I don't see how this network (L1=150n, C=62p) will oscillate when put in a feedback loop with an ideal amplifier. Please explain.Hi again,
This discussion is interesting.
Roff:
Ok, but i dont think we can be that critical on what will and will not oscillate
in the feedback path of an ideal 360 degree amplifier. This is especially true
with the claim that a resonant network which does not pass through zero will not
oscillate, because you can take the network we have been working with and change
L1 to 150nH and although its phase does not go through zero behold the circuit
still oscillates, albeit at a different frequency of course.
I don't see how this network (L1=150n, C=62p) will oscillate when put in a feedback loop with an ideal amplifier. Please explain.
No disrespect intended, but I think you're blowing smoke.Hello again Roff,
I havent gone through the equations for this yet, but i assume that
it is because sufficient conditions for oscillation exist which dont have
to include an exact phase shift of 0 degrees in the network because
a circuit can oscillate at other points when the loop gain is sufficient.
Yes a circuit might oscillate fine with a perfect 360 degree phase
shift, but that's not the only time it can oscillate. I think the simplest
way of looking at this is that sine waves can add (reinforce) even
when there is a phase shift and they dont have to be perfectly in
phase to add up to a higher value, depending on the gain too.
We can push a swing with a small force F when it swings far back
toward us, right at the crest, or we can wait until it starts back
in its forward path and push a little harder. Either way, it keeps it
moving. I think we can even push when the chain goes through its
perfectly vertical position, although we would need a lot more force
(gain) then to keep it going.
I tried this with the original circuit values by using a CCVS (H) as the amplifier (with and without a diode limiter in the loop), but the DC feedback (see edit at the end of this post) always made the output run off to teravolts. A cap in series added a third frequency where the phase passed through zero, so it wanted to oscillate at that frequency. As a test, I removed L2, making the network a simple damped series LC circuit. It still has a DC path, but I was able to get oscillations at the zero phase shift frequency:I'll see if i can get the equations into the proper form and analyze for
oscillation, but in the mean time, stick this network with a zero degree
phase shift amplifier and gain of 1 into a circuit simulator and see what
frequency the output of the amp puts out.
I had tried ringing the circuit's bell with a fast step also, but I could not get enough consistency in time domain period measurements or FFTs to be certain that the ring frequency was different from both the others.BTW, i hate to add more confusion to the confusion, but i guess it's
never too late <little chuckle>, as we have yet to discuss a THIRD
type of resonance which hasnt been mentioned yet.
Hint: i can get the network to oscillate at a frequency other than
the Im(Z)=0 frequency or the Max|Z| frequency by driving it with
a short pulse. This oscillation is damped but nonetheless still exists,
and does not even require a feedback amplifier.
It's sometimes called the "free oscillation" resonant frequency.
This now leaves us with three candidate resonant frequencies for
this circuit:
Im(Z)=0
|Z|=max (or min)
FreeOsc Z
Just to note that with this particular circuit, i get these three actual
frequencies approximately:
Im(Z)=0: 124MHz
|Z|max: 128MHz
FreeOsc Z: 127MHz
EDIT: The "oscillator" went to teravolts on the original circuit because the phase shift and impedance of the network are both zero at DC.Of these three, |Z|max seems to be the most important in electrical
resonant circuit calculations other than a pure LC circuit, but yes we
dont want to totally dismiss the other two.
BTW, you might be familiar with the w0 frequency used in many simple
circuit calculations. This w0 is actually the Im(Z)=0 frequency and
can be used in other ways too and is used in much literature,
but the importance of this variable drops off as the network becomes
more complex. In fact, i think that is what also happened here by
simply adding another inductor to the classic parallel RLC circuit!
Well, it cant be that because i quit smoking years agoNo disrespect intended, but I think you're blowing smoke.
Ok, when we say 'ideal' amp we mean one with a perfect gain constantI tried this with the original circuit values by using a CCVS (H) as the amplifier (with and without a diode limiter in the loop), but the DC feedback (see edit at the end of this post) always made the output run off to teravolts. A cap in series added a third frequency where the phase passed through zero, so it wanted to oscillate at that frequency. As a test, I removed L2, making the network a simple damped series LC circuit. It still has a DC path, but I was able to get oscillations at the zero phase shift frequency:
Fres=90.394MHz (1/(2pi√LC))
Fpeak=90.356MHz (shift from Fres due to 139Ω damping resistor)
FΦ=88.4873MHz (zero phase shift)
Fosc=88.4877MHz (period measurement)
Fosc=88.4722MHz (FFT)
It's not easy to measure, but perhaps you can try some other values where the |Z| max isI had tried ringing the circuit's bell with a fast step also, but I could not get enough consistency in time domain period measurements or FFTs to be certain that the ring frequency was different from both the others.
Ok, mine broke into oscillation (150nH) but as i said i cant yet explainEDIT: The "oscillator" went to teravolts on the original circuit because the phase shift and impedance of the network are both zero at DC.
I don't understand what "this" is in the above quote (do I sound like Bill Clinton?).I believe this is why a parallel resonant crystal has to be 'loaded'
when used in an oscillator.
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