FM Transceiver

Discussion in 'The Projects Forum' started by jan.rover, Jun 18, 2011.

  1. jan.rover

    Thread Starter New Member

    Jun 18, 2011
    19
    0
    Hey, I need to make an FM transceiver (walkie talkie) with 3 channels that works for a couple of meters only. I've done some research and only got to stumble upon lectures about Tx and Rx separately; I don't have any idea how to combine the two to make a transceiver. What should I use to provide the carrier frequency?

    please help me...
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,150
    3,058
    Old cordless phones used the FM range, just like baby monitors. Channels were selected using PLL control of a crystal oscillator. I believe there were hacks that could decouple the phone functions, giving you a 10-channel walkie-talkie. Of course, you could also find a multi-channel walkie-talkie.

    You can learn more looking at the old datasheets, such as this one for a transmitter and this one for a receiver.
     
  3. HarveyH42

    Active Member

    Jul 22, 2007
    425
    5
    Walkie-talkies are dirt cheap these days, even with multiple channels. Would check into that first. Would want to spoil the fun and expense of building your own though, but if you don't have the needed test equipment, a meter or two will most likely be the range you achieve, and not so clear or stable either.

    I'm not saying it can't be done cheap or free, from salvaged parts, and basic equipment, but the cost is made up for in time and patience, not a quality I posses a surplus of myself. Many people do though, and you might enjoy such challenges. I prefer to get to a working device, as quickly, and cheaply, as practical.

    There are also wireless intercoms, which are short range, and channel selectable. Cell phones have really driven down the price of these sorts of devices, since they aren't typically needed, unless you are in remote areas without service.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,150
    3,058
    One thing to consider is whether you can live with press-to-talk (like a walkie talkie) or if you need simultaneous send and receive (like a phone). The latter requires a different frequency for each. The simpler walkie talkie disables the receiver when the transmitter is engaged.
     
  5. jan.rover

    Thread Starter New Member

    Jun 18, 2011
    19
    0

    It has to be Press-to-talk, 'coz my prof required a half-duplex. I don't have old phones to salvage I need to make the circuit from scratch. How do I get the frequency for it? I know FM has 88-108Mhz range, how do I choose the frequency to use? :(
     
  6. jan.rover

    Thread Starter New Member

    Jun 18, 2011
    19
    0

    This is actually a school project, not for personal use since cellphones do all the job. MAybe it would be easier to build it out of salvaged parts but I'd want to build it from scratch for better understanding.

    My biggest problem right now is how to "merge" the Tx and Rx. I understand the basic concept of transmitting and receiving, I just don't know how to implement it on the circuitry in order to come up with a working 3channel FM Transceiver. :(

    Do you have some ideas?
     
  7. jan.rover

    Thread Starter New Member

    Jun 18, 2011
    19
    0
    My prof said that there has to be only one circuit for the FM Transceiver, and the Tx and Rx common element is the crystal that would provide the carrier frequency. I've found separate circuits for both Tx and Rx, but I don't know how to merge the two. :(
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,150
    3,058
    The application notes for those old Motorola chips (and the related ICs) contain examples. I don't have the exact link to provide, but I know the information is out there, as I was once pursuing a similar project. But that was 20 yrs ago! :eek:
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Any other secrets? It's very frustrating when we get requirements and - or information in drips and drabs.
     
  10. jan.rover

    Thread Starter New Member

    Jun 18, 2011
    19
    0

    It has to be 3 channel, works in at least 2m apart, and in FM. THere are no other restrictions...:confused:
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Building two complete 3 channel FM transceivers is not a minor task. IMO you would be well advised (if permissible) to build them from kits.

    How much time is he giving you?
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Here is another student who knows nothing about radio circuits and his assignment is to design a transmitter-receiver on 3 channels using FM modulation.

    He does not know that FM modulation is not only used on the 88MHz to 108MHz FM broadcast band but can be used at any carrier frequency.
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Check out the accredited courses these days and it becomes obvious how this is happening. Do you remember the commercial "It's not your fathers Oldsmobile anymore"? Well, it's not College as you knew it any longer either! :rolleyes:
     
Loading...