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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