On Wed, 13 Sep 2006 20:26:41 -0400, Ted
wrote:

On 9 Sep 2006 11:23:33 -0700, "Professor"
wrote:


K7ITM wrote:
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,
---snip---

Aarrgg! This is very disappointing! I usually check at least my
values for LC circuits against the LADPAC programs In "Experimental
Methods in RF Design", however. By the way, when I click on
Tools/DesignAttenuator, the schematic that is in the pop-up window for
default 10 dB shows the three resistor values as 71.151 and 96.248.
When I go through the process you did, I get the same results. I
saved the file, and closed and reopened the program. Then, I loaded
the saved 10dBAtten.cct file, and of course got the same -102.69 dB
for S11. Then, I Tuned the values back to 71.151 (which immediately
displayed only 71.15), and tuned the other two to 96.248, (which
displayed 96.25). When I simulated, S11 was now -107 dB!

Regards,
Ted KX4OM

I just ran the simulation in the LADPAC programs. I first built the
attenuator in Ladbuild, with the resistors rounded off to two decimal
places, and saved the result. In GPLA, I got (after resetting the
scale to -120 dB) S11= -107.3. Then I edited the values to the 3
decimal place values from the original attenuator pop-up window of
RFSim99, and got S11=-121.67.

The circuit review feature of GPLA still showed 2 decimal places,
rounded off, but saving that version of the attenuator and opening the
saved program in Ladbuild showed three decimal place values when
clicking on the component.

It's surprising how a couple of thousandths of an ohm can change the S
parameters that much.

Ted KX4OM