Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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PIM and other tests

Posted by Jacques (A) on September 7, 2006 at 11:22:30

In Reply to: Re: Answers for what I know and technical comment on PIM posted by jneutron on September 7, 2006 at 09:00:51:

I do not believe the current buffer stage capability can be ignored, nor the load reactance that it is required to drive.

I don't believe either; I just think that PIM measurement is a poor tool to optimize the current buffer. Because it should be extremely hard to relate actual measurement to its physical cause. Using IIM and even DIM measurement is better aimed at current buffer assessment.(in fact, IIM, with its abuility to exert the PA in the four quadrant is the tool)
By contrast, PIM is easily related to pitfalls in the first and second stages designs.

I do not believe the current buffer stage capability can be ignored, nor the load reactance that it is required to drive.

A similar analysis I applied onto the 1st and 2nd stage could be done. However, since voltage gain is about 1, we would expect a small dependency of roll-off frequency with voltage or current. To be checked, but currently, I smell peanuts.

I find fault with load resistors that do not accurately load a voltage output at the higher frequencies, and the trusting of the voltage across them when driven by high slew rate currents.

Easily solved if you apply correct layout methodology to minimize loop areas and use correct non-inductive resistors. It's an issue to check, but only at the prototype level. Designing an amp where these factors would change significnatly from unit to unit would show a massive incompetency...
BTW, makes me think to these simplistic distorsion measurements where people just connect the analyser at the PA output, without thinking of making a bridge measurement with a copy of the (complex) output impedance in the other branch... They are just expecting the damping factor to be infinite, which is rather unlikely at 20KHz furthermore if the PA feeds a reactive load... But it's another topic, way OT. A thread on the subject, maybe?

I do not believe the current buffer stage capability can be ignored, nor the load reactance that it is required to drive.

I don't believe either; I just think that PIM measurement is a poor tool to optimize the current buffer. Because it should be extremely hard to relate actual measurement to its physical cause. Using IIM and even DIM measurement is better aimed at current buffer assessment.(in fact, IIM, with its abuility to exert the PA in the four quadrant is the tool)
By contrast, PIM is easily related to pitfalls in the first and second stages designs.

I do not believe the current buffer stage capability can be ignored, nor the load reactance that it is required to drive.

A similar analysis I applied onto the 1st and 2nd stage could be done. However, since voltage gain is about 1, we would expect a small dependency of roll-off frequency with voltage or current. To be checked, but currently, I smell peanuts.

If a current buffer has any problems with either the voltage swing, the current slew, the current demands, reactive load power return, or SOA boundary approach, your (nicely detailed) analysis will not model it.

As explained above, I never expected it to. Not its business.

I'm not sure if we do "well" to pidgeonhole all this into one distortion category. As you point out, there are a few to "blame".

Well, several different set of measurements, and test procedures. One for the design state, you know, the trilogy of pairs

• analytical study <--> calculations from (feasible) equations
• modelisation <--> simulation
• prototyping <--> measurement,

and comparing the three.
With audio, we will add subjective testing.
Then industrialisation, monte-carlo simulations, and automatic testbench programmation.
Later, tests accessible to servicemen.
Different sets of tests for each need, I don't think it's a problem.
Categorization is also done in subjective tests ( many different sets of different instruments...)

ps...I'm tossed between the 75 dollar membership, vs the 20 bucks per article..hmm..

I do remember there is an erratum following this paper. Check whether it's free, those guys are such audiosharks...

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