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Drake instability & Dielectric Filter Simulation



Folks,

I seem to have some instability in my Drake 2880 which I believe
may be related to the Toko dielectric filter I installed.  Recently,
I let the Drake run for 24 hours with a small dipole antenna for the
input and ran the output into my DSP-10 transceiver.

The waterfall display on the DSP-10 was set for 24 hours.  Examining
the 24 hour DSP-10 display revealed three distinct modes.  The three
modes appeared as shifts in the waterfall pattern and none seemed to
be favored over any other.

In order to diagnose the problem I thought I would first run some
simulations with Radio Designer.  I wanted to see just what effect
adding a dielectric filter to the output of an LNA would have on
its stability.

I asked Toko for a file of S-parameters for the TDFS8A-2450T and
received them by email the next day.  However, when I tried to use
the data in Radio Designer (simulating only the filter first) I
got results that were not even recognizable.

I then noticed that the data columns in the Toko file were titled
"REAL" and "IMAG" rather than "MAG" and "ANGLE".  In addition, S12
came before S21 (I'm used to seeing it the other way around).

I changed the "DATA" block argument in Radio Designer from "S" to "Z"
and got closer.  However, the entire attenuation curve showed 30dB
more attenuation than spec (see below) (the Toko data is there also):  
http://people.we.mediaone.net/billwashburn/dielectric_filter.htm

I spent most of yesterday with MathCAD running transformations on
the Toko data.  I tried Z to S, Z to MAG & ANGLE and so forth with
no success.

Can anybody tell me how to transform Toko's measurement data into
50 Ohm S-parameters so that I can run some simulations?

Thanks,
Bill, WA6QGR




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