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From: Joerg on 15 May 2010 10:49
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.

In case this is an active EMI filter in a power bus watch out for hard
load change reactions and input turn-on spikes. Can be simulated.


> Just a chicken farmer trying to be a electronic consultant,
> Harry


Nothing better than free range products, enjoyed it from a coworker with
a farm while at an ultrasound company :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: MooseFET on 15 May 2010 19:14
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.

From: Joerg on 15 May 2010 19:19
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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: MooseFET on 16 May 2010 09:42
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.

From: Harry D on 16 May 2010 15:44
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
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