Jeff K, Chris Whittle
Following on from aLOG 29400, which suggests a very large negative bias voltage zero-crossing on ITMX, we have repeated ITMX charge measurements at excitation frequency 20.1 Hz in an attempt to produce results more comparable to those in aLOG 27752 (zero-crossing at 3k counts). Defying expectation, we found the bias voltage zero-crossing to depend on the excitation frequency. Repeating this measurement for multiple frequencies gives the first attachment, which shows the bias voltage zero-crossing as a function of excitation frequency. This plot seems to suggest a f-2 dependence. The second attachment shows results from a separate measurement in which we sweep over multiple frequencies at different bias voltages. The two plots seem consistent in that dips in the transfer function seem to correspond to the frequencies with the associated bias voltage zero-crossing.
Limited studies of ITMY show similar behaviour:
| Excitation frequency [Hz] | ITMX bias voltage zero-crossing [cts] | ITMY bias voltage zero-crossing [cts] |
|---|---|---|
| 11.5 | -140k | -119k |
| 16.55 | -65k | |
| 18.0 | -56k | |
| 20.1 | -43k | -37k |
| 28.0 | -22k | |
| 36.0 | -13k | |
| 43.0 | -5.7k |
These results are also reproducible day-to-day, varying by <10%. We have not looked closely at these fluctuations, however.
Further investigation is required to determine the cause of the frequency dependence, and to understand the large difference between current measurements and those taken in June.
