As EQ pointed out recently, we're injecting into the FSS error point just after an RF pi filter, but before the VGA. We wondered what the weird filter impedance was doing to our signal if we inject after it. I used LISO to model this FSS common section and attach the plots.
The first plot shows the TF between the Test 1 input and the AD602 VGA input. This is NOT the input that we are actually using.
The second plot shows the TF between the IN1 port (which we are actually using) and the VGA input.
Neither of them shows the 1 MHz bump that we see in the measurements, so I suspect that the board has been modified...the hunt continues. We've got to pop the top of the TTFSS and take photos and measure from IN1 to VGA input.
** FSScomm.fil is now in the LISO SVN. The following command line will run it with two different cases and cat the PDF files into one. If you use an auto-refresh PDF viewer like Okular or Mac Preview, its a nicer display than the usual GNUplot window:
./mfil FSScomm.fil; sleep 1; pdftk FSScomm_run*.pdf cat output FSScomm.pdf