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buddypoor

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Re: Harmonic Oscillator with ideal Opamp models
Reply #30 - Dec 05^th, 2011, 1:03am

Hello Loose-electron,

With posting #4 you gave a recommendation to read Lundbergs contribution to oscillation conditions (http://web.mit.edu/klund/www/weblatex/node4.html).

With posting#6 I have written some critical comments to Lundbergs statements.

Since it was your recommendation I would be very interesting for me to hear about your opinion. In particular, do you know something about an oscillation criterion that is sufficient?
Thank you.

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Lex

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Re: Harmonic Oscillator with ideal Opamp models
Reply #31 - Dec 5^th, 2011, 1:30am

buddypoor wrote on Dec 5^th, 2011, 12:17am:

....
And - coming back to the original core of discussion - I still think that in case of simulation problems (false or unexpected results) the user will be the only cause and not the simulation program.
I think the circuit as mentioned in one of my former postings (opamp with positive resistive feedback) can serve as a good example:
The simulation results are correct by 100% - based on the assumptions and conditions that exist during the simulation. Problems can arise only if the user transfer the results to real-world conditions (noise, stable supply, no switch-on,...). � �

I don't agree completely. Surely, the designer needs to have some knowledge on the tool he (or she) is using, but you cannot expect him (or her) to know as much as the software/math guru's who actually made the simulation tools. In that sense, some trust/distrust is always needed. If you don't trust the (or any) simulator at all, your productivity will be fairly low, I expect. But as well, too much trust is not okay: I've seen some junior designers look for hours at a circuit, while the problem was purely simulation (e.g. numerical noise, minimum time step, voltage resolve resolution).

Switch-on/mismatch/current pulse in general is a good way to make sure you don't get stuck in a meta stable point. Switch on is probably the least invasive. For mismatch you could to do a MC analysis w/o altering the schematic.

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buddypoor

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Re: Harmonic Oscillator with ideal Opamp models
Reply #32 - Dec 5^th, 2011, 2:47am

Lex wrote on Dec 5^th, 2011, 1:30am:

I don't agree completely. Surely, the designer needs to have some knowledge on the tool he (or she) is using, but you cannot expect him (or her) to know as much as the software/math guru's who actually made the simulation tools. In that sense, some trust/distrust is always needed. If you don't trust the (or any) simulator at all, your productivity will be fairly low, I expect. But as well, too much trust is not okay: I've seen some junior designers look for hours at a circuit, while the problem was purely simulation (e.g. numerical noise, minimum time step, voltage resolve resolution).

Wow...don't agree completely!
Hi Alexandar, thanks for replying.
Perhaps you have misunderstood something.
I did never claim that the designer should "know as much as the software/math guru's who actually made the simulation tools"
My only concern was (and still is) that - in case of some ideal conditions (simulation environment or models) - the designer should know that a simple direct transfer to real conditions can lead to false results.
And I gave an illustrative exaple for that (ideal opamp with pos. resistive feedback). This has nothing to do with general trust/distrust of simulation results. It means simply that the designer should know what he is doing. So - I really don't know what you are arguing against.
And regarding your last sentence: I think it really supports my standpoint. If somebody specifies a minimum time step that is still to large for the expected signal variation - who has made an error? The simulator or the user?
Regards

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Lex

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Re: Harmonic Oscillator with ideal Opamp models
Reply #33 - Dec 5^th, 2011, 4:05am

buddypoor wrote on Dec 5^th, 2011, 12:17am:

...
And - coming back to the original core of discussion - I still think that in case of simulation problems (false or unexpected results) the user will be the only cause and not the simulation program.
...

I cite "I still think that in case of simulation problems (false or unexpected results) the user will be the only cause and not the simulation program."

This is what i don't agree to. The story is never black and white. Loose-electron already mentioned modelling. I am mentioning the fact that the user of a piece of software cannot (and is not expected) to know all the details of the simulator. It is the task and responsibility of the software engineer to make its use trivial.

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buddypoor

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Re: Harmonic Oscillator with ideal Opamp models
Reply #34 - Dec 5^th, 2011, 7:20am

[quote author=Alexander link=1322576364/30#33 date=1323086745]buddypoor wrote on Dec 5^th, 2011, 12:17am:

I am mentioning the fact that the user of a piece of software cannot (and is not expected) to know all the details of the simulator. It is the task and responsibility of the software engineer to make its use trivial.

Perhaps this discussion is not very fruitful - however, now I strongly disagree.
Do you really think that the use of a high-quality simulation program should and can be "trivial"? Don't you think it is up to the user only to set a correct and realistic simulation environment? �
Think of items like:
Simulation time, max. time step, initial conditions, power switch-on transients, frequency resolution, operational point (offset), stability aspects,.....
Even "modelling" is a task that belongs to the users side (and is not part of the program)
Most important: The user should have in advance an idea how the simulation result should look like. Otherwise, he believes everything - even false results because of errors during schematic entry.
I am working with several different simulators since more than 30 years - and I cannot remember one single case where the simulation program itself was responsible for an unexpected and false result.
(This applies to analog circuits only; I have not much experience with digital circuits). �

Please, can you give me such an example? � �
Regards.

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loose-electron

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Re: Harmonic Oscillator with ideal Opamp models
Reply #35 - Dec 5^th, 2011, 9:19am

buddypoor wrote on Dec 5^th, 2011, 1:03am:

Hello Loose-electron,

With posting #4 you gave a recommendation to read Lundbergs contribution to oscillation conditions (http://web.mit.edu/klund/www/weblatex/node4.html).

With posting#6 I have written some critical comments to Lundbergs statements.

Since it was your recommendation I would be very interesting for me to hear about your opinion. In particular, do you know something about an oscillation criterion that is sufficient?
Thank you.

I don't overthink this.

Sufficinet gain in the loop
suitable phase relationship in the loop

you can get fancy with Blacks Law, feedback system analysis poles, root locus etc etc etc etc but for designing things in the real world, the above two items get it done for me.

The open loop Bode plot - both gain and phase tell me I got an oscillator or not. Academics may find fault with that but for me it works for all the designs I have done.

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loose-electron

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Re: Harmonic Oscillator with ideal Opamp models
Reply #36 - Dec 5^th, 2011, 9:25am

Simulators are tools.

You need to know how to use the tools properly and what are the approximate results you would expect from those tools.

You can either use the tools to build houses, or you can cut your fingers off.

In silicon, I have seen the multi-million dollar slicing off of fingers a few too many times.

Nowadays, the model being simulated is much more often in fault than the simulator itself.

Garbage In = Garbage Out

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boe

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Re: Harmonic Oscillator with ideal Opamp models
Reply #37 - Dec 5^th, 2011, 11:32am

Back again.
buddypoor wrote on Dec 2^nd, 2011, 9:19am:

...
This sounds reasonable - but the problem is: How to know in advance (and for unknown circuits) if the simplification is "excessive" (means: not allowed and not applicable) or not?

This is a very good question. I don't think there is a general answer (applicable to all circuits of any complexity) to this question. But having loose-electron's "capable designers" certainly helps.

buddypoor wrote on Dec 2^nd, 2011, 12:34pm:

...
in response to your last reply I accidently have found a paper from an author (Czech Republic) describing a simple simulation procedure to check if the dc operatimng point of a circuit with feedback will be stable or not.

Buddypoor, could you post the reference, please?

buddypoor wrote on Dec 5^th, 2011, 7:20am:

...
I am working with several different simulators since more than 30 years - and I cannot remember one single case where the simulation program itself was responsible for an unexpected and false result.
(This applies to analog circuits only; I have not much experience with digital circuits). �

Please, can you give me such an example? � �

Although they are rare, simulator bugs do occur; e.g. http://www.designers-guide.org/Forum/YaBB.pl?num=1181226897 (missing @cross events in a mixed-mode simulation).
However, I agree, usually unexpected results are caused by errors on the user's side (models, setup, design, ...).

I also agree with loose-electron's last post (nice wording!).

- B O E

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buddypoor Community Fellow Offline Posts: 529 Bremen, Germany	Re: Harmonic Oscillator with ideal Opamp models Reply #38 - Dec 5^th, 2011, 1:58pm Hi Loose-electron, thank you for the two replies. I think, we are on the same track (or line?) - more or less, in particular your last posting.
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loose-electron

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Re: Harmonic Oscillator with ideal Opamp models
Reply #39 - Dec 5^th, 2011, 3:03pm

buddypoor wrote on Dec 5^th, 2011, 1:58pm:

Hi Loose-electron,
thank you for the two replies.
I think, we are on the same track (or line?) - more or less, in particular your last posting.

Not a problem -

Also - making the use of a simulator trivial is a bit risky IMHO, electronics is not Lego's or point and click internet shopping.

People do simple things in electronic block assembly all the time and later get bitten due to not understanding the implications of what they have done. I see this all the time.

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Lex

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Re: Harmonic Oscillator with ideal Opamp models
Reply #40 - Dec 6^th, 2011, 12:26am

Well let me expand a bit on making the simulator trivial to use. If you make it is easier to use, people will make fewer mistakes. This is a matter of organization, clarity and transparency. In my opinion software guys are not working according to these principles.

If important simulator parameters are into 3 menus and 5 sub-menus deep down some archaic software, some people will have a hard time finding them. Or if you use very cryptic names, it takes a lot of time to dig up some PDF file of 700 pages and search for the phrase to finally find out what it means, and which one they need to alter (everybody must have been there, I guess Wink

). This is very unproductive and hence leads to errors.

The fact that the matter is very complex, doesn't mean you can apply some organization, some clarity and transparency. It breaks down the complexity, allows some overview, inducing fewer errors. I am talking about the "A" in CAD.

Down to earth example: if you make your circuit look like spaghetti, obviously it will be difficult and take long for someone else to decipher how it works. Might take half an hour. Some organization and notes might reduce that time to 10 seconds.

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Harmonic Oscillator with ideal Opamp models Reply #41 - Dec 6^th, 2011, 2:20am usability of a software interface is a useful thing, however most EDA tools are not "Apple PC iMac" user friendly. its a problem, I agree, but EDA is not shrink wrapped Microsoft type products, and getting improved ease of use is often not a priority for the vendor. I do agree that it is an issue.
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HdrChopper

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Re: Harmonic Oscillator with ideal Opamp models
Reply #42 - Dec 6^th, 2011, 6:19pm

[quote author=buddypoor link=1322576364/30#32 date=1323082079]Lex wrote on Dec 5^th, 2011, 1:30am:

My only concern was (and still is) that - in case of some ideal Hi conditions (simulation environment or models) - the designer should know that a simple direct transfer to real conditions can lead to false results.
And I gave an illustrative exaple for that (ideal opamp with pos. resistive feedback).
�

Hi Buddypoor,

that should not be a concern since it is design experience what builds designer's knowledge and intuition. This knowledge is the one that will make the designer weigth and take into account the limitations of the model he's running. A good and experienced designer will be aware of such limitation. I think your concern is simply part of the natural process of learning...

Best
Tosei

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Keep it simple

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buddypoor

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Re: Harmonic Oscillator with ideal Opamp models
Reply #43 - Dec 7^th, 2011, 12:19am

[quote author=tosei link=1322576364/30#42 date=1323224381]buddypoor wrote on Dec 5^th, 2011, 2:47am:

Lex wrote on Dec 5^th, 2011, 1:30am:

My only concern was (and still is) that - in case of some ideal Hi conditions (simulation environment or models) - the designer should know that a simple direct transfer to real conditions can lead to false results.
And I gave an illustrative exaple for that (ideal opamp with pos. resistive feedback).
�

Hi Buddypoor,

that should not be a concern since it is design experience what builds designer's knowledge and intuition. This knowledge is the one that will make the designer weigth and take into account the limitations of the model he's running. A good and experienced designer will be aware of such limitation. I think your concern is simply part of the natural process of learning...
Best
Tosei

Hi Tosei,

I am afraid that I can only partly agree. As you know - in particular in the area of opamps - it is common practice to idealize the gain and to use approximate formulas, which are exact enough for nearly all cases.
We know the limits/conditions for these simplifications and are, thus, aware that the formulas must not be applied for frequencies beyond a certain limit.
But the example I have presented is quite different, I think. Using the idealized opamp model the results don't only deviate up to a certain degree from the correct behaviour, but there is a fundamental difference beween real and ideal: works vs. works not at all. � �
Thus, there is no chance for the designer to "weigth and take into account the limitations of the model he's running".
I will tell you something about the background of my posting:
There is one author -very well known in the analog world - who has published a paper in an international electronic magazine (also with very good reputation) dealing with an "oscillator circuit" (how he calls it) that never will work in practice. It is a modification of the classical WIEN oscillator - however, all analyses are based on an ideal opamp model only. In detail, he is discussing oscillation condition, frequency, amplitude stabilization - for a "circuit" that never will oscillate in case of real opamps (immediate saturation due to net positive dc feedback).
And my original question was if such a combination of active and passive parts may be called "oscillator". �
What do you think?
Regards

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loose-electron

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Re: Harmonic Oscillator with ideal Opamp models
Reply #44 - Dec 7^th, 2011, 9:28am

papers and journal papers get published all
the time that have errors, make incorrect
assumptions and similar.

It is not a perfect error free world.

One of the basic premises of science -

1. Publication of work
2. review of work by peers
3. Reproduction of results by peers
in an independent manner from the
original work.

A lot of the material that you read in
engineering is your
participation in step 2.

Just because it has been published
in a trade magazine, or a peer reviewed journal
(IEEE and others) does not mean that it is
perfectly correct, or even technically feasible.

Welcome to reality!
Cheesy

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