3 channel FM transmitter/receiver

Discussion in 'The Projects Forum' started by jan.rover, Jul 17, 2011.

  1. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    hey, I need to construct an FM Transceiver and I want to know if I can use a separate FM transmitter and FM Receiver with the same frequency and try to 'merge' them.
    But I the schematic I have only caters one channel, and I need to modify it to three.

    I want to ask if I have to make 3 separate circuits for both Tx and Rx?
    In the Rx circuit, pins 5 and 6 are the LC connections right? for the frequency channel? HOw do I modify it so that I can come up with 3 channels?

    This is the Rx image:
    [​IMG]

    This is the Tx image:
    http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/88-108MHz_25mW_FM_TX.gif

    And this is the proposed connection for Tx and Rx 'merging'. Is this correct?
    http://2.bp.blogspot.com/_FLJi_sW6vA0/TJLp_xS-3TI/AAAAAAAAAWU/FLrWaX3ugrw/s1600/trans2.JPG



    Pls...!!! I really need some help with this ASAP. T_T
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Can you send the RX-link again? It's not there.
     
  3. jan.rover

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    Jun 18, 2011
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  4. praondevou

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    If this is a walkie-talkie scheme with half-duplex communication, yes you'll use one receiver and one transmitter on each side, all being adjusted to the same frequency.

    Neither your transmitter nor your receiver has an option to switch channels/frequencies, there are certainly options to achieve this with the circuits you have but it won't be simple. So you will probably have to build 3 of those circuits and adjust them accordingly. Not a good idea if you ask me. Your circuit is going to be huge...

    Aren't there any ICs designed which already have a multiple channel option?

    Do you really want to work on the frequencies from 88MHz to 108Mhz? There are so many broadcasting stations, if you live in a city, this won't work on a bigger distance.
     
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  5. PeeSeeBee

    Member

    Jun 17, 2011
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    For walkie talkies you need a crystal controlled TX & RX. LC tuned circuits are simply not stable enough for walkie talkie use.

    If you find a circuit for crystal controlled RX & TX, you can change channels by simply switching in different crystals.
     
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  6. wayneh

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    Sep 9, 2010
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  7. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    yes, I've thought about that one option, and it really is going to be huge and troublesome.
    There are ICs for multiple channel option,T've read about them; but I haven't seen a schematic that used one.
    The distance doesn't matter much, because this is only for a school project. Even minimum of 2m is enough. I just hope there's enough frequency in between FM radio stations that could cater our Tx/Rx without interference. I live in a city.
     
  8. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    Ah, yes. I've read about LC not being stable enough for changing channels. I've seen this schematic a few days ago, and I just noticed the different crystals at the lower left of the picture.

    Will this one do for an 88-108Mhz FM TxRx? It does use multiple crystals for switching...
    here's the pic:

    [​IMG]


    ??
     
  9. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    Pls see the picture above...will this one do for 88-108Mhz FM TxRx? Its quite complex though...

    ?_?
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    WOW, if this is your first RF-project.. good luck ;)

    It says "2m". If he means the 2m amateur band (144MHz to 148MHz), then this is not meant to work on frequencies between 88MHz - 108MHz. And there are certainly legal restrictions for this frequency band, too.
    Somehow I doubt that you will find a schematic for a transceiver working in this frequency band.

    The site you found is great ! Doesn't any of the other transceivers in different freq bands fit your needs?
     
  11. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    ..yeah, I think he meant 144Mhz. But it's still Frequency right? I've read something about Frequency Modulation being used not only in the 88-108Mhz range, is that correct?

    The site I've found is really good, quite helpful if as it offers a lot of options. But so far the schematic above is the closest I've got.


    how about these?
    [​IMG]

    [​IMG]

    http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/144MHz_SSB-FM_TRX_DC6HL.gif


    this is my first RF project indeed. Pls reply... I really need every help that I can get. :l
     
  12. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    Hey, this one is a Hand-held transceiver, but I'm not sure if it operates using FM... pls help. T_T

    [​IMG]
     
  13. praondevou

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    Jul 9, 2011
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    ??? You think you will be able to build this??? It doesn't even fit on my screen!!! And it's in german...:confused:

    I think we should really be trying to find something else.
     
  14. wayneh

    Expert

    Sep 9, 2010
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    Those are really complex and would require a lot of experience and equipment to test if/when anything went wrong. A simple circuit times 3 would be far simpler and raise the odds of success. That one circuit uses the MC3362. I've got some of those but I'm pretty sure they're obsolete.
     
  15. radiohead

    Active Member

    May 28, 2009
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    This kit is available from Ramsey Electronics. Bulky, but it has decent transmit distance.
     
  16. radiohead

    Active Member

    May 28, 2009
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    Here is an old magazine article. The parts might still be available.
     
  17. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    I've seen a lot of transceiver schematics, but I'm not confident if they fit the requirements. About the 88-108Mhz range, I think I've read something about FM being used even in other ranges right. Is that right?


    :l
     
  18. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    hahaha! you're absolutely right. hmmm. I'll refer the first schematic to my instructor, maybe it'll do.
    hey, FM can be used even outside the 88-108Mhz range, right?
     
  19. jan.rover

    Thread Starter New Member

    Jun 18, 2011
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    It's really helpful, 'cause there's a PCB layout with it. But we're not allowed to use Programmable IC; which, I think, made the project quite more difficult since I've encountered other schematics for FM transceiver that's simpler but utilized PIC.
    thanks, though. :)
     
    Last edited: Jul 20, 2011
  20. praondevou

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    Jul 9, 2011
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    To be honest I like more the schematics Radiohead posted, even though I didn't analyze in depth. There is for example a PIC involved.. software??

    The schematics you post, do you have a part list too?? There is no specs of the inductors / capacitors, wattage of resistors, instruction on how to adjust everything.... apart from the (inexistent) layout where it becomes more and more important to follow specific rules when it comes to RF.

    Really if this is the first RF project u should stick to something simpler.

    edit: just found this one: http://www.pira.cz/entx3.htm It's not a small project but it has a complete instruction how to build it, part list, schematics, pcb. It works on FM 88-108MHz, frequency can be adjusted via dip switches (PLL). The dip-switches could be replaced with another digital circuit and then you have your multichannel fm transmitter.. Receiver should be easier to accomplish.

    found the link on this site: http://www.discovercircuits.com/F/fm-tranms.htm
     
    Last edited: Jul 20, 2011
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