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Maxim Integrated - Integration Nation
Dennis Feucht

Z Meter on a Chip? Impedance Meter Oscillators

Dennis Feucht
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davebirdieee
davebirdieee
12/2/2013 8:25:13 PM
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Teacher
Re: Oscillators/Feedback Analysis
Dennis:


Thanks for the reply. I agree with all of your points. I've encountered them and agree. Yes, I'd like to see your paper. I have toyed with possibly using z transforms to do some of this, but haven't spent any time really looking at it.


Is there some special way to communicate my email or is it available through this board?

Agree with your comments re IEEE docs. Such a pity, and a loss. But, someone does have to pay.

Thanks again,

Dave

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D Feucht
D Feucht
12/2/2013 6:16:59 PM
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Re: Oscillators
Victor,

First, thanks for the encouragement in regard to hand-drawn circuit diagrams. They are sloppier than those from CAD editors or in published literature, but they also convey a sense of engineering as an art, which for the better engineers, it certainly is.

Your several points regarding the benefit of analog vs DSP are well-taken. One of the challenges of good design is knowing which combinations of analog and digital implementation will result in an optimum design.

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D Feucht
D Feucht
12/2/2013 6:13:12 PM
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Re: Oscillators/Feedback Analysis
Dave,
Not long ago, as part of my Z meter research, I spent a couple of weeks or more working on just this problem and have a detailed document on it. It could be submitted to Planet Analog for a short series on Oscillator Amplitude Control (which I probably will do), though if you are keen to see it, I can send you a MS Word copy if I receive your email address. It is about 341 kbytes in length and requires that the Word equation editor be installed.

What makes this a difficult control problem is that the loop is inherently nonlinear because multiplication in some form is present within the loop. My analysis is based on a JFET gain control mechanism, though the loop dynamics section is more general. Detection of the amplitude is not a trivial problem either.

The analysis assumes that the reader has a good familiarity with classical continuous control theory and with some DSP. (Hint: sinx/x shows up in the analysis of the peak detector.) Nonlinearity is handled by linear approximation around an amplitude loop operating point.

James Roberge, in his classic book, Operational Amplifiers, presents a method for analysis, but I think mine is more refined (nearly 40 years later!) and makes explicit some of the subtler assumptions in his scheme, though it is a good place to start.

I also do not have access to IEEE documents and will not pay the rather large amount an individual would have to shell out to have it. Perhaps the IEEE is intending to encourage originality and creativity, with perhaps some redundancy too, in making access to its papers unobtainable for so many.

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Victor Lorenzo
Victor Lorenzo
12/2/2013 4:09:27 AM
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Re: Oscillators
@D. Feucht, thanks for the article, I have to admit I still love seeing those hand drawed schematics ;).

"However, the analog schemes still have much going for them because they produce many bits of resolution for a few cheap analog parts which cost far less than a > 16 bit DAC"

One more thing to add is output filtering to remove the OOB noise introduced at converting signal from digital to analog domain (DAC), it will also add cost.

And even one more aspect to take into account is the DSP floating/fixed point precision, depending on the algorithm used for modelling the oscillator they could lead to instabilities that could lead to output saturation. Some generators simply use a bunch of memory to store one pre-generated pattern and then simply play it back.

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davebirdieee
davebirdieee
12/1/2013 3:18:52 PM
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Teacher
Oscillators/Feedback Analysis
Dennis: Amplitude regulation of linear oscillators seems to be done exclusively through empirical methods. Do you know of any way to analyse the regulation path so as to be able to design similar to the usual feedback analysis? I've seen an article or two in the IEEE, but only as abstracts since I'm retired and not subscribed to their digital archive, and even then it's not completely clear that those articles address what I'm looking for.


Appreciate any insight you might be able to throw on this.


Thanks,

Dave

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D Feucht
D Feucht
10/13/2013 9:15:21 PM
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Re: Oscillators
Hey Kendall,

As good engineers, we strive to be objective and suitably dispassionate in our analysis of the alternative solutions, and yes - DSP-based methods are quite in the running. (I just didn't want to give away any Really Good ideas in this series about them! - just the basics.) Hence the inclusion near the end of the article of "The final category of sine-wave generators is digital, ...".

There is a category of DSP techniques based on incremental function generation  that might be considered an outgrowth of the digital differential analyser (note correct spelling) work that went on, mainly in Britain after WW II. When implemented so that truly stable sinusoids are generated digitally, the result is quite attractive and would be one of the first methods to be considered in a Z meter IC implementation.

However, the analog schemes still have much going for them because they produce many bits of resolution for a few cheap analog parts which cost far less than a > 16 bit DAC. Happily, not too many bits of precision are needed for the Z meter oscillator unless one is pushing the state of the art in performance, and DSP generation is a front-line contender. So your point is well made.

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B_Albing
B_Albing
10/8/2013 10:59:31 AM
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Editor
Re: Oscillators
@kendall - well, it does feel a bit heretical, but I agre that some of the digiatl frequency synthesis methods are pretty slick: flexible and accurate. Hard to argue with that.

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kendallcp
kendallcp
10/7/2013 11:57:47 PM
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Newbie
Re: Oscillators
I shall probably be shot down in flames and accused of heresy but... all these nice olf analogue oscillator circuits are lovely, but why not do it digitally with some form of synthesis technique.  I have been leaning towards filtering in the digital domain rather than the analogue domain for some while, and gerating sustained oscillation is another one of those things that can be done quite well digitally these days.  OK, I know it's not as much fun.  But it works well, and can be done with very low component count.

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antedeluvian
antedeluvian
10/5/2013 9:53:48 AM
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Re: Oscillators
Man21

I presume 'antedeluvian' meant prescribed rather than proscribed.

Given how badly I did at the subject, perhaps it was more proscribed than prescribed  for me.

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Man21
Man21
10/5/2013 8:04:46 AM
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Re: Oscillators
I presume 'antedeluvian' meant prescribed rather than proscribed.

 

From even more antediluvian.

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