AM Radio carrier modulation

Discussion in 'The Projects Forum' started by wayneh, Jul 6, 2011.

  1. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    I'm thinking of making the simple AM transmitter circuit described here. The idea is to use a crystal oscillator to produce the carrier (as a square wave) and then to modulate that by putting audio AC onto the power pins of the oscillator. An audio transformer is used to provide the isolation and impedance matching between the sound source and the oscillator. A better explanation of the same circuit is here.

    Anyway, I'd like to replace the audio transformer by simply using a coupling capacitor as shown. I think this should work fine, maybe even better than the transformer. I'm not convinced the power at the clock output will vary as much using the transformer as I might be able to get by limiting the current to the clock into a range where the AC signal affects the output. I'm guessing that's at supply voltages below its 5v rating.

    But I'd like some opinions. Do you see any problems with this strategy? Any advice? I want to be sure to not overload the headphone output of my iPod, which I think is 32Ω impedance. Should be safe, right?
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  2. CDRIVE

    Senior Member

    Jul 1, 2008
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    This is the second square wave AM transmitter thread to appear in a matter of weeks. It was a bad idea then and it's still a bad idea now. What's the deal with square waves? They're harmonic generators! Marconi's spark transmitter was received all over the frequency spectrum because of the same reason. A clean sine wave was considered a great improvement. So why are we regressing?

    All that said, I think you'll deliver more modulation with the transformer.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I don't think your iPod is in trouble, but if you have a local "Dollar Store" see if they have a cheap radio or something else that you wouldn't mind killing in the process.

    The transformer version I believe uses a Radio Shack 1K to 8Ω transformer which will step the voltage up 1000/8 times so has an advantage.

    Oh, and for all you nay sayers here, I actually tried the 555 version for a giggle, it worked, in fact worked from one end of the AM spectrum to the other as I found no dial frequency it wasn't at, and transmitted for a total of about 5 feet. Past that all was quiet.
     
  4. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    I'm not so sure, because there is some confusion regarding which way the transformer is oriented. But if the 8Ω side is connected to the audio source as I think it should be, I have the additional problem of a mismatch to the iPod output.
    Five feet would be just about perfect. I'm using it to find a break in the perimeter wire of a dog fence.
     
  5. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Laziness. If you've got a better, essentially one component RF transmitter circuit, please share. The pre-packaged oscillator just solves a number of problems, at the expense of the shotgun frequency spread that I don't care much about because of the short range I'm shooting for.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Wayne, if I could give you something as simple as that I would. Personally, I would use an admittedly more complex modulated Colpitts Osc but I'm an RF purist. :D

    Chris
     
  7. wayneh

    Thread Starter Expert

    Sep 9, 2010
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    Well, my oscillator parts arrived :D and I'm having good luck with the circuit I posted using capacitive coupling to replace the audio transformer.

    The resistor in series with the oscillator needs to be 2.2K at a supply voltage of 5v and 10K at 12v, to get the voltage down to around 3 across the oscillator. So the oscillator is only drawing about 1 mA and 3mW. The level of modulation increased as the voltage across the oscillator dropped. Results were poor if voltage dropped below ~2V however. It was hard to see a range difference between voltages in the range of 2.5 to 5v at the oscillator. The modulation increase with dropping voltage was more obvious than the loss of range that "should" be observed.

    For the coupling capacitor, more is better and I can tell a difference between 0.47µF and 3.3µf (more volume and bass with the latter), but larger value electrolytics don't make much additional difference. A 0.27µF is working fine for me, as I don't care about low end. The fidelity with the larger cap is quite good. At 0.1µF or less, volume fades away steeply.

    With a ~2ft wire antenna and a breadboard circuit, I'm getting 5-10 feet of range. My cheap AM handheld receiver has a digital tuner, and I'm surprised that I get NOTHING except at the nominal 1000kHz signal. I expected a broad spread but I'm not seeing that at all, even inches from the antenna.

    So I'm almost done with a two-channel build. I intend to use two tones, one on each channel, both transmitted at the same frequency. A break in my perimeter wire antenna should show up as a change from one tone to the other across the break. Fingers crossed.
     
    Last edited: Jul 9, 2011
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