We did some testing of the new Fiske step software yesterday. To see how the device (the SIR chip) behaves, we first ran a plot where we set the FFO bias current and read out the FFO bias voltage.
Some plots of an area with Fiske steps, where the Y axis is the FFO bias current and the X axis is the FFO voltage:
If we make a much finer scan, it looks like this:
What is basically seen, is a cloud of points that is formed by setting the bias current on the FFO and then reading out the voltage. Each line means a different current setting on the FFO control line (FFO CL). (For an explanation of the SIR including FFO control line, see entry 2006-04-24 SIRs for dummies).
Note that we've scanned for a limited number of control lines.
Now if we want to have the FFO beam at a certain frequency, we calculate which voltage we need by dividing the frequency with the Josephson constant. To make it easy to understand, say we want to find a Fiske step at 0.7 mV.
Some research was done by the Russian researchers and what came out is that the procedure to find a good Fiske step must be done by setting the FFO bias, then proceding to increase the FFO CL. If no good Fiske step is found, the FFO bias must be lowered, and again, the FFO CL must be reset and increased again until a certain point.
So there are two loops going on; we loop the FFO CL from high to low and get a bunch of value pairs -- FFO bias voltage and FFO CL current. For each loop, we lower the FFO bias current. Basically, you get a horizontal cut from the plots seen above.
You could just follow the lines that are drawn above, which connect one FFO CL setting. If you would do that, you'd get results with the same FFO CL setting. This migh seem logical when looking at the plots above, however, we follow the advice of the Russian team on this point.
Let's see if we can find some numbers that a Fiske step procedure should use. I've graphically extrapolated picture 1 as follows:
The blue lines are extrapolated clouds of points. The green line is a possible combination of FFO bias current and FFO voltage. The fat green line could be a possible scan area where we want to find a good Fiske step.
What you can see is that if you start looking at 32 mA for a good Fiske step, you will keep scanning down until you hit 27 mA. If you had begun at 32.5 mA, you would immediately have hit a good point. Scans should thus cover at least 5.5 mA.
However, there's another input we must keep track of: the setting of the FFO control line. I haven't displayed the plot here, but for each milliampere change in the FFO bias, we upped the FFO CL 1.2 mA.
Right, so how do we know we should stop the Fiske step procedure? Then we'll have to look at the second plot again and see how wide those clouds are. Roughly it looks like it's 2.5 uV (yeah that's microvolts) wide. If we do a sweep of at least 10 settings on the FFO CL current where we make sure we have a result of the FFO voltage with a width of 5 uV, we can see if the points that come out are centered around the target voltage (e.g. frequency).
Some questions remain:
- Is our measurement accuracy good enough for getting results with a width of a couple of hunderd nanovolts? What FFO CL current do we scan with in that case? Is our DAC accurate enough to set that current?
- Is that FFO CL increment of 1.2 mA (each "outer loop" of the scan) a good idea? Why not more or less? How is this influenced during flight?
- Should we read out the SIS voltage and compare it with its optimum?
- How many uA must the FFO CL be increased to measure -- within 10 uV -- a number of ten points? (The points being FFO bias voltage measurements.)
Those questions might be answered as follows:
- This is device-dependent and must be characterized by making FFO plots and seeing at which FFO CL the SIS mixer has the correct voltage.
- The SIS mixer has one optimum voltage which can be seen by pulling a SIS plot. There already is a routine which optimizes the FFO control line and we need to think about how this relates to the Fiske step procedure.