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Radio Signal With No Resonator?

  1. Nov 20, 2013 #1
    I am a beginner at electronics, and wanted to make a simple (non-audio) radio transmitter and reciever, with no inductor-capacitor resonator, since I don't understand it and I am not making an audio radio. Here is my transmitter, with the input wire to the BJT being the signal: f8svJPJ.jpg
    Here is my reciever:
    9NnnSxV.png (the circuit is not complete) I put the serial number on both crystals to show that they are the same and oscillate at the same frequency. My radio signal is either 3V ON or 0V OFF, which will represent digital signals. I am a beginner, and am posting this here to ask if my circuit would work (if connected together and closed, obviously), and if not to explain why. Thank you very much for your advice.
     
  2. jcsd
  3. Nov 20, 2013 #2

    Bobbywhy

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    Gold Member

    What would the output of a “simple (non-audio) radio transmitter” look like? You write: “My radio signal is either 3V ON or 0V OFF”. If I understand your words correctly you would transmit RF pulses of some duration (fixed or variable), and at some pulse repetition frequency (PRF).

    What would your receiver’s output be to show it was receiving the pulses?

    Here is Wiki’s overview:
    http://en.wikipedia.org/wiki/Pulsed_radiofrequency

    Here are Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:
    http://discovercircuits.com/R/rftrans.htm

    NOTE: If you are in the USA the Federal Communications Commission restricts all radio transmissions according to frequency band, power out, type, etc. You can be “busted” and fined if you interfere with others. Be sure your transmitter is low enough power so as not to travel beyond you lab. Other countries have similar rules.

    One final note: “Square” transmitted pulses, radio frequency or acoustical, generate side harmonics. This means that the total radiated power is not concentrated in the narrow band of your crystal oscillator, but is spread out among upper and lower harmonic frequencies. This can wreak havoc on requirements like interference with others, maximum range, and more. A gradually rising and falling pulse helps avoid this pitfall.

    Edit: Here are hundreds of examples for you to have a look at: https://www.google.com/search?tbm=i...=1366&bih=683&q=rf pulse transmitter circuits
     
    Last edited: Nov 20, 2013
  4. Nov 21, 2013 #3

    berkeman

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    Staff: Mentor

  5. Nov 21, 2013 #4
    Thanks so much Bobby for the great resources. I had suspected something odd would happen with square signals, but I didn't deeply think about it. I guess my circuit will have to be a lot more complex!
     
  6. Nov 22, 2013 #5

    Baluncore

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    Science Advisor

    VarietyBook. The the crystal resonator, your transmitter output would generate broadband “keyclicks”. The crystal would act like a trap to prevent transmission of only a very narrow band at resonance. Energy would be radiated throughout the rest of the spectrum, although influenced by the tuning of your TX and RX antennae.

    Around about 1897 Sir Oliver Lodge in London invented tuning and wireless communication began a great leap forwards. Now we are using more spread spectrum which is the antithesis of tuning.

    SS spreads the energy over a wide band of the spectrum for transmission and then despreads it back in the receiver. Because the spreading – despreading employs a digital code there can be many signals in the same band with different key codes.

    In effect spread spectrum has no resonant elements, but it does need a couple of reference frequencies. Firstly the centre carrier frequency, which is usually suppressed and so not transmitted. And secondly a chip rate clock that decides the rate of the spreading or hopping code.

    Your first radio design was a spread spectrum system but you had no provision to synchronise the transmitted carrier centre frequency and chip rate at your receiver.

    https://en.wikipedia.org/wiki/Spread_spectrum
    https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum
     
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