Previously, I described how we measure the transfer of the AC bias.
To recap, we assume the user has characterized the DC biasing of the SQUID(s). When we measure the transfer of a single Array-SQUID or a dual-SQUID (i.e. pre-SQUID plus array-SQUID), we need to take the following steps:
Why do we put a test AC signal on the feedback coil, instead of sending it straight through a pixel on the sensor? Because each pixel has a hardwired LC filter, we could say. But that feedback coil is there, because the SQUIDs hardly have a dynamic range.
Let me put this in other words, because it's extremely important to understand the whole setup.
The SQUID is a very sensitive magnetometer. Thus the smallest change of flux will cause it to change its output. The output looks like a sine wave, thus it has an extremely small dynamic range (namely, the flank of a sinus). This means that we need a feedback loop that holds the SQUID locked to an output range. Because we have more than one pixel, we would run the SQUID out of its dynamic range. Thus the feedback loop ensures that, within its dynamic range, the SQUID can add all the pixel carrier waves.
To make this measurement, we use a set of registers that allow us to configure the Demux board to start spewing data over the second optical network link. It's very flexible and can be viewed as a number of digital data taps located on several points of the Demux board. You can set the data source, the amount of data, the trigger source, and finally whether you want a specified number of data blocks or just continuous data.
The data source can be configured to the ADC, the DACs, or the demodulated output of pixel 1 to 16. The ADC is of course what we need currently, but you can also read out the DACs. This is useful to check that the FPGA algorithms (more on that later) are correctly working. Finally to read the demodulated output of pixels is hugely useful, because (1) (see below).
The trigger source is worth a different topic, which involves explaining what the sensor actually measures (X-rays or infrared signals). The options currently are: testpulse generator (2), level triggering of selected pixels, both of these options, or just auto (get all the data).
For our purpose, we need to configure it as following:
Footnotes:
(1) Demodulation saves us bandwidth as well as processing on the PC.
(2) The testpulse generator is used to generate fake detector signals, to test the complete signal chain. You can test demodulation and/or test the electronical system. It generates the ideal detector signal. You can send the testpulse generator signal to the feedback coil of the SQUID and measure the PPM drift of the signal chain.