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From: MooseFET on 16 May 2010 16:20
On May 16, 12:44 pm, Harry D <har...(a)tdsystems.org> wrote:
> On May 16, 6:42 am, MooseFET <kensm...(a)rahul.net> wrote:
>
>
>
> > On May 15, 4:19 pm, Joerg <inva...(a)invalid.invalid> wrote:
>
> > > MooseFET wrote:
> > > > On May 15, 7:49 am, Joerg <inva...(a)invalid.invalid> wrote:
> > > >> Harry D wrote:
> > > >>> On May 14, 6:17 pm, John Larkin
> > > >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> > > >>>> On Fri, 14 May 2010 18:01:08 -0700 (PDT), Harry D
> > > >>>> <har...(a)tdsystems.org> wrote:
> > > >>>>>  How do I attach a PDF (Bode plot) to this message so I can ask
> > > >>>>> questions. I am using Google groups and they do not have
> > > >>>>> alt.binares.electronic.design. I know I am asking to be totally
> > > >>>>> harassed and Joerg will tell me how much smarter he is but I would
> > > >>>>> like some help.
> > > >> I am not smarter. Except maybe when it comes to a few barbecue tricks :-)
>
> > > >>>>> Cheers, Harry
> > > >>>> Park it in one of those free photo hosting sites, photobucket,
> > > >>>> imageshack, like that.
> > > >>>> Joerg can provide the sound effects.
> > > >> *KABLAM* .... phssseeeeeooouuuu ... phut
>
> > > >>>> John
> > > >>> Ok JL, let's give this a try. Attached Bode Plot PDF.
> > > >>>  http://www.keepandshare.com/doc/1913125/bode-plot-gp-30k?da=y
> > > >>> (Google groups not like links)
> > > >>>    This is an open loop SPICE plot of a multi stage system. The gain
> > > >>> crosses 0 at 26.3MHz  with the phase at -675 degrees and the phase =
> > > >>> -720d at 45.2MHz with the gain at -3.2dBv. Looking at the gain
> > > >>> crossing zero at a low slope makes the system appear stable but the
> > > >>> phase crossing is at -675d, not near -360d. But how does the system
> > > >>> know how many spins the phase vector has rotated greater than 360d? Is
> > > >>> the system  not just comparing the feedback phase to the reference
> > > >>> phase on an instantaneous basis?  If true, I can then subtract N*360
> > > >>> (N= integer) from -675 and get -315d which gives, 360 - 315 = 45d
> > > >>> phase margin. This collaborates the low slope of the gain crossing
> > > >>> zero and the (Low) 3.2dBv gain margin.
> > > >> If it is a feedback system it would have already become instable and
> > > >> hung up in an oscillatory manner at the first point where the phase
> > > >> turned past 180 degrees and there was any positive gain.
>
> > > > Feedback systems stability is controlled by the phase margin at the
> > > > last point where the gain hits 0dB.  It can have all manner of phase
> > > > shifts at frequencies below that point.
>
> > > > You can have 3 integrators inside the loop so long as there are at
> > > > least a couple of zeros placed to get the phase back up before you hit
> > > > the cross over point.
>
> > > But his phase didn't go back up, it's all downhill.
>
> > Yes so the phase at the cross over will be such that it
> > oscillates.  I was trying to point out that he could make
> > a closed loop that works by fixing the phase only at the
> > frequency that matters.- Hide quoted text -
>
> > - Show quoted text -
>
> MooseFET,
>  I agree with your axiom, "Feedback systems stability is controlled by
> the phase margin at the last point where the gain hits 0dB.  It can
> have all manner of phase shifts at frequencies below that point"
>   Does this mean that we cannot subtract N*360d from the total phase
> shift to obtain an equivalent phase shift at the gain zero crossing?
>  If we have on open loop gain and phase that does not meet the above
> axiom, at what frequency will the system  oscillate when the loop is
> closed?
> Cheers,   Harry

The 12th commandment is thou shall not subtract one full
cycle from the phase for doing so is an abomination and
surely your designs shall be doomed.
From: Okkim Atnarivik on 17 May 2010 06:46
MooseFET <kensmith(a)rahul.net> wrote:
: On May 15, 7:49�am, Joerg <inva...(a)invalid.invalid> wrote:
: >
: > If it is a feedback system it would have already become instable and
: > hung up in an oscillatory manner at the first point where the phase
: > turned past 180 degrees and there was any positive gain.
:
: Feedback systems stability is controlled by the phase margin at the
: last point where the gain hits 0dB. It can have all manner of phase
: shifts at frequencies below that point.

That's risky. If there is large enough fluctuation which hits
the amplifier non-linearity (typically the power switch-on), the amplifier
gain gets suppressed and the whole gain plot effectively shifts downwards.
Then the 0 dB crossing may momentarily move to the region where the
phase margin is inadequate. And once the system starts oscillating
it definitely hits the non-linearity - that's what limits the
oscillation amplitude - and goes on oscillating.

Probably it is possible to construct an additional circuit which
detects the oscillation and kills it by tweaking momentarily the
open loop response in a suitable way. Or a clever version might
intervene smoothly: use non-linear elements in the frequency response
defining elements so that the large-signal response is stable at
all loop gains even when the small-signal response might have
exessive phase shift. On a second thought maybe this is not a good
idea, the system probably finds a suitable amplitude for oscillation
somewhere between the extremes. Maybe there is a non-linear generalization
of the causality (Kramers-Kronig) relation, which prohibits a
smoothly-intervening version.

The there are harmonic generation and intermodulation as possible
sources of instability.

Regards,
Mikko
From: Phil Hobbs on 17 May 2010 09:02
Okkim Atnarivik wrote:
> MooseFET <kensmith(a)rahul.net> wrote:
> : On May 15, 7:49 am, Joerg <inva...(a)invalid.invalid> wrote:
> : >
> : > If it is a feedback system it would have already become instable and
> : > hung up in an oscillatory manner at the first point where the phase
> : > turned past 180 degrees and there was any positive gain.
> :
> : Feedback systems stability is controlled by the phase margin at the
> : last point where the gain hits 0dB. It can have all manner of phase
> : shifts at frequencies below that point.
>
> That's risky. If there is large enough fluctuation which hits
> the amplifier non-linearity (typically the power switch-on), the amplifier
> gain gets suppressed and the whole gain plot effectively shifts downwards.
> Then the 0 dB crossing may momentarily move to the region where the
> phase margin is inadequate. And once the system starts oscillating
> it definitely hits the non-linearity - that's what limits the
> oscillation amplitude - and goes on oscillating.
>
> Probably it is possible to construct an additional circuit which
> detects the oscillation and kills it by tweaking momentarily the
> open loop response in a suitable way. Or a clever version might
> intervene smoothly: use non-linear elements in the frequency response
> defining elements so that the large-signal response is stable at
> all loop gains even when the small-signal response might have
> exessive phase shift. On a second thought maybe this is not a good
> idea, the system probably finds a suitable amplitude for oscillation
> somewhere between the extremes. Maybe there is a non-linear generalization
> of the causality (Kramers-Kronig) relation, which prohibits a
> smoothly-intervening version.
>
> The there are harmonic generation and intermodulation as possible
> sources of instability.
>
> Regards,
> Mikko


What Mikko said. Third-order PLLs are sometimes used in things like
satellite tracking, where it's useful to have zero phase error due to a
frequency ramp. To avoid the nasty nonlinear oscillations, you short
out one integrator while unlocked, and turn it back on some time after
acquiring lock.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Joerg on 17 May 2010 09:38
Phil Hobbs wrote:
> Okkim Atnarivik wrote:
>> MooseFET <kensmith(a)rahul.net> wrote:
>> : On May 15, 7:49 am, Joerg <inva...(a)invalid.invalid> wrote:
>> : >
>> : > If it is a feedback system it would have already become instable and
>> : > hung up in an oscillatory manner at the first point where the phase
>> : > turned past 180 degrees and there was any positive gain.
>> :
>> : Feedback systems stability is controlled by the phase margin at the
>> : last point where the gain hits 0dB. It can have all manner of phase
>> : shifts at frequencies below that point.
>>
>> That's risky. If there is large enough fluctuation which hits
>> the amplifier non-linearity (typically the power switch-on), the
>> amplifier
>> gain gets suppressed and the whole gain plot effectively shifts
>> downwards.
>> Then the 0 dB crossing may momentarily move to the region where the
>> phase margin is inadequate. And once the system starts oscillating
>> it definitely hits the non-linearity - that's what limits the
>> oscillation amplitude - and goes on oscillating.
>> Probably it is possible to construct an additional circuit which
>> detects the oscillation and kills it by tweaking momentarily the
>> open loop response in a suitable way. Or a clever version might
>> intervene smoothly: use non-linear elements in the frequency response
>> defining elements so that the large-signal response is stable at
>> all loop gains even when the small-signal response might have exessive
>> phase shift. On a second thought maybe this is not a good idea, the
>> system probably finds a suitable amplitude for oscillation somewhere
>> between the extremes. Maybe there is a non-linear generalization
>> of the causality (Kramers-Kronig) relation, which prohibits a
>> smoothly-intervening version.
>>
>> The there are harmonic generation and intermodulation as possible
>> sources of instability.
>>
>> Regards,
>> Mikko
>
>
> What Mikko said. Third-order PLLs are sometimes used in things like
> satellite tracking, where it's useful to have zero phase error due to a
> frequency ramp. To avoid the nasty nonlinear oscillations, you short
> out one integrator while unlocked, and turn it back on some time after
> acquiring lock.
>

And woe to those who forget. Once it's in the satellite it's too late
and NASA is hanging up the triple-A service trips up there ...

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: VWWall on 17 May 2010 12:44
Phil Hobbs wrote:
> Okkim Atnarivik wrote:
>> MooseFET <kensmith(a)rahul.net> wrote:
>> : On May 15, 7:49 am, Joerg <inva...(a)invalid.invalid> wrote:
>> : >
>> : > If it is a feedback system it would have already become instable and
>> : > hung up in an oscillatory manner at the first point where the phase
>> : > turned past 180 degrees and there was any positive gain.
>> :
>> : Feedback systems stability is controlled by the phase margin at the
>> : last point where the gain hits 0dB. It can have all manner of phase
>> : shifts at frequencies below that point.
>>
>> That's risky. If there is large enough fluctuation which hits
>> the amplifier non-linearity (typically the power switch-on), the
>> amplifier
>> gain gets suppressed and the whole gain plot effectively shifts
>> downwards.
>> Then the 0 dB crossing may momentarily move to the region where the
>> phase margin is inadequate. And once the system starts oscillating
>> it definitely hits the non-linearity - that's what limits the
>> oscillation amplitude - and goes on oscillating.
>> Probably it is possible to construct an additional circuit which
>> detects the oscillation and kills it by tweaking momentarily the
>> open loop response in a suitable way. Or a clever version might
>> intervene smoothly: use non-linear elements in the frequency response
>> defining elements so that the large-signal response is stable at
>> all loop gains even when the small-signal response might have exessive
>> phase shift. On a second thought maybe this is not a good idea, the
>> system probably finds a suitable amplitude for oscillation somewhere
>> between the extremes. Maybe there is a non-linear generalization
>> of the causality (Kramers-Kronig) relation, which prohibits a
>> smoothly-intervening version.
>>
>> The there are harmonic generation and intermodulation as possible
>> sources of instability.
>>
>> Regards,
>> Mikko
>
>
> What Mikko said. Third-order PLLs are sometimes used in things like
> satellite tracking, where it's useful to have zero phase error due to a
> frequency ramp. To avoid the nasty nonlinear oscillations, you short
> out one integrator while unlocked, and turn it back on some time after
> acquiring lock.
>
Way back in the days of carrier telephone systems, there was an
amplifier used by Bell that had to be turned on in a special way. You
removed one tube, (remember them?), turned on the power and waited for
the rest of the tube's heaters to warm up. You then replaced the
removed tube and the amplifier worked as intended.

--
Virg Wall
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