I was wondering if it is possible to make a transmitter that doesn't have a fixed frequency. It is as simple as that, lol. I would like to transmit audio, but be able to alter the freq. Thanks for your suggestions

 

It's possible and easy to do if you're not bothered about stability.

Just build a normal LC oscillator and use a variable capacitor and switch between different taps on the inductor.

Unfortunately this isn't very stable, even if you add a buffer amplifier, the frequency will drift all over the place. Building a stable wide range variable frequency oscillator isn't easy, the best method is to use a crystal oscillator reference and a phase locked loop multiplier.

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You might take a look thru amateur radio publications. Yes, commercial equipment is available but I am guessing you're looking for something you can home-brew. Keep in mind that whatever you transmit is subject to the rules of your country and region.

Typically amateur radio is confined to segments of the spectrum and quite often the bands are integral multiples of a fundamental frequency - so an oscillator might be followed by doublers and triplers.

You'll notice that even what the amateurs do is not all that simple- speaks to Hero's comments about stability, among other problems.

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stevez

 

Yes, of course it is possible to transmit without using a fixed frequency. But if you want a receiver somewhere to receive your signal and deliver the audio to a speaker, it has to know what your frequency is at any one moment.

Many phones do as you ask. They transmit audio without using a fixed frequency by using what is called frequency hopping spread spectrum. In this kind of system, the transmitter's frequency jumps around through a range of hundreds or thousands of channels at a high rate of speed. The receiver that is listening to this transmitter happens to know exactly what frequency the transmitter is going to jump to next and it synchronizes so that they jump together to the next channel.

Another kind of system avoids using a fixed frequency by spreading their modulation so wide that it doesn't matter any more what their "frequency" is. This is called Ultra Wide Band modulation. It is theoretically possible to make an ultrawideband transmitter that uses the entire radio spectrum from 1 MHz to 2000 MHz as one big channel and if you do this, you could say that your not using a fixed transmitter frequency.

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RadioRon

 

Try a varicap in the tuned section and switch frquencies via resistors. The really hard thing is to get a transmitter to stay on a fixed frequency, only the most expensive come close.

 

That seems like the most plausible idea so far... would you mind pointing me in the right direction to start making one? Thanks, Jag

 

Check out "spread spectrum"