2013-04-11 measure the AC gain

A little intro

Another routine in the system is the AC gain curve measurement.

This routine is different from the previous one, 2013-04-09 Characterizing the transfer of a voltage biased SQUID. In that routine, we just looked at the DC part of the amplifier chain.

In this measurement, we assume those measurements were successful, and that the amplifier chain was set according to optimal parameters.

And since our instrument basically uses AC biased TESes (as explained here), we want to measure the transfer of the AC bias, after the SQUID has been configured optimally. This is very interesting, because that means you see the instrument as a whole, instead of characterizing a small part.

The sensor:
safari sensor.jpg

The measurement

So what we're going to do, is sweep the AC bias and then measure back the result.

To do this, we have to send the FPGA the appropriate commands. First, some schematics:

On the Demux board, there are two DACs that can drive the SQUID in the cryostat. There is also an ADC to measure back the result. The FEE board is in between here, but for now we can ignore it. The cryostat is where the above sensor is located. It has an array of TESes, each with its own LC filter.

If we want to feed in an AC bias, we don't want to use DAC2, because we don't want it filtered through the LC filters. Instead, the FPGA exposes a register where we can switch the input line of both DACs, so you can feed the AC bias signal to DAC1. Note that this AC bias signal is still digitized, otherwise we couldn't feed it into a DAC :-)


The user needs to be able to adjust the AC bias signal. The FPGA exposes a number of parameters for this, but we just need the frequency and amplitude, it's a really small signal: 2 MHz at 1 uA. Both are adjustable, and I'll add a setting for 1 and 5 MHz as well.

The data

We retrieve the data asynchronously. This is rather different from our usual way of getting data; i.e. normally we'd just polling a particular register.

In this case, the hardware has a special function called the "science switch". A possible analogy could be that this science switch acts as a number of water faucets, where you'd just let the raw data stream out of the board, from specific points. Since water and electronics don't mix much, it's actually a number of registers. I'll write more on that later.