thread sounds perfect! I was amazed when I tried it. It has pre-emphasis (treble boost) like FM stations have and very low distortion. It has a voltage regulator and an RF buffer amplifier to keep its RF frequency from drifting all over the place like simpler designs do. Its range is over 2km line-of-sight to a very good quality FM radio.
     

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  9. mr.gone

    mr.gone New Member

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    Another reason I don't like that design is: Notice I said it will vary with frequency and amplitude. That means the amplitude of the audio signal will effect the collector current thereby producing AM components as well. In this case you will need a limiter circuit to get rid of the AM. Now you run the risk of audio distortion. I would much rather modulate the tuning section of the oscillator. Then you won't have any unwanted AM components.

    Edit after I saw your design:

    I assume your pre-emphasis is C9. I don't have a problen with that but where is the buffer amp you speak of? A buffer consists of an emitter follower or darlington pair amplifier and yes you are right, buffers a very good idea! I use buffers all over the place. Especialy following the all important oscillator.
     
    Last edited: Nov 13, 2006
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The AM from my FM transmitter is fairly small.
    The IF amplifiers of all FM radios have a limiter that rejects AM.
    I can't hear distortion from my FM transmitter on my very high quality stereo tuner nor on my cheap clock radio.

    If I wanted to be an FM broadcast station operator then I would have made this frequency-synthesized MICROMITTER one with a PLL and varactor diode. It is stereo and also has pre-emphasis.
     

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  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    C9 is a supply bypass capacitor.
    C4 provides pre-emphasis (boosts the high audio frequencies). C4 has a different value for the different amounts of pre-emphasis around the world.
    Transistor Q3 buffers the oscillator so that things near the antenna don't change the oscillator's frequency. It also amplifies the RF power.
     

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  12. mr.gone

    mr.gone New Member

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    OK buddy! I don't want to get off on the wrong foot here. It's just that I got attacked about the condensor mike and I have done too many designs with them, but like I said, I'm not gonna go there with you. Now there is something to be said for cheap and dirty. I have designed PLL FM transmitter using PIC16F84 and Motorola MC145149's. I also have designed stereo multiplexers with discrete components because at the time a common chip (the RHOLM BA1404) was not redily availble. You simply run your left and right adio into the MUX and you get a composit signal that will modulate the tranmitter oscillator or in my case the VCO with a stereo signal. I would post the cotton picker but I did it so long ago it is only in Orcad SDT which runs in DOS.

    The only IC is a divider used to obtain the 19KHz pilot and carrier for the 38KHz double sideband signal.
     
    Last edited: Nov 13, 2006
  13. mr.gone

    mr.gone New Member

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    Here's one of my FM broadcaster designs:

    Crap! can't figure out how to add an attachment. :D
     
    Last edited: Nov 13, 2006
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The Rohm BA1404 was full of problems and has been replaced by Rohm's BH1417 that is used in the Micromitter that I posted.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It looks very faint.
    What are the functions of transistors Q7 to Q159?:D
     
  16. mr.gone

    mr.gone New Member

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    What are you talking about? I didn't even attach anything. It won't let me.
     
  17. mvs sarma

    mvs sarma Well-Known Member

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    Hi Mr.Gone

    after you finish typing, if you go further go down the screen you see additional options where you find attachment possibility

    pl use it
     
    Last edited: Nov 20, 2006
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Maybe your stereo FM transmitter uses many vacuum tubes (valves)??
     
  19. mr.gone

    mr.gone New Member

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    Very funny Scroogy duck. Yeah it was being stopped by my pop up blocker.
     

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  20. mr.gone

    mr.gone New Member

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    Yeah I remember that design. There is a mistake on it. The output Q2 is actually taken off the collector and not the emitter. Q2 is a DC amplifier for loop gain.

    edit: Also U1 is a MC145170 not x149. They are available now from Landsdale Semi-conductor but were originally Motorola.

    Also while I'm at it. The display is Vitronix VI-422-DP. It's connected to the LCD driver AY0438L which is a Microchip display driver.

    All transistors are 2n3904 except the JFET which is 2n5486 and the final amp Q12 which is an NTE473 and it puts out about 2 watts.
     
    Last edited: Nov 13, 2006
  21. mr.gone

    mr.gone New Member

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    Hey audioguru? I know they may not post it in rules but I think this site can get sued if your avatar is a copy writed image. Since your an audio guru. I have a question for you. Can you tell me of a way to boost my audio gain over the audio output level of an LM386 without increasing my voltage power supply of 12 volts? I've tried some push-pull designs but it seems I just need more voltage to increase the audio DB gain? Got any suggestions, because I don't want to change the power supply.

    I'm talking class A only. It has to be class "A" for SSB reception.
     
    Last edited: Nov 13, 2006

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