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Simple FM Transmitter Circuit

  1. Jun 5, 2008 #1
    Hello everyone, I'm relatively new to dabbling with electronic circuits, so I decided to build my own FM transmitter using a circuit I found on the internet.

    The schematic has been attached as an image file.

    However, there are a few things I'd like to clarify.

    What exactly does the 'ground' in the circuit represent? Am I supposed to physically attach a wire and stick it onto the ground?

    And also, since the circuit doesn't use a variable capacitor, how is the frequency adjusted?

    Lastly, the power source (1.5V) is connected through a single point on the circuit? I don't quite get this either. I thought the battery had to be connected on the positive and negative terminals? Quite simply, I don't understand how the battery will be wired to the circuit. I assume it has something to do with the grounding, I just don't know what.

    That's about it. Thanks for your time.
     

    Attached Files:

    Last edited: Jun 5, 2008
  2. jcsd
  3. Jun 6, 2008 #2

    NoTime

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    First this is an AM transmitter, not a FM transmitter.
    It may work with a cheap (and old) FM receiver, but probably not with a modern one.
    This circuit design is very old and originally used a CK722 in the AM band.
    There are also legal issues with running an unlicensed transmitter.
    You may want to consider joining a Ham Radio club in your area.
    You can get lots of help including better circuits and proper licensing.

    The ground symbol shown indicates the common power supply connection.
    In this case the negative battery terminal, if you are using a battery.
    It will make more sense when you start using power supplies with multiple voltages.
    The stick a wire in the ground symbol is different, usually called earth symbol.

    Frequency is determined by coil construction.
    Its not meant to change and usually there should be some notes on making the coil.

    Also the physical parts layout is going to be important at FM range frequencies.
     
  4. Jun 6, 2008 #3
    thanks for the input.

    From where I originally got the circuit, it was supposed to be a FM transmitter. Could I somehow get it to transmit onto the FM band?

    And quite honestly, I doubt this circuit will be able to transmit a signal for more than a few meters, so it shouldn't be a legal issue.

    Also, to make things clear for me, do I connect the two parts represented by the ground symbol together to the negative terminal of the battery?

    What I'm trying to do here is cultivate a personal interest in amateur electronics, so I thought this would be a good starting project to build up on. It's certainly tougher than it looks, but thanks for the help!
     
  5. Jun 6, 2008 #4

    NoTime

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    Sure, it will transmit in the FM band. Problem is that most modern FM receivers use balanced detectors which will ignore AM. Older ones used a simple FM detector that was much more sensitive to AM. It might be better to pick a coil for the AM band. Wiring layout will be less critical as well.
    Probably.
    Yep.
    Good for you :cool:
    Besides the Ham Radio club, I might suggest a subscription to "Nuts & Volts".
     
  6. Jun 7, 2008 #5
    Hmm, I'm not sure I understand that. It transmits in both the AM and FM band? So if it does transmit into the FM band, there should be no problem with the receivers since they ignore AM, which isn't what is being produced.

    Nuts and volts, haha. I just hope there's a ham radio club somewhere near here. You don't hear of any very often in Malaysia, so that might be a bit of a problem. I think the closest we have to finding a ham radio club would be an amateur electronics group.

    Well, anyway thanks again. All I'm trying to do is to get the darn thing to work. I've soldered it all together and checked the connections some 5 times over, but still no luck. Any ideas?
     
  7. Jun 7, 2008 #6

    NoTime

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    AM=amplitude modulation
    FM=frequency modulation
    A band is a range of frequencies assigned for a particular purpose by government agencies.

    Just because you transmitter can transmit in a particular band doesn't mean that it is going to magically switch its modulation mode to match the band.

    Since the physical arrangement of parts is important and coil construction is critical.
    Best guess is that it doesn't work at all due to the first reason or you're not transmitting in the FM band due to the coil or you are not using a receiver that will recognize what you are transmitting.
    If you have an RF probe you could test for the first reason.
    A frequency counter could test the second.

    Berkeman the Moderator of this section is a Ham. Perhaps he might know of a group in Malaysia or knows someone that does.
     
  8. Jun 8, 2008 #7

    berkeman

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    That circuit doesn't look too useful. I'd recommend instead that for your first attempt, start with a kit. They are inexpensive, generally work, and in the case of building transmitters, the kits for FM transmitters do meet the low-power requirement (in the US at least) so as to not interfere with licensed transmissions. Check out the FM transmitter kit halfway down this page of kits, on the right hand side:

    http://www.transeltech.com/kits/kits1.html

    I do know that HAM radio is active in Malaysia, but I don't have any direct links. jonmah just needs to search on "ARRL" in his area, to get more info.
     
  9. Sep 16, 2010 #8
    what is the capacity of the inductor,band of frequency transmitted
     
  10. Sep 16, 2010 #9
    can anyone tell me the working of the FM transmitter circuit which i have attached..
     

    Attached Files:

  11. Sep 16, 2010 #10

    vk6kro

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    Looks like a good circuit.

    [​IMG]

    It might need a bypass capacitor of about 0.01 uF from the top of the coil to the negative supply. ie across the battery but located right near the top of the coil.
    You would build it on a piece of printed circuit board (unetched) and connect the negative end of the supply to this copper surface. Then you can bypass the top end of the coil to this surface.
     
    Last edited: Sep 16, 2010
  12. Sep 17, 2010 #11
    Thanks a lot ...
    please explain me in detail about the second part (modulation and transmission)
     

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  13. Sep 17, 2010 #12

    berkeman

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    Can you please give us some context to your questions? Where is the circuit diagram from?
     
  14. Sep 17, 2010 #13

    vk6kro

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    The audio amplifier puts a varying audio signal on the base of the oscillator transistor.

    This will vary the amplitude of the oscillation and hence the output signal. This is amplitude modulation and not really wanted in an FM transmitter.

    The varying voltages on the transistor must cause some small variation in the internal capacitances of the transistor and, because of the high frequency in use, this is enough to cause a small amount of frequency shift in the oscillator frequency.
    This gives enough frequency modulation to let the audio be heard in an FM receiver.

    These capacitances in the transistor are very small, but 5 KHz frequency shift of a 95 MHz signal is only 0.005 % so I guess such a variation is quite possible.
     
  15. Jun 19, 2011 #14
    Hello everybody. I'm interested in this field too and would like to know how these transmitters work. From what I have learned, fm transmission is done by "piggybacking" the audio signal onto a radio wave. In this circuit, where does this pure radio wave come from? does simply having a transistor placed in parallel to a capacitor with an inductor connected to the collector create oscillations in the radio frequency range? at least that's what I understand from the attached picture (from wikipedia)[​IMG]
     
  16. Jun 19, 2011 #15

    vk6kro

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    The circuit in the first post in this thread is easier to understand.

    The top 10 pF capacitor forms a parallel tuned circuit with the inductor across it.

    This is a high impedance at one frequency. In this case, about 95 MHz.

    The other 10 pF provides feedback to the emitter which in this case is an input of the amplifier.

    This feedback causes the transistor to oscillate at the resonant frequency of the tuned circuit.
     
  17. Nov 18, 2011 #16
    Thanks. It's clear now.
     
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