Maybe this is a stupid question, but could some electronics guru clarify this a bit ? Current voltage controlled oscillator designs, based on fixed frequency oscillators - e.g., Colpitts have a resonator and the feedback amplifier. The resonator typically consists of a LC tank circuit, with varactors and fixed value capacitors. This is followed by the feedback amplifier stage. The resonator is continuously fed by the tuning voltage, so whatever losses that occur in the tank are compensated by the tuning voltage. In this case, is the feedback amplifier essential ?
Where did you get the idea that tank losses are compensated for by tuning voltage? It's not the case.
Yes, the feedback amplifier is essential. The varactors are only used to change the resonant frequency of the tank; the only compensation for loss in the tank circuit that they can do is to shove the resonant frequency back if losses move it.
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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
This seems to be a followup to your previous question about ring oscillators as VCOs, but I'm not sure of what the connection is.
There's nothing magical about a VCO. Take any oscillator, and figure out some means of changing the frequency with voltage, and you've got a VCO. Sometimes it's done with a circuit that you'd never see elsewhere, but most of the time it's a basic and familiar oscillator circuit, with some element added to change the frequency with voltage.
Note that while it's not practical for most of the times you'd need a VCO, you could connect a slow motor to the variable capacitor of an existing oscillator and that would be a VCO, how much voltage applied to the motor would determine the amount the oscillator's frequency is changed, and the polarity of the voltage would determine whether the oscillator goes up or down in frequency.
Given this, the feedback stage is vital, since it's the feedback stage that makes the circuit oscillate. No feedback, and you've just got an amplifier. No tuned circuit, and where it oscillates is pretty variable.
Instead of that motor connected to the variable capacitor, you'd replace the variable capacitor with a voltage variable capacitor, a varicap. The varicap changes capacitance as the voltage to it is varied, so when it goes into the oscillator circuit, you can change the frequency with voltage.
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