> > Determining by measurement that one instance of a circuit fails to
> > oscillate does prove false the assertion "every instance oscillates".
But
> > the domain here is production, where there are hundreds or thousands of
> > such instances; one cannot measure them all.
> One can not model an RF system without measurements.
> > The question is whether, with some combination of reasonable parameters,
> > some instances oscillate. Modelling is a pragmatic way to answer that
> > question -- it allows evaluation of the multiple combinations of
> > parameters likely to be encountered during a circuit's production.
> You don't know those parameters without measurements.
> > Models and modelling results can also be shared via the net, permitting
> > independent verification.
> You won't know the models are valid unless you make
> measurements, and analyze the model step by step to be sure it
> agrees with the measurements.
Agreed - measurement is necessary to validate a model. This does not
diminish the value of modeling.
> With measurements and calculations, you can determine the
> feedback and gain limits. From that, you can tell if it's possible to
> oscillate.
> You can't do a model without those measurements and some of
> the calculations needed to make the model.
> All the model will do is handle the "math" as you change various
parameters.
> You can do that without a model just as easily as with a model.
Given a significant number of parameters and the need to assess circuit
performance over multiple values for each parameter, the resulting number of
cases can be large. Varying each of 50 parameters through a 20% range in 1%
increments yields 1000 cases. And while some parameters can easily be varied
while measuring, others cannot; how does one step a tube through its
tolerance range for contaminants, or through its tolerance ranges for
inter-element spacings?
> Let me give you an example of how people assume models are
> correct.
Most engineering techniques produce erroneous results when badly practiced.
The fact that some practitioners model poorly does not invalidate the
virtues of competent modeling.
> IMO, if modeling programs can't handle a simple inductor
> properly...they need to be carefully watched with other components.
Are you asserting that no modeling program properly models an inductor? The
fact that some do not hardly means that all do not. There are plenty of
low-quality scopes about, but that fact doesn't invalidate the measurements
made with high-quality ones.
73,
Dave, AA6YQ
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