# 2013-04-09 Characterizing the transfer of a voltage biased SQUID

Currently, I'm finishing a software routine to characterize the transfer (see below) of a voltage based SQUID. Simplified, our electronics could schematically be shown as follows:

On the left, you see a single SQUID. This is the pre-SQUID. In the middle, you see an array of SQUIDs. We functionally treat that last one as a single squid, calling it "the" array-SQUID. These two types of SQUIDs are biased differently; the pre-SQUID is voltage-biased, and the array-SQUID is biased with a current.

You can view this combination as one "dual-stage SQUID".

So what we do, is: we set a current over the shunt on the left. This will put a voltage on the pre-SQUID and a magnetic field will form on the inductor next to the array-SQUID. We'll read out an output voltage on the right.

Now normally, the amount of voltage we put on the pre-SQUID would overdrive the array-SQUID. Thus, we put it in flux-locked loop. This is a mode where the array-SQUID simply passes current that results from the changes in flux.

Because the array-SQUID is in flux-locked-loop (FLL), we can measure the output current of the pre-SQUID without a dynamic range limitation (+/- 500 uA). This flux-locked-loop is nothing more than a feedback circuit which forces the bias of the array-SQUID down to zero.

Note that this is all DC-biased. The SQUIDs are part of the amplifier chain, and the whole amplifier chain is DC-biased. After we've characterized the amplifier chain, our primary purpose is to read out the sensor, consisting of a TES array that's AC-biased, see also 2012-07-02 Minimizing amplifier chain offset.

I said "characterize the transfer". The transfer means: what is the effect of the input on the output. If you draw this function, you'd ideally see a sinus. We want to know in which voltage range our pre-SQUID behaves the best (i.e. where it has the highest dynamic range. In other words, you want to pick the voltage that's right in the middle on a flank. Because that way, with a little bit of input, you get a nice stretch of usable output.

Compare it to a radio that can receive a lot of stations.