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Old 29th June 2012, 10:34 PM   #2631
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Originally Posted by BenY View Post
You are right,
Time shift description makes the feedback limitation much more easily understandible...
Phase shift issues in feedback are much more complicated to explain as they are also time shift but are also frequency depandent and involve additional compexity..
No! It's just the other way around. I repeat, on top of that, without amplitude roll-of a NFB system is by definition unstable.
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Old 30th June 2012, 01:28 AM   #2632
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Sure it's unstable... If for we eases shake hold the delay or time shift through the devices constant, then the phase-shift increases with frequency..when that phase-shift reaches 180 degrees the feedback becomes additive instead of subtractive and it would thus make the amplifier unstable. It would go into wild oscillation. This is in return prevented by introducing frequency limitations, so the gain is less than 1 at 180 degrees. Off-course by introducing level compensation we add another pole and thus introduce additional phase issues making the understanding a little more complex.. as the level is not only time-shifted from the components in the signal path, but also by the level filter components that with increasing frequency also adds an increasing phase-shift.

Last edited by MiiB; 30th June 2012 at 01:35 AM.
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Old 30th June 2012, 09:03 AM   #2633
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Originally Posted by MiiB View Post
time is phase...or..??
If you look at it in the time domain, one can speculate that phase change can be represented by a delay in time over a single cycle, over a series of cycles phase delay and time has no significance at all.

If we "mix" two identical wave forms (like we do in negative feed-back and the input stimulus) the vector result of the phase change is a constant amplitude directly proportional to the phase angle between the two wave forms.

For instance if the phase angle is zero the constant component will be zero, as the phase angle progresses through 90 the constant would be a maximum and as it shifts though to 180 degrees it decreases again to zero.

If the phase angle between the two signals were varying continually a resultant signal will be developed as described by the Doppler effect.

Pure negative feed back does not contribute to any form of distortion at all.
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Old 30th June 2012, 09:25 AM   #2634
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Quote:
Originally Posted by BenY View Post
You are right,
Time shift description makes the feedback limitation much more easily understandible...
Phase shift issues in feedback are much more complicated to explain as they are also time shift but are also frequency depandent and involve additional compexity..
No no no! Phase shift is NOT time shift!
Are we surrounded by morons here??
Did anybody read this: Simple Symetrical Amplifier
If you don't agree with that, why not, pray tell?
If, otoh you Don't Get It, stop regugitating nonsense!

jan
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Old 30th June 2012, 09:40 AM   #2635
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Originally Posted by janneman View Post
No no no! Phase shift is NOT time shift!
Are we surrounded by morons here??
How would you make signal time shift in 100% analogue circuit? Explanation please.
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Old 30th June 2012, 09:45 AM   #2636
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How would you make signal time shift in 100% analogue circuit? Explanation please.
Simple, compare a direct stimulus with the same stimulus that has passed through 10 km roll of cable there will be propagation delay (time delay).
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Old 30th June 2012, 10:09 AM   #2637
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There's a delay line in almost all analog scopes. Google is your friend.

jan
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Old 30th June 2012, 11:23 AM   #2638
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Quote:
Originally Posted by janneman View Post
No no no! Phase shift is NOT time shift!

jan
In absolute terms it`s not the same ... I agree.

Phase shift @ a constant frequency produces a certine time shift or a delay in time.. if you call it that.

A 30 dergree shift in 1khz can be measured in time, it`s always delayed, it never appears before of the input signal...

A 1mhz 30 degree of shift would represent a much smaller time shift/delay.

So the element of time / shift / delay is always present here even though that shift / delay is frequency dependent.


And that does not mean that I claim you to be wrong.
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Old 30th June 2012, 11:49 AM   #2639
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Originally Posted by janneman View Post
There's a delay line in almost all analog scopes. Google is your friend.

jan
... and this analog delay circuit is what, 10 km long transfer link or simple LC circuit?

Fixed frequency signal phase shift with attached circuit is time delayed or what?
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Old 30th June 2012, 12:09 PM   #2640
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A very high order lowpass can look a lot like a time delay. I have often wondered about this. Cables AFAIK at their frequency limits have a very steep rolloff (or is this just an artefact of simulated cable models?). So it does not surprise me to draw a connection between high order behavior and time delay. As far as the feedback loop is concerned, they have similar effects. It is as simple as: at any frequency where a positive input signal causes a greater positive feedback signal, there will be oscillation.

The difference is that, theoretically, phase shift can be restored by compensation, but time delay cannot. However, with very high-order phase shifts, the distinction may become moot, because in the real world only so much compensation is possible.

Is this reasonable?

Even if there is phase shift, there is no time delay. What you see may look like it, but no. Even the phase shifted signal responds immediately to input, just the high frequencies are attenuated so perceptually it is different. If you are in the habit of seeing all signals as a collection of sine waves, you may not understand this because the sine wave is a very theoretical concept. Think about it. We interpret phase based on where the crest is. Is this a sane way to look at it at all times though? After all a lowpass does not move the crest by shifting all spectral components back in time - it simply responds to the integral of the sine wave (or whatever the opposite of derivative is). So I think sticking to the high school description of a sine wave will cause confusion because it compromises accuracy in return for practicality.
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