2012-07-02 Minimizing amplifier chain offset

For the SAFARI project, I'm refactoring some code, and one part of that is a routine to minimize the offset of the front-end electronics or FEE. For the place of the FEE in the setup, see also 2012-03-29 Extensions to Safari library.

The overall setup

Here is a picture of the lab setup.

lab setup with cryostat.jpg

From right to left: the PC with our lab software, the DEMUX board and some other measurement equipment in the rack, and the cyostat with on top the FEE board.

Zooming in on the cryostat; we're looking at a closed cryostat now. It's filled with helium because the scientists are doing a measurement. Before the test, the cryostat was opened:

opened safari cryostat.jpg

The sensor

The type of sensors used by the SAFARI instrument are TESes: Wikipedia article on transition edge sensors. As you can read with in this article, this type of sensor uses a SQUID in its readout electronics.

The TES basically consists of a tiny surface ("pixel") which is biased by a current source from the DEMUX board. When a photon hits the pixel, its resistance increases and the voltage drops. You detect this via a SQUID, whose output is amplified outside of the cryostat.

The TES its setpoint is 20 mOhm and if it's hit, the resistance increases to 100 mOhm. It's superconducting, so it's hard to get it out of its setpoint with heat, so flux is used.

The SQUID is controlled with a DC voltage, meaning we set and characterize it via a DC voltage. It's supplied by our DEMUX electronics board. Reading out happens with an AC voltage. It's supplied by our FEE electronics board, which also amplifies the output so it can be digitized by the DEMUX board. See also this schema:


The signal

We currently use a single SQUID for all the TESes. The SQUID combines these in one signal, which the DEMUX board then splits up again so we can read out each individual pixel. To see what I mean, here is the sensor:

safari sensor.jpg

Description: at the top left, we have the TESes. On the bottom left, we have the LC filters (see below). On the right, we prominently see the black connectors that bring the signal out of the sensor. On the bottom right, we see the single SQUID. Note the connections in the top and bottom middle. If we increase the number of pixels, these connections will stay the same because the TES signals are multiplexed (put together). To get an idea of the physical scale, this thing has a width of 12-13 cm (I haven't measured it exactly, just an estimate).

There is one LC filter for each pixel. Basically, with the LC filter you pick out a certain frequency. Thus the LC filter controls which information the pixel gives us; it makes sure the spectrum we're measuring is nicely divided over the pixels. Wikipedia has an article on LC circuits which is hard to read for me, but do notice the applications header which I've linked to. First entry says that the most common application is tuning, which is exactly what's being done here.

As an aside: two or more SQUIDs

SRON bought the SQUID from elsewhere, namely Magnicon, and then bonded to the silicon substrate. As for the SQUID, there are other possibilities. Per pixel row, you could use one SQUID. Or you could still use the above setup, but with an extra SQUID, which we call the dual-SQUID configuration. This configuration has been designed, but for this to work, we need to characterize both SQUIDs and since they influence each other, we'd need to use a flux-locked loop to lock the first SQUID in its set point, then characterize the other. I might have to expand on this sometime later.

Looking ahead

What I've described above, is just the lab setup. With this setup, we can only detect power intensity, but the SAFARI instrument is a spectrometer. Thus, the instrument consists of an FPU (focal plane unit), which is a box containing mirrors, shutter, filter wheels, a cooler, an FTS and three FPAs (focal plane assemblies). These FPAs are basically little cubes with the above described sensor in there. The focal plane unit is cooled to 1.7 Kelvin, and the focal plane assemblies are cooled to 50 milli-Kelvin via an intermediate 300 milli-Kelvin stage. Outside of the focal plane unit, we also have two "warm" units. Basically these have the function of the current DEMUX and FEE boards, but our current boards are meant for a lab situation, so new electronics will be made.

So the sensor measures power intensity, and the FTS will give us intensity as a function of wavelength. In other words: the instrument will send us power plus mirror position (of the FTS), and these can be transformed mathematically to create a spectrum. Since SAFARI is an infrared instrument, we are looking in the 34 to 210 micrometer wavelength.

Current task

So far I've described the setup. Now on to my current task.

To accurately set the DC bias on the pre-amplifier chain that consists of a single SQUID, an amplifier, and an integrator (??), we need to measure the offset that's introduced by the FEE.

The basic schematic of the SQUID, the amplifier and the integrator look as follows:
schema single squid.png

On the left, we have the single SQUID. It's biased in a range between 22 and 44 uA. On the right, we have two functions. The top right shows a function of the FEE board, and the botton right (with the DC voltage part) shows a function on the DEMUX board; the DAC which sets a voltage, and the integrator that keeps the voltage over the setpoint of the SQUID. Below the SQUID is

This offset can be adjusted via the integrator-offset bias generator (noted as DAC in the above schema (??)). It's also adjusted via:

Note: the ?? indicates I have to check this statement


SPICA - SAFARI Link to the SRON homepage concerning the project