Simple fm receiver with strange problems

Discussion in 'Wireless & RF Design' started by Lolrapa, Mar 31, 2019.

  1. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    Hello, I've made this circuit out of a circuit I found in the web.
    rad.jpg
    It is made in a pcb.
    The values insinde the boxes are the real components i've used since I dont have that component value.
    The idea is to make a channel to speak like a walkie talkie, but since I have no experience with RF I've decided to make a simple fm radio to start.
    The antenna is 4 naked wires of a common copper cable twisted, it's 75cm long (2' 5").
    It kind of works, I can hear the radio stations really soft under a lot of noise. Besides the radio station frequency changes really easily (when I move or when i touch the antenna). Even hovering my hand over the circuit changes the station.

    VC1 Is a trimmer capacitor from 5.2 to 30pf
    L1 is made of 8 turns of 22swg wire with 4.7mm inner radius air core.

    Why is my circuit so susceptible to interference?
    Thank you!

    Edit: I've been tested and I notice I can use the antenna as a dial. If I press the antenna and move my finger along it I can change between stations and if I find the right spot I can hear the station really clear.
     
    Last edited: Mar 31, 2019
  2. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    Aditional Info:
    Sometimes I can hear more than one station at the same time
     
  3. sghioto

    Well-Known Member

    Dec 31, 2017
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    I think you have made a wideband AM radio receiver.
    SG
     
  4. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    Maybe I'm getting some AM interference (? But I was able to hear and find the FM stations in another radio
     
  5. sghioto

    Well-Known Member

    Dec 31, 2017
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    In another radio with the same design?
    SG
     
  6. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    In a commercial radio
     
  7. sghioto

    Well-Known Member

    Dec 31, 2017
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    I'm confused. Do you mean that some of the stations you picked up on your circuit were verified on the commercial radio?
    SG
     
  8. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    Exactly, for some sations i even heard when they said their frequency 101.5 for ex.
     
  9. sghioto

    Well-Known Member

    Dec 31, 2017
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    OK then it's working.
    Main reason for the interference is the low gain and no filtering or selectivity at the antenna input.
    You must realize that by touching the antenna the circuit is reacting to your body capacitance, changing the tuning and using your body as an antenna.
    SG
     
  10. Ylli

    Active Member

    Nov 13, 2015
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    162
    Definitely non-conventional, but I think I have seen that circuit before long ago. Do you have a link to the original? Do they really specify 2N3904's? Those wouldn't be my choice at 100 MHz.
     
  11. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    You are right, in the original circuit the designer used BF494 and BF495, I couldn't get those, so I checked for similar gain and band width and came up with 2N3904. Is that a mistake?

    Here is the link to the original circuit
    https://electronicsforu.com/electronics-projects/simple-fm-receiver
     
  12. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    I've been doing some research and I found a similar reply to a similar problem. It said the antenna was too bad and the signal from it to low thats why when I touch it I interfere with the signal.
    Should I make a dipole antenna with coaxil? The idea behind all this was to make a device to comunnicate using fm, handies doesn't use dipole I think.
     
  13. sghioto

    Well-Known Member

    Dec 31, 2017
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    I would try it but remember It's a "Simple FM Receiver".
    SG
     
  14. Ylli

    Active Member

    Nov 13, 2015
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    I threw it into a simulator (LTSpice) and the front end worked just fine with the 2N3904.
     
  15. Lolrapa

    Thread Starter New Member

    Sep 19, 2018
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    Ok, I'll try to improve the antenna somehow.

    Another question, the low pass filter conformed by R1 and C4 (wich I think is filtering high frequency noise), with the capacitor I had (10nF) has a cutoff frequency of 1600~ hz, is that too low? I mean it is filtering audible spectrum and thus reducing the output volume?
     
  16. sghioto

    Well-Known Member

    Dec 31, 2017
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    Yes too low. I would try and stay with the values in the schematic.
    SG
     
  17. MisterBill2

    Distinguished Member

    Jan 23, 2018
    2,970
    582
    It appears to be a minimal implementation of a regenerative detector, which is a circuit either just oscillating or on the verge of oscillating , that is varied a bit by the incoming signal from the antenna. They used to be quite common around 1950 and that era. It works BUT it is also radiating a signal that will interfere with the reception of the more modern and more sensitive receivers.
    I am amazed at all of the guesses already posted.
     
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