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Telecommunication question

  1. Aug 14, 2011 #1

    I hope I'm typing this in the right place. I'm trying to build a 4 channel communication system using FM. The system should enable only one person to initiate a call to three others and the channel should be duplex. That is, the remaining three cannot make a call, they can only receive. Now, I have some idea but as this is my first serious and involving project I can't organize my ideas.

    What I have in mind is:

    (1) I first create my base frequency using a Colpitt oscillator and get the frequency I want which is about 10 MHz (suitable for my oscilloscope).

    (2) Then I have to shift this frequency by about 200Khz to provide the necessary channels to communicate with the others separately.

    (3) Then since the controlling station has to choose who to communicate with, he has to have some sort of a switch which can pressed to allow the frequency corresponding to one of the remaining three channels to modulate his audio frequency. That can be done using an analog knob or an encoder with a multiplexer.

    (4) The frequency which has be chosen then gets into an FM modulator which it is encoded with the audio signal.

    (5) Then transmit.

    (6) The receiving end gets some notification when a call has been initiated.

    (7) The receiving end needs to have a transceiver which has a receiver operates at the downlink frequency and a transmitter which operates at the uplink frequency.

    I guess that is all. I’m not yet an engineer and really have a limited practical knowledge and I hope someone with experience in the telecomm field can tell me if my ideas are in the right plane. What I’m afraid of is the transition from theory to practical because things which tend to work on paper don’t necessary work in reality like you can easily calculate the necessary RC values in a Phase Shift Oscillator to make it work in the high frequency band and in reality has a max bandwidth of 1MHz.
    I hope someone can help me.
    Thank you.
  2. jcsd
  3. Aug 14, 2011 #2


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    Staff: Mentor

    Welcome to the PF.

    Sounds like a fun project. What is the context of this project? Even if it's a schoolwork project, I think we can keep this thread here for now.

    Can you post the exact wording of the project statement? When you say that once the call is initiated by one caller, no others can initiate a call, do you mean that the others can participate on a "conference call", or do you mean that the others cannot hear each other and can only hear the person who initiated the call?

    My first reaction is that it would be best to use digital communication for this project, with microcontrollers (uCs) at each station to handle the decoding and arbitration tasks. You say you are supposed to use "FM" for the channel communication, but can you use packetized digital communication to do the overall task?
  4. Aug 14, 2011 #3
    Thanks for the reply. Actually, this is a mini project in which I'm participating in.
    Well, the summary of it all is to design a 4 way communication system that will make one (head) person initiate a call to 3 others who can only receive but cannot call.

    It doesn't matter if I can create a system for 2 or 3. What matters is I have to demonstrate that I can make a call to at least two others.

    I have design my oscillator (which was simple enough). Lets say it outputs at 10MHz. I want to find a way which can produce three or four different variations on it, like 10.2 MHZ 10.4 MHZ etc so that where I select which person of the 3 I want to call, I'll just press a button (encoder and a multiplexer) making the frequency I've chosen pass and modulate my voice. I was wondering how to do that and if there is a more sober idea than mine.
  5. Aug 14, 2011 #4
    I'd like add this circuit which is an FM transmitter with inbuilt oscillator which is at the left of the aerial. What I have in mind is since it will be so difficult and uncomfortable to continuously change the inductance or capacitance in order to change the FM frequency so that I can call someone, then I'd like to use a separate Colpitt oscillator which gives me a constant frequency and get some circuit (which I don't know what) to do the constant shifts.

    [PLAIN]http://www.electronics-project-design.com/images/3VFMTxSch.GIF [Broken]
    Last edited by a moderator: May 5, 2017
  6. Aug 14, 2011 #5
    Duplex usually means transmitting and receiving simultaneously like a cordless phone. Is that what you had in mind? In order to do that you'll probably need to separate your transmit and receive frequencies by more than a few hundred kilohertz.

    Your choice of a Colpitts oscillator at 10 MHz is a good one as Colpitts oscillators are easy to frequency modulate with varactors.
  7. Aug 14, 2011 #6
    Well, I want to know how to create the frequency channels that I should use to communicate separately with the others.
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