[Fujimoto, Tanaka, Komori]

Abstract:

We measured the transfer function and dark noise of the CARM CMS.
The results appear reasonable, and we have now completed the measurements necessary to perform the CARM noise budget.

Details:

We carried out the same set of measurements as those performed for the IMC CMS, as reported in klog:35669.
For the CARM CMS, we focus only on the fast path and ignore the slow path for now, because the slow path is used for MCE feedback and is negligible for the CARM noise budget.

The dark noise of the CARM CMS fast output was measured with a ×100 amplification using the SR560, and the result is shown by the blue line in Fig. 1.
We measured the spectrum in two frequency ranges, similar to yesterday’s IMC CMS measurements, and this time there was no discrepancy around the 1 kHz boundary.

The measured transfer function (blue dots) is compared with the modeled one (cyan line) in Fig. 2, and they agree well.
Using the modeled transfer function, I calculated the input-equivalent noise of the CARM CMS fast path (Fig. 3).

We also investigated the mismatch between spectra measured in two frequency bands reported in klog:35669.
Today, we found that one of the terminal resistors had a poor connection and was introducing excess noise. After replacing it, we successfully measured the IMC CMS slow-path dark noise with good agreement between the two spectral measurements (red in Fig. 1).

On the other hand, we observed a mismatch in the dark-noise measurement of the IMC CMS fast path (magenta in Fig. 1), even though it matched well in previous measurements.
We found that the dark noise is sensitive to mechanical stress applied to the CMS board connector.
Since the high-frequency measurement is reproducible and the IMC CMS fast-path dark noise is negligible at low frequencies, we will use the previous result.

The mismatch in the Mokulab dark noise is likely due to a degraded noise floor in the low-frequency band caused by additional downsampling.
Furthermore, we found that the overall gain discrepancy between measured and modeled transfer functions originated from the impedance setting of Mokulab: we should have divided the output by two. With this correction, the measured and modeled gains now match well.

We have completed the necessary measurements for the CARM noise budget and will proceed to the calculation phase.