Simple Receiver for my simple transmitter

Discussion in 'The Projects Forum' started by BLOBY, May 15, 2011.

  1. BLOBY

    Thread Starter Member

    Mar 10, 2011
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    Hello,

    The problem I'm facing is very basic, but I'm sure it will make me more interested in radio if it worked...

    Ok... I got my very basic colpitts oscillator circuit ( in attachment ) for which i hooked up an antenna (a 5 feet wire in vertical position )... Now, I want ideas on designing a receiver which would be placed at about 3-5 meters away from the transmitter circuit.. The receiver should just detect the transmitter signal and nothing else and this signal should get amplified and the reading should be detected in the voltmeter... I do have some ideas on the receiver circuit (in the attachment) which I tried but didnt work want help on...

    * Can i design a receiver with the bandpass filter having coil and caps having the same values as in the transmitter?

    * Is the length of the transmitter antenna (5 ft) good for the frequency of 410kHz?

    Im really short on ideas on the receiver part.... especially on the tuning and stuff.. Thanks for ur help in advance... ;)
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    You really need to modulate this transmitter somehow. The reciever would pick up this modulation, and verify you are getting a signal from it.

    You may be sure it is oscillating, but until you can measure it you aren't really sure. You only think it is.

    A reciever can be as simple as a Schottky diode/capacitor connected to an antenna and a DVM. If the antenna's are very close you should be able to see a couple of µv on the DVM.
     
    BLOBY likes this.
  3. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Thanks Bill for quick reply... But what is the need for modulation?? Say after making some adjustments I get about 20-30mV at the receiver... Now I remove the battery connection of the transmitter and reading reduces to zero... wont that be enough? Why modulation? I do plan on making FM transmitter sooner or later though... And what is the difference between the colpitts oscillator I hav posted earlier and the one I posted now (version 2)... I have read in other post that this type is of common base type.. All the various schematics I hav seen of FM transmitters use the design which i hav posted below .. Which one is better for RF frequencies? Is my way of hooking up the antennas correct? :confused:

    (sorry, lots of questions in one post... )
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Using your first RF generator (not the second, which is different) inject low level audio at the emitter. It should become an AM transmitter (I think).
     
  5. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Yes... I liked the name you gave it.. RF generator... And I got one cool idea after reading it...
    I will do as u said and try to listen music in my old AM radio.. is 410kHz under AM band??


    I wil try this only to ensure that my transmitter transmitting signals.. but my question is on receivers....
     
    Last edited: May 15, 2011
  6. Audioguru

    New Member

    Dec 20, 2007
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    In North America and Europe the AM broadcast band is from 540kHz to 1600kHz.
    It sounds awful.

    The frequency is so low that a very long antenna is needed or a ferrite loop antenna with lots of gain in the radio circuit.
     
  7. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Ok.... I found this oscillator circuit which says it can transmit waves in fm band... http://circuitdiagram.net/basic-rf-oscillator.html... But I tried this circuit in simulator and it doesnt seem to oscillate at all!! I also tried for different values of inductance, capacitance and stuff but no.. no oscillations... Could anyone please explain me why my simulation failed and what kinda oscillator circuit is it... Im waiting....
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    You're not trying to breadboard this on a protoboard are you? If you are, that's probably your problem. ;)
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    the FM transmitter is an extremely simple Colpitts oscillator.
    The transistor operates as a common base amplifier (because the base has a capacitor to the positive supply) and the 4.7pF capacitor provides positive feedback from the collector to the emitter.

    A simulation program frequently needs to be KICKED for it to begin to oscillate. The KICK in a real circuit is produced by noise in the transistor.

    Because the circuit is extremely simple then its performance is very poor. Its frequency changes due to many reasons.
     
  10. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Ok,
    I did as Audioguru said and it showed some oscillations but dampened out... Leaving the other design of the oscillator aside, I returned to my original circuit and simulated it in multisim... ( the simulation pic is in attachment )..
    I kept two probes one near the inductor and one near the negative battery terminal... It was found that the inductor carried a current of 300mA whereas the battery only supplied about 1mA !!!! (both currents in peak to peak).. Now how is that possible?? I really believe that this is some sort of a simulation pitfall... plz guide me where It went wrong..

    Also, I somehow feel that though the inductor might not be carrying 300mAmps of current, it is still carrying quite a bit of current (in my practical circuit that is :cool: maybe 70% of supply current) and this inductor itself is acting like an antenna and also my attached antenna seems useless.. So plz tell me what is going wrong.. I just want to make my antenna effective propagator of radio waves...
     
  11. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Yes I used breadboard but the connections are made super small.. No probs there...

    I realised that my antenna was not effective after I placed the oscillator which was oscillation at 410kHz near the AM radio but i could not hear any buzzing sound.. I had read somewhere that if one places the antenna extremely close, it should happen.. actually it happened when i brought the inductor near... so.. how to make my antenna effective? or is it effective and i am just not realising it??

    Sorry for asking multiple questions....
     
    Last edited: May 19, 2011
  12. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    nice hard work, Mr. Bolby........
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Length matters, especially to low frequencies like your dealing with. The resonant length for AM signals is measured in miles. It is why antenna farms for AM is so tall.

    Just a suggestion, use cheap AM receivers bought off the shelf to check out your transmitters. If the transmitters are not powerful (and likely they won't be) you can do you experiments.

    Crystal radios (or radios with audio amps and crystal front ends) are as simple as it gets, and the web is full of kits, schematics, and how to's. You want something more advanced it isn't simple anymore.

    A basic 25 in 1 electronics kit will give you everything you want.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    The frequency of your transmitter is too low to be received by an ordinary AM radio.
    Your radio circuit has no gain so its range from the transmitter might be only a couple of mm.
     
  15. CDRIVE

    Senior Member

    Jul 1, 2008
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    The circulating current within a resonant tank is very high but it's nearly 90° out of phase with the voltage across the tank, so it doesn't compute to real power. The only real power is disipated by the resistance of the inductor and the ESR of the cap, which determines the 'Q' of the tank.

    Just one of the fascinating attributes of RF circuitry that makes it a separate field of study. I like to think of it as the 'Magic Factor'! ;)

    Edit: Originally posted 180° but edited it to 90°. Should not have edited it.
    Edit-2: Changed it again to 90°. What should have been stated is that when a circuit is resonant Xl and Xc are equal but since they are opposite reactances they're 180° out of phase with each other.
     
    Last edited: Jun 6, 2011
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yea, it's even below his 455KHz IF.
     
  17. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Thanks Bill, CDrive and Guru...

    Yes, I think I should get some cheap AM receiver for the frequencies I am working with..

    I will be asking simple questions like these so plz don't get annoyed by me... (All related to my transmitter or receiver ) :cool:
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    We don't generally get annoyed with questions. Keep em coming.
     
  19. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Ok...

    I simulated this RF oscillator in Multisim (In Attachment) which Audioguru said needed a KICK to start oscillating.. I hav encountered a new problem.. There is oscillation when I insert an AC source in the circuit which is in no way connected to the actual oscillator circuit..it is just kept somewhere near.. But there is no oscillation when the AC source is removed!!! :confused:

    Now.... Is this AC source acting like a noise source?? Strange Multisim is... Plz explain what happened!!!

    And also an explanation on how this oscillator works and how feedback is achieved would help...
     
  20. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
    2,831
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    Last edited: May 23, 2011
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