"Aleksandr Timofeev" wrote in message om...
"George Dishman" wrote in message ...
"Aleksandr Timofeev" wrote in message om...

Whether you can describe physical principles of operation of the RC-oscillator
(capacitance-resistance oscillator) from a point of view of a resonance?

Although a capacitor stores energy an RC circuit is not a
resonant system. The voltage and current in an RC circuit
decay exponentially and have no natural oscillation. The
differential equation is first order while resonance
requires second order.

Then you should explain the physical mechanism of the RC-generator,
which one ensures on an output of the RC-generator (RC-oscillator)
only sine-wave oscillations. ;-)

You mean as I mentioned in the next paragraph? ;-)

To make an oscillator using an RC requires a separate
non-linear gain stage (a 'relaxation' oscillator)

This doesn't give a sine wave but just for completeness:

http://www.ee.polyu.edu.hk/staff/een.../ee251lab2.htm

or you
need multiple RC stages.

This is the one you are thinking of and does give a
sine wave:

http://home.earthlink.net/~doncox/wec/Oscillators.html

Note that you need at least three stages. You need to get
180 degrees phase shift from the RC delays to produce an
in-phase signal into the inverting amplifier. In theory a
single RC stage will produce 90 degrees shift but only
when the gain is zero. The oscillator as a whole can have
a similar frequency characteristic to a resonant system
but the initial energy in the capacitors is lost as heat
in the resistors and base of the transistors and has to
be continually replaced from the power supply. The
individual RC sections only provide phase shift, not
useable power storage so it is not resonant in the
physical sense. (Some electronics texts may describe it
as resonant because of the response, not the mechanism.)

As Craig pointed out a simple LCR circuit can be resonant
provided the resistance is low enough (series mode) or
high enough (parallel mode) to avoid excessive damping.
This is a good description of that (9 page PDF):

http://faculty.washington.edu/maniso.../resonance.pdf

The key difference is that in an LC circuit, energy can
be stored in the magnetic field in the inductor and in the
electric field in the capacitor. The collapse of the
magnetic field induces a voltages that charges the
capacitor, the voltage across the capacitor then builds
the current in the inductor in the opposite sense to the
original and so on. Without resistive losses this could
go on indefinitely.

With just an RC, the energy in the capacitor is turned
into heat in the resistor and that is the end.

George