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I saw a FM transmitter radio circuit, which was an easy one and had a 20 MHz quartz crystal, on internet. It promised to fix the frequency at 100 MHz. I soldered the components together and tried at home. It worked as expected. As the transmission was online, there was silence in radio. I integrated a sound source to circuit and I could listen to it in radio. The efficiency of the transmitter was about 10 meters.

I know that quartz crystals behave in harmonic frequencies too, like for 20 MHz so in 40, 60, 80 MHz ... I decided to change the 20 MHz crystal with an 8 MHz one. I hoped to get an 88 MHz, which is an 11th harmonic frequency of 8 MHz. This time I had no luck. All I got was a strange sound when I was scratching the pins of my microphone.

After all, I was in doubt of choosing the wrong resistor and capacitor for the circuit. In the example below, the resistor is 47K and the capacitor is a 68pF. The quartz crystal is a hc-49s one.

I know the calculation of cutoff frequency.

$$fc = 1 / (2 * pi * R * C)$$

My question is about the relevance of resistor, capacitor and crystal in circuit. How can this circuit work with these components without problem? Is there a shortcut formula to calculate them? Do these components work like a filter?

The circuit I soldered together

lastime
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  • Where did the circuit come from? – Andy aka Mar 16 '23 at 18:26
  • The circuit comes from YouTube. Zafer Yıldız is introducing this one. – lastime Mar 16 '23 at 18:28
  • Does the video also say how or why the circuit works? Or is this one of those free energy joke channels that present weird circuits and fake that they work? – Justme Mar 16 '23 at 18:31
  • No, he said just that the components of this circuit are part of engineering and i tried it at home, as i predicted it works like a charm. – lastime Mar 16 '23 at 18:33
  • It isn't an FM transmitter so what circuit did you use? 100 MHz is an FM band so no, it isn't "part of engineering". – Andy aka Mar 16 '23 at 18:37
  • Yes, It is a 100 MHz FM band. The engineering part is about the harmonic frequencies of the crystal, i think so. – lastime Mar 16 '23 at 18:37
  • I said **it is** an FM band but, your "oscillator" isn't a well-engineered frequency modulator. It's a type of colpitts oscillator but, the antenna will shift the frequency depending on how long it is and what surrounds it. – Andy aka Mar 16 '23 at 18:39
  • The problem is that, it works but how ? – lastime Mar 16 '23 at 18:40
  • Ask Zafer Yıldız is my advice. – Andy aka Mar 16 '23 at 18:41
  • He told something about band pass filters but did not make any comments. – lastime Mar 16 '23 at 18:42
  • I researched a time long, but could not make any useful and practical information, so i landed here for more clarity. – lastime Mar 16 '23 at 18:43
  • In one video, the person seems to build a transmitter by powering a 10 MHz crystal oscillator with 12V battery through a 100 ohm resistor. And microphone outputs modulates by connecting to oscillator square wave output. That's not going to work but end up with fried oscillator, therefore the videos are fake. Which is why it's not worth believing any random videos you happen to see, just like movies from TV don't happen for real and are just acted performances. – Justme Mar 16 '23 at 18:55
  • I know what i did. It worked like expected. I do not know what you have been watching last time and i do not know every video of this person, but this one is working. – lastime Mar 16 '23 at 18:57
  • I would advice to take a look into the circuit. I am not in offensive. – lastime Mar 16 '23 at 18:59

1 Answers1

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Having thought a bit about this circuit, I don't think it is a proper overtone oscillator but, because the OP says "it works" there has to be a reason. Now, it's no use trying to simulate this (unless someone has a model of a crystal that can produce overtone operation) so it's partly guesswork...

...Anyway, the upshot of my contemplation is that you may think that it produces a clean overtone frequency at 100 MHz but, it's more than likely, that the distorted waveform it generates will feed harmonics directly on to the antenna.

This is not the same as an overtone oscillator BTW.

So, there will likely be an infinite diminishing series of "odd" harmonics based at the 20 MHz fundamental that could be picked up at an FM broadcast frequency of 100 MHz (the fifth harmonic). And, there's a pretty good chance that if you could find a receiver that could tune to 60 MHz (the third harmonic), you would also pick-up a signal. You might also receive "even" harmonics (typically 80 MHz) at a lower level.

As an experiment, try putting an oscilloscope on the antenna or the collector (via a 1 kohm resistor) and, look at the waveform. I'll bet it's a distorted 20 MHz square-ish wave. This means that the fifth harmonic (100 MHz) will be quite significant and, will "blank" that region on an FM broadcast receiver.

I integrated a sound source to circuit and I could listen to it in radio.

This has happened so, why is the big question. Well, the oscillator is basically a badly designed Colpitts oscillator that uses the antenna's capacitance as part of the phase shift network. But the OP also said this: -

I integrated a sound source to circuit and I could listen to it in radio

It's quite feasible that the BJT's miller capacitance is pulled up and down in value by the audio modulation signal and, this will give rise to a small amount of frequency modulation i.e. the crystal can be pulled and pushed off-centre a little. This is well-known to anyone looking into Colpitts oscillators.

I hoped to get an 88 MHz, which is an 11th harmonic frequency of 8 MHz. This time I had no luck.

The 20 MHz crystal oscillated but, the 8 MHz crystal probably didn't oscillate until you "was scratching the pins of my microphone" and, that may have introduced just enough capacitance to cause it to sporadically oscillate and get something at 88 MHz (11th harmonic).

And, as I said above, if you get an oscilloscope you can check this out. You might also add 20 pF from the antenna connection to ground to see if it kick-starts it into oscillation at 8 MHz.

Andy aka
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