I made transmitter from this question with some minor modifications (100 K pot for resistor between 8 and 7 and 22K for resistor between 7 and 6. 22 pF for the capacitor). Here's the picture:


When transmitting this song, it is almost normally transmitted. On the other hand, when transmitting this song, almost nothing is audible. For example from 2:15 to 2:50 noting can be heard.

My home-brewed sound card oscilloscope (96 kHz, 16 bit) sees first song like this:

scope 1

and the second like this:

scope 2

If I'm right, on the second image only artifacts from the carrier wave can be seen (I'm running radio at 612 kHz. I guess that's OK, since it has range of about 5 cm).

I've heard such effects on radios with bad antennas before. What could be the technical explanation, or at least where could I start looking for it?

Just to be clear, the my question isn't about improving the transmitter, its about the effect I'm describing.

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    \$\begingroup\$ What is the dc bias path for Q1's emitter? \$\endgroup\$ – markrages Mar 30 '11 at 0:04
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    \$\begingroup\$ I just checked and there is audio on both channels on the second song all the time, so the problem isn't with bad stereo mixing. \$\endgroup\$ – AndrejaKo Mar 30 '11 at 14:17
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    \$\begingroup\$ It looks as though that circuit is very roughly on-off-keying the transmitter based upon the input signal, and would spew all sorts of harmonic garbage. The circuit is going to switch on when the signal is above a certain threshold and switch off below that threshold. I would expect that moving the +5 end of R2, to the wiper of a pot that goes between +5 and ground, would allow you to tweak the threshold to yield much better success, but I find it hard to believe that transmitter would be legal at any power level. \$\endgroup\$ – supercat Mar 30 '11 at 14:50
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    \$\begingroup\$ @AndrejaKo: I've seen designs for transmitters using one transistor, a transformer, and some resistors and caps. I think one could also build one with two transistors, some resistors and caps, and no inductors. Even if one wants to use a 555, I would think that one should let the 555 run undisturbed and have the audio input modulate the output transistor. \$\endgroup\$ – supercat Mar 30 '11 at 15:34
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    \$\begingroup\$ @AndrejaKo: A transistor needs a DC bias to amplify. In other words, there needs to be a path between the emitter of Q1 to ground. Such as a resistor. With the emitter floating linke that I suspect that Q1 is doing nothing of value in this circuit. \$\endgroup\$ – markrages Mar 30 '11 at 17:22

Three observations

(1) With any kind of transmitter, to get consistent output, you need consistent input levels, so that's the first thing to check. If the level in the second file are significantly different, it would be worthwhile to adjust it. This transmitter doesn't appear to have any way to accommodate variations in input level, so the easiest thing to do is try adjusting the output level from the PC playing the file. Commercial broadcasters use special hardware to automatically adjust levels into the transmitter, a technique called compression in the audio world. If you play a signal with a lot of dynamic range into this transmitter, you may find that the level setting that makes one part of the song sound better will make louder/quieter sections sound worse.

(2) That's definitely not amplitude modulation. It's essentially pulse width modulation. If you LPF pulse width modulation, you can recover an approximation of the input signal. An AM receiver will respond to the fundamental component, so apparently the fundamental's envelope closely approximates the input, but I'd be surprised if there wasn't significant distortion. You wouldn't call it high fidelity, but you could probably recognize voice through it. Using the 555 to do pulse width modulation is going to be especially sensitive to input levels.

(3) Transistor Q1 is probably not doing anything helpful. As shown, Q1 will be either completely off or .. completely off. The only signal getting to the antenna will be from capacitive coupling between the base and emitter. Two alternatives to try - you could just connect your antenna directly to pin 3, i.e., omit Q1 altogether, and get more signal onto the antenna that way, OR - leave Q1 where it is, but connect a 75 ohm resistor from its emitter to ground. This would make Q1 into an emitter follower (common collector) amplifier, and might give you some power gain, but pin 3 of the 555 is already a push-pull output, so just wiring pin 3 to the antenna would probably work just as well.

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    \$\begingroup\$ come to think of it, it might be that the fundamental isn't even in the std AM broadcast band, your receiver could be grabbing on of the harmonics. \$\endgroup\$ – JustJeff May 6 '11 at 20:39

It's possible that the frequency that you're trying to produce is out of the range of the 555.

I'm also not entirely sure that this signal as actually pure AM. I may be wrong, though. I've been trying to figure it out for myself. It might be a form of direct modulation.

P.S. Nice use of LabVIEW.

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  • \$\begingroup\$ The 555 I'm using should be able to take up to 2.7 MHz and I can get the frequency I expect on the output. \$\endgroup\$ – AndrejaKo Apr 5 '11 at 8:11

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