Short version: we think CPS's are limiting st2 performance above 10hz. We have modfication ready to try that we think will be quick and painless to try that can be easily backed out.

So after sorting out configuration issues with the end stations, Jeff and I (mostly Jeff) have come up with a small modification to try on the St2 BSC 250mhz blends. We noticed on my BSC performance plots from the the other day that turning on the St2 loops reduced the St2 performance at high frequency. We think this is because the CPS isn't rolled off fast enough in the blend filter, so we messed around with a couple ideas in foton, see attachment. The current blend is shown in red The first idea was a simple pole a 5hz, blue on the plot. This rolled the CPS off quickly, but lost phase around the blend frequency. Next we tried a couple pairs of poles at 10zh, which improved things some, losing less phase at <1hz, but introduced some gain peaking at 10hz (green and brown traces). Finally we used a low-q elliptical filter and it was just right. Or less bad. The combined fitler is shown in cyan(?), the ellipse only is shown in black.

In foton the zpk of the ellipse is : zpk([17.782+i*26.674;17.782-i*26.674],[5.148+i*9.424;5.148-i*9.424],1,"n"). The filter isn't loaded yet, it's not  being used, but first thing Monday morning, I hope install and test it for each dof on st2.