2007-08-09 Fiske steps

I've previously explained the SIR chip, so I'll keep it short and say that currently, we're implementing a procedure to automate the setting of the frequency with which the FFO (Flux flow oscillator) beams.

This frequency is determined by the voltage that's set on the FFO. If you multiply that voltage with the Josephson constant (483 597.9 * 10^9 Hz V^-1), you get the frequency.

But we can't set that voltage straight away. We first set the FFO current. We then measure the resulting voltage to see if we're on the right way.

There are two circumstances here. We have on the one hand a Josephson junction (a special superconducting circuit); the SIR chip its temperature is brought to about 2 Kelvin. On the other hand, a magnetic field envelops the FFO. That is due to the control line. This is a conducting line which is etched below the FFO on the SIR chip. When we set a current on the FFO, a magnetic field results.

When you combine these two circumstances at a certain FFO bias voltage (and thus a certain frequency), Fiske steps can occur. From what I've gathered so far, a Fiske step is a certain voltage range that cannot occur when you set a certain current and a certain magnetic flux on a circuit. 1)

So my electronics colleague created a macro, which is a list of instructions for the Telis FPGA. This procedure does the following:

  1. Determine which frequency we need; using Josephson constant, determine FFO voltage
  2. Establish a lower and an upper boundary voltage in which we will search
  3. Set the FFO control line current
  4. Set FFO current
  5. Read FFO voltage
  6. Compare readout with the wanted FFO voltage
  7. If it's too big: quit
  8. Start lower boundary loop (see below)
  9. Decrease FFO current in small steps, reading out FFO voltage; we probably skipped the lower boundary and want to get close to it
  10. Start upper boundary loop (see below)

Lower boundary loop:

  1. Increase FFO current in large steps
  2. Read back FFO voltage
  3. Continue until we've passed the lower boundary

Upper boundary loop:

  1. Increase FFO current in small steps
  2. Read back FFO voltage
  3. Continue until we've passed the upper boundary

We now have a set of points. These must be looked at to see whether we need to choose a new value for the FFO control line and whether the procedure must be started again.

Below is the output of the oscilloscope, where the X-axis displays time and the Y-axis displays the FFO voltage. This is a test situation where a simple resistance is used instead of the FFO.

FISKE Steps test 01 small.jpg

Larger picture

1) Problematic in this case is that there is some hysteresis. If you lower the FFO control line, other Fiske steps occur. If you raise it again to the previous level, the Fiske steps are not the same anymore. So you'll have to steadily work your way down, assessing the merits of each control line setting and stopping when you think you've reached a correct setting.