Pole analysis

Dear All,

Is it possible in SPECTRE to find out poles given between different
nodes of the signal path. To clarify, suppose I have a three stage
amplifier. Now I want to calculate all the poles added by each stage's
input-to-output path. Is it possible to do? It will be better if
corresponding OCEAN script syntax is provided.

Thanks & Regards
Samiran.

 

Samiran,

It sounds like you're analyzing an LTI system. Assuming that, suppose
your three-stage signal path consists of signals A, B, C, and D, where
each signal is some voltage or current. I don't know of any magic
syntax that provides the poles and zeroes directly. Even if there was
such a thing, you'd still need a way to associate those poles and
zeros with circuit elements. So, one useful thing you can do is find
the transfer function from A to B, and analyze it separately. Repeat
for the transfer functions from B to C, and C to D. This will at least
break up the problem, and let you know the contribution from each
stage of your signal path.

Best Regards,
Stephen Greenwood

 

Hi Stephen,

You are right as I am analyzing a 3-stage LTI system...but unable to
trace the contribution (in terms of poles of the composite transfer
function) of individual stages. Using SPECTRE's pz analysis I am able
to find out all the poles and zeros of the actual input-to-output
transfer function (t.f.). Now as you are suggesting to find t.f. of
each intermediate stages I know how to plot voltage gains, but would
that also give me the poles added by that particular stage. If
possible please clarify with syntax [I know there is "no magic
syntax" ], but at least if you can guide me it will be very
helpful.

BR
Samiran

 

Hi Samiran,

You'd simply use the mag() and phase() functions on quantities B/A, C/
B, D/C to obtain their (complex) transfer functions. For each of
these, you should know the number to poles and zeroes, and then you
could infer where they are. If they're simple transfer functions
(e.g., single pole per stage), you could make the inference directly
(e.g., -45 degree phase shift). For more complicated functions or ones
in which the poles and zeroes are not widely separated, you could
probably also make the inference analytically using multiple points,
but you might find it easier to post-process the data with curve-
fitting software. In case you're not already familiar with them, the
outfile(), ocnPrint(), and close() functions will allow you to print
your waveform data to a file for use by a post-processing tool. (If
you don't like ocnPrint() format, use fprintf() and make whatever you
want. In times past I have created files for Matlab and Excel for this
purpose.)

Best,
Stephen Greenwood

 

HI Stephen,

Thanks for your valuable suggestion. I am aware of MATLAB's curve-
fitting software as I am already using them. Actually, I have started
doing whatever you suggested...

BR
Samiran