interference from transmitter

Discussion in 'Wireless & RF Design' started by mik3ca, Apr 6, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    [​IMG]

    I assembled the above FM transmitter on a breadboard and modified some part values.

    my electret microphone is the audio out on my computer coupled through a 4.7uF capacitor. My C1 is omitted. My R1 is 5.6K. I tied a 220pF capacitor across R1. My R2 is 68 ohms. My C3 is 10pF my C2 is 3 - 20pF (sprague GKU18000 I think. and it came from Sayal Electronics) tuned at around 4 - 10pF. My L1 is fixed at 0.1uH. My C4 is 220uF and the supply is 6V.

    What I want to do is make my transmission happen at around 109Mhz for a couple of street blocks for a short period of time, as a test.

    I am almost successful. I tried tuning the trimmer the smallest amount possible and every time the screwdriver is on it or the trimmer is in the wrong position, my mom complains that her TV has interference. This happens on channel 59 and channel 60 in Ontario, but only to her TV. Her TV is about 3 meters away from my transmitter.

    I want to avoid transmitting to the TV.
    For now, I will try a smaller cap range.

    Can anyone tell me how to reduce the bandwidth or amount of radio space I am using?
    I don't want to flood everyone's radio for several blocks. I just want one measley small section for me, and that's it.

    Thanks.
     
  2. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I decided to experiment some more.

    I modified C2 and C3 so that they match closer to my receiver circuit. C3 = 33pF, and C2 = 50 - 70pF.

    I removed the capacitor that coupled the resistor. (that was in parallel with it).

    So I tested my units again. The sound quality was somewhat distorted, but that's OK.
    It seems that the range is halfed. More like 2 blocks distance instead of 4.

    I tried to couple the resistor with the capacitor again, but in order for the transmission to occur, I had to add 1 or 2 10pF capacitors to the tank circuit. That part made no logical sense to me.

    I tried again, and still, 2 blocks max distance.

    It seems that the capacitor across the resistor helps a bit.
    It also seems that hitting the frequency dead-on eliminates TV interference.

    But the problem is that the range is reduced.

    What value(s) or part(s) do I need to change in order to increase frequency without increasing power supply voltage?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You assembled a red X?

    Because that's all I see.

    You have to link to the image, not the entire page.
     
  4. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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  5. thingmaker3

    Retired Moderator

    May 16, 2005
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    Try a Google search on "oscillator Q" for an explanation of why your power drops off when you veer from the design frequency.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I'm not sure why you built an FM transmitter in the first place, since your superregen receiver is AM, not FM.

    The transmitter design is very primitive, and it's going to have a lot of sideband/ harmonic emissions.

    This isn't something you're going to be able to fix by merely changing a cap or two.
     
  7. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I was wondering, is it because the Q of my receiver tank doesn't match the Q of my transmitter tank?

    My transmitter tank has a 0.1uH inductor and my receiver tank has a 0.47uH inductor.

    I'm going to see if matching Q's will help.
     
  8. Xray

    Well-Known Member

    Nov 21, 2004
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    1

    Actually, superregenerative receivers CAN demodulate FM. I recall many years ago when I first started building electronic gadgets that were published in the hobbyist magazines like Electronics Illustrated, and Radio Electronics, I built a very simple, but very sensative one-tube superregen receiver. It was tuned to the FM broadcast band and, although the audio quality was not that great, it did in fact demodulate the FM carrier. Superregenerative detectors are very sensative, but not at all selective. And the worst attribute about their design is the fact that they radiate RF interference from the very same antenna that they use for receiving signals! It's an interesting and cheap design, but not very practical.
     
  9. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    Don't all radios do that?
    A radio contains at least one oscillator, and doesn't that oscillator, when tuned to the right frequency produce a transmission?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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  11. Xray

    Well-Known Member

    Nov 21, 2004
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    Sure, nearly all receivers that utilize an oscillator of some sort will radiate some RFI, and some more than others. But in the ubiquitous Superheterodyne receiver (which is the design for nearly all modern receivers) the oscillator is isolated from the antenna by either an RF amplifier stage, or by a mixer stage. In a superregerative detector, the antenna is coupled directly to the detector stage which is going in and out of oscillation at a high audio frequency. That type of detector is notorious for radiating RF noise across a wide spectrum.
     
  12. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    my research tells me to increase the value of my transmitter inductor. I just have to watch the value because If I go too high, then I will have to fine-tune the trimmer to the point where I might not be able to fine-tune it.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    The author of the circuit mentioned a 1/4" threaded bolt/screw, but not what the threads were.
    1/4" NC (National Coarse) is 20 threads/in., which would result in a coil 0.4" long, and about 0.2 uH.
    1/4" NF (National Fine) is 28 threads/in., which would result in a coil 3.5" long, and about 0.24uH.
    If you squeezed the coils so they were practically touching, you'd wind up with around 0.35uH.

    But you can calculate this yourself - look up Wheeler's Formula. It's for single-layer air coils. It's not exact, but close.

    You can get an idea of what increasing capacitance will do by placing a finger near the cap in question. Your finger will add to the capacitance.

    When you were using a screwdriver to change the tuning cap, if it was made of metal it would have drastically lowered the frequency of the circuit.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    A super-regen radio has an AM detector. It picks up FM if you tune it to one side of the station then it "slope-detects" because its output amplitude changes as the input FM swings up and down the slope of its tuned circuit.

    Simple FM transmitters change their frequency if a metallic screwdriver adds stray capacitance to the tuning capacitor. Use a plastic screwdriver to adjust the frequency.

    A super-regen has its oscillator connected directly to its antenna and it radiates the RF as an interfering signal.
    A super-heterodyne FM radio has its oscillator buried inside its circuit so that very little radiation occurs.

    EDIT: Here are the changes that were made to the FM transmitter:
     
  15. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I forgot to mention that my antenna was connected to the transistors emitter instead of the collector.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    Why?
    Then the signal is much weaker to the antenna.
    Then the capacitance and loading of the antenna messes up the oscillator.

    I simulated the transmitter. Its RF waveform is saturated since the transistor has way too much base bias current.
    The signal at the collector is about 3 times more than at the emitter which is 9 times the power.
    The signal at the emitter has many harmonics that cause interference.

    EDIT:
    In the simulation, the green trace is the clipping collector voltage and the red trace is the distorted emitter voltage.
     
  17. thingmaker3

    Retired Moderator

    May 16, 2005
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    Put it back on the collector. Signal across Q1+R2 will be much larger than signal across R2 alone. To understand why this should be, Google "Ohms Law" and "BJT" in separate searches.
     
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