Jim, Sebastien

We've been investigating a little more on the HAM3-ISI issue. Quick sum up: we see a 0.6Hz sharp peak on all DOFs when the Z sensor correction is on (see logs here and here).

First we checked the electronics. We swapped the STS input cables of the AA chassis between HAM2 and HAM3. This didn't do it, and the issue stayed on HAM3. This exclude the whole STS electronics chain except the AA chassis and ADC.

But, back in the December, the sensor correction was implement with a different seismometer (which means different AA chassis and different ADC), and the peak was already here. Thus we can say with confidence that it's not an electronics issue.

From there, couple of observations:

- The unity gain frequency of the nominal blend filter is 0.6Hz.

- When we switch from the nominal blend filter to a 750mHz blend, we don't see the peak anymore.

Given that, the nominal configuration of HAM3 is the same that the other ISIs, and the problem appears only on HAM3...

Also, by looking at the local transfer functions of HAM3, we see a lost of coherence around 0.6Hz (see files attached). Rubbing might explain the whole story... A soon as we have access to the platform, we'll do a driven transfer function in this bandwitdh and double check that hypothesis. In the meantime, we keep investigating on the blend configuration.