A question regarding voltage controlled oscillators

[Daku]

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 ?

 

[Tim Wescott]

On 05/23/2010 09:40 PM, Daku wrote:

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.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

 

[Michael Black]

On Sun, 23 May 2010, Daku wrote:

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 ?

Huh?

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.

Michael