Hi all,

Every once in a while I come across an anomaly in RFSim99. I'd be
happy if someone could tell me that I'm doing something wrong in the
following:

Fire up the simulator. Tools--Design--Attenuator. Pi Section, 50R
input impedance, 50R output impedance, 10dB attenuation. Simulate.
!Current schematic will be overwritten; OK. Now note that the graphics
window displays S11 as 2E+31dB.

In the circuit window, edit the values of the two 96.25 ohm resistors;
they have apparently been simulated to more decimal places than shown.
Just manually enter 96.25 ohms for each. Now the magnitude of S11 and
S22 show up as -102.69dB; that seems reasonable. The phases for S11,
S12, S21 and S22 are all zero. Now, from the graphics window,
File--Save results as S Parameter File, and save the file in some
convenient place, as type S parameter files (*.S2P).

Now delete the resistors from the schematic and instantiate a two-port
in their place. In the 2 port properties window that comes up, select
"Load File" and load the file you just created from the attenuator
simulation. Notice that the phase is 89.994 degrees for each of the S
parameters, at each frequency. Or at least that's what I'm getting.
Continue on and connect up the schematic and notice that it simulates
as 89.994 degrees. Edit the .s2p file with your favorite text editor
and notice that it really does have 89.994 degrees in it.

Boy, I wish I could make a frequency-independent 90 degree phase
shifter that easily in real life!

More similar fun: create a 10dB 50 ohm attenuator as before and
simulate it. Add an 80nH inductor across the top, from port 1 to port
2, and put a 32pF capacitor between the bottom of the two 96.25 ohm
resistors (connected together) and ground. Simulating that should give
you an S21 that's 0dB at low frequency, smoothly transitioning to -10dB
at high frequency, with -5dB at about 120MHz. But S11 and S22 have
strange magnitude and phase behaviour. Change the 96.25 ohm
resistors to 96.2 ohms, and the S11 and S22 magnitudes behave OK,
though their phases are still strange at very low frequencies. Save as
an s-parameter file again. For convenience, start another copy of
RFSim99 so you can compare the results of a new simulation with the one
you just did. In the new window, instantiate a 2-port and load the
file you just saved into it. Notice in the file that S11 and S22
angles appear to match the simulation you just saved from; there's no
90 degree phase shift this time. But when you connect ports 1 and 2 to
the 2-port you just placed and simulate it, you find the phase of S11
and of S22 are all messed up. Huh?

Has my copy of RFsim99 gotten corrupted, or do others see this
behaviour too?

Cheers,
Tom