Electronic – An alternative way to make a VCO


From what I am able to understand, a voltage controlled oscillator is "controlled by a voltage" when this voltage is applied to vary the value of the tank circuit: e.g. this could happen by applying this voltage on a varicap diode, which change its capacitance according to the voltage.

However I also noticed another thing: I have assembled a very simple oscillator based on a transistor and a handful of components: it is very basic and weak (I found the schematic in a book for beginners,) and I have noticed that if I vary the value of the power supply, the oscillator will vary/drift the working frequency (I have verified this, since a friend of mine has good equipment at his lab.)

So, can this oscillator be called VCO, or is just a "feature" due the weakness/instability of the circuit? Also, why will the frequency vary according to the value of the power supply? I was also thinking, if I apply a sawtooth waveform as power supply, will the oscillator automatically drift on its frequency range? I ask this also because I'm studying this kind of modulation.

Please consider the fact that I am a beginner, so I will appreciate an exhaustive answer also with practical examples/schematics.

Unfortunately, since the schematic of my oscillator is only in the book, and I don't have a scanner in my printer, I can't post the exact schematic.

However my circuit looks very similar to this one, except for the fact that I don't have a mic, and in parallel with C1 there is another resistor.

Oscillator schematic

Best Answer

The problem with varying power supply is that you'll vary amplitude as well, which will create a big deal of intermodulation and parasitic frequency modulation; an undesirable heterodyne effect. So, therefore a good VCO should have the amplitude independent from the controlling voltage.