2006-04-24 SIRs for dummies

Previously, I showed some pictures on the electronics that are controlled by the software I maintain and enhance. Now I'll explain what exactly is controlled.

We're basically commandeering a SIR, which stands for Superconducting Integrated Receiver. It's a tiny (4 by 4 mm) chip with which signals are measured in the 500 to 650 GHz range. The chip is set behind a silicon lens, is put in a container which is supercooled to about 2 to 4 kelvin.

Below is a picture of the SIR chip, a Russian product from the Institute of Radio-engineering and Electronics of RAS:

SIR chip.jpg

Legend: between the zero and one, the flux flow oscillator (FFO) is located. The Harmonic Mixer is located below that. More about these in a later entry. Right in the middle, a SIS mixer is etched (superconductor-insulator-superconductor). If you look at the SIS mixer from the side, you could sketch it (very badly) as follows:


The F2 signal comes in from the atmosphere, which is a very weak signal. The F1 signal comes from the FFO, which generates a very steady known signal. Out comes a signal which basically is F1 minus F2 and which can be amplified enough to read out. When a signal hits the SIS mixer, the resistance changes slightly and you can read this out using the current.

The base as well as the edges left and right are conducting. When the chip is supercooled, they become superconducting. The edges are separated from the base by an insulation (hence the name SIS). A constant voltage bias is put on the edges (denoted in the sketch by Bias) and the current I is varying to keep the voltage constant. Below this construction, another conducting line is etched, the Control Line (denoted by CL), about which I'll tell more below.

We can draw a curve with the values of current C and V, which is called an I/V curve:

SIR IV curve 2.png

If you want to get a clear signal, your curve should be like the thick red line. (Note that the derivative of the curve is the resistance of the SIS mixer.) However, that is not the case: we get the grey line. It wiggles a bit, even goes down a bit before it rises and with a bend continues in a linear fashion. To get the thick curve, we need the control line on the SIR chip.

More about the control line. This line is conducting and etched below the SIS mixer. It must generate a magnetic field, drawn as the small circle around the control line and extending over the base. To generate a magnetic field, the current must be kept constant.

Besides the control line, the FFO is operated (more about that later). The FFO generates a clean signal with a certain power, which lifts the grey line up.

When combined we get the red line, but it still can go a bit down like the grey line. We need to find a couple of values to get a clear signal: the current and the bias voltage on the SIS, and the current on the control line. So what do we do: we set a certain current on the control line, then for that current, vary the bias voltage on the SIS from, say 1 to 10 mV.

SIR bias cl values.png

In the meantime, we read out the current on the SIS. We can then draw the I/V curves as shown above and see whether we get a nice straight curve.

Note: to keep the voltage constant on the SIS mixer, we need some kind of circuit:

sir schakelingen.png

The left circuit is a voltage source. If the current changes due to radiation hitting the SIS mixer, this circuit measures the change and with some filtering adjusts for this so the voltage stays equal. The right circuit is an alternative, an adjustment of the left circuit so it delivers a constant current instad of a voltage.

What is eventually done with the resulting signal? Well, when the signal comes in from the atmosphere, you want to analyze the spectrum (Spectroscopy) and see what kind of gases are there.