Bode Plot help

Park it in one of those free photo hosting sites, photobucket, imageshack, like that.

Joerg can provide the sound effects.

John

Maybe try a Google Docs thing.

Ok JL, let's give this a try. Attached Bode Plot PDF.

-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=3D integer) from -675 and get -315d which gives, 360 - 315 =3D 45d phase margin. This collaborates the low slope of the gain crossing zero and the (Low) 3.2dBv gain margin. Just a chicken farmer trying to be a electronic consultant, Harry

It says EMI active filter as the title, so you are showing the response of a filter, right? Unless you are going to close a loop around this filter, why do you think it is necessary to do Bode analysis (gain/phase margin)?

I am not smarter. Except maybe when it comes to a few barbecue tricks :-)

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.

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

=3D

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.

:-)

ain

=3D

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.

s :-)

gain

e =3D

Is

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

sk

uld

cks :-)

he gain

ase =3D

he

em

d? Is

nce

N*360

nd

ase

hit

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.

MooseFET wrote: : On May 15, 7:49 am, Joerg 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

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 ...

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.

In my experience there's nothing in general to rule out the possibility of a smoothly-intervening nonlinearity to eliminate hard limit cycles*. At least in the systems that I've worked on the nonlinearities could be applied as simple memoryless limits to integrator range -- essentially fancy anti-windup measures.

You have to understand the root cause of the hard limit cycle, which isn't always easy. But my prejudice is that such a smoothly-intervening nonlinearity is superior than something that deduces that there is a problem and changes the control mode. Switching controller modes is an opportunity to launch oscillations itself, and discriminating oscillations from noise isn't trivial; trying to put them together would be like standing before God and telling him he's lame, and you don't believe that his f***ing lightning bolts really work anyway.

• In nonlinear systems parlance a "limit cycle" is the characteristic of a system's nonlinear behavior that makes it oscillate. A "soft" limit cycle is one that will always happen, i.e. the system state gets attracted to the limit cycle no matter where it starts, or at least always from rest. A "hard" limit cycle lurks "out there", waiting for something to set it off.

For example, a properly designed oscillator circuit has just one soft limit cycle. Most properly designed pendulum clocks have a very well defined hard limit cycle (wind a stopped mechanical clock, or very carefully wind a stopped mechanical watch, and it doesn't start -- you have to pull the clock's pendulum over to the side and let it go, or give the watch a shake, for the ticking to start).

_Poorly_ designed oscillator circuits often have more than one limit cycle, or they have a chaotic attractor in their makeup that leads to "squegging".

Not true in general, although almost certainly yes for this circuit. I often design motion control systems that have three integrators active at DC, with two zeros active below the loop closure to bring the phase down to between -90 and -180 at 0dB gain, then rising phase lag after that. They work just fine if you mind your P's and Q's.

OTOH, with this particular circuit, it goes through over 360 degrees of lag in the interval between the two gain-crossing points. Close a loop around it, and if it doesn't oscillate at 10kHz then I expect it'll oscillate at 500kHz, where the phase hits -540 degrees.

This sort of thing is much more clear if you do a Nyquist plot and look for encirclements of -1.

Yup -- although I don't think it'd be necessary in this case, 'cause I think it'll oscillate no matter what!

These days it's all done in DSP, and the necessary decorations to the PLL could be done without extra circuitry (just extra execution time), possibly without mode changes in the controller.

(I dislike controller mode changes, intensely. It's like putting a sign in your system, taunting the Stability Gods.)

nd

se

e quoted text -

The first plots were the open loop response with an embedded feedback amp to add 22dB at 30MH for closed loop crossing at that point. Here is the modulator (plant) response.

-26k?da=3Dy As you can see I am good at designing gain but piss poor at phase control. Given -266d in the modulator and at least 34d phase margin leaves only 60 degrees left for my 20dB error amp. Thanks, Harry

quoted text -

It's certainly way down on phase margin, if not well beyond a chance of being stable.

The phase offset in the plot doesn't help much, either -- I still see a different phase offset every time I look.

What are you using for an amplifier?

And is there any way you can get your tool to cough up a Nyquist plot?