[AMPS] Modelling and its failures

[Fred Fliss]

Gents, I shall reply to two posts at once, and far more brief than my 
initiating exposition.

Tom wrote (including my initial comments):

>>It has been written that an amplifier either oscillates, or it doesn't.
>>That is a half truth.  It is possible that an amplifier is on the verge -
>>that is, with a slight change to some parameter, such as valve gain, a
>>resistor value, or stray capacitance, it will oscillate.  This is
>>ABSOLUTELY a measurable, and predictable condition.

>Which is my point. Please don't take what I say out of the context
>it was in. There is enough at that going on Fred.

I do not recall that you had written this, Tom.  Somebody had, and I 
recalled not who it was, which is why I did not attribute it to any 
particular individual.

Tom also wrote:
>Also, if feedback is so low that the oscillation only sustains with
>excitation, it is unlikely to have a level that does any more than
>cause TVI. It is also possible to make that oscillation sustain
>without drive, through manipulation of bias, HV, and load or source
>impedances.

I believe the last part of the comment, "load or source impedances" is 
possibly the biggest variable.  Consider the chap who moves from one QTH to 
another and erects altogether new antennas.  The load impedance outside of 
the normal bandpass of the amplifier is now different from anything the 
amplifier has experienced in the years at the original QTH, or at least the 
original antenna configuation.  Ditto if the operator changes exciters.  The 
change in impedances can be sufficient to make a seemingly stable amplifier 
oscillate.  However, an amplifier which is this marginal to begin with will 
indicate such with responsibly-made measurements, such as the ones you refer 
to when you say, that "there are test procedures and measurements that can 
predict problems."  I am in complete agreement with you on this issue.

AA6YQ enquired:
>Are you using a commercial modeling program, or rolling your own?<
Were I competent to write that sort of software, I might have made it in 
Silicon Valley, but I did not.  For purely research purposes, I use ARRL 
Radio Designer.  It's quick, has an easy to use interface and gives accurate 
results.  However, as my work is in commercial amplifier design, I cannot 
use it professionally, due to the license restrictions it carries (not to be 
used for commercial purposes).  Hams, of course, can use it to their heart's 
content for their own amplifiers.  If I am fortunate, the company I am 
contracted to will have one of the better analog packages such as eeSOF or 
SuperCompact.  I do run a copy of IntuSoft's version of SPICE.  This is not 
an RF-optimized package, so it will not calculate microstrip and stripline 
impedances from physical dimensions.  However, that functionality is really 
not required below several hundred MHz anyway.  When I am engaged in 
microwave work, I insist that my employer provide a useful package.  In 
microwave work, I also insist on an optimizer, but that is an entirely 
different matter.

I have, at times, used the software by EagleWare, but have encountered 
enough oddities that I doubt its accuracy, and they do not detail their 
component models in the manuals well enough to really trust them.  However, 
it gets the nod for fastest execution.  It is trustworthy as long as you are 
not on the edge of a design.  But the edge of a design is exactly where you 
need a modelling program to let you explore "what if" changes to the circuit 
that would take days in hardware, assuming you had a well-enough supplied 
parts bin.

Many regards,

FF
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