2007-11-28 Testing optimization

After running the Fiske macro for the setup part, we have a nice value for the FFO Control Line (CL) current. Funny enough when we use this value, the macro immediately returns, saying that setting this CL will make the FFO voltage higher than requested.

Thinking about it, this can either be a measurement error or something else. I tried to rule out measurement error by running the 'measure offsets' routine.

Later, I found out several things. Firstly, the FFO bias and CL weren't set to 0 before attempting to run the Fiske macro. This is important because otherwise with a second scan we would get different results. This turned out to be the culprit.

The next problem was that the macro reported not being able to find the FFO voltage start. After checking the output of the macro, I found this funny: the macro reported that it tried to use the previously found settings.

In other words: the previous run found a certain voltage when setting FFO bias and CL. Trying again with a slightly lower FFO bias and it fails -- while the physics tell me this shouldn't happen.

I tried upping the Find Step Up value, but this didn't help. After discussion and a cup of coffee, it turns out that I wasn't making the same settings at all: the FFO start voltage in setup step 2 is NOT the same as the FFO start voltage in setup step 1...

When that was done, I still found out that the macro returned value 1.0237 as the first result, while I had put the limit at 1.0231. It was besides the point, but I needed much more values so I made the FFO CL scan step not 40 times bigger than normal scan, but 10 times. This resulted in not 13 pairs (of FFO voltage/CL current) to choose from, but up to 32. This again was pushing the limits of themacro, since the macro is limited to 32 and if it hits that limit, it'll return an error code. So I switched it back to 20 times which seems enough for now.

Still I encountered the following:

FFO CL FFO Bias Resulting FFO voltage

So: setting the same FFO CL but a different lower FFO bias resulted in a higher FFO voltage! That's physically not possible so something had to be going on here. After adding debugging,... the problem disappears. This is good for speed, but not so for understanding.

The next problem seems to be that the macro output is misinterpreted; there's a different code for getting the right FFO voltage: with stepping up and without. That last situation means: the first FFO bias/control line we set, immediately results in a good voltage. I didn't account for that.

The macro now had the problem that after a number of succesful scans with not enough points, results came up where the Vstart couldn't be found. A programming error caused the FFO bias to be constant, when it actually should be lowered with a small step. After fixing that, it turned out that the current stepsize wasn't good enough either. This is because if you keep the FFO CL the same, but you lower the FFO bias, you get a slightly lower FFO voltage as a result. Thus with each FFO bias step down, the stepsize should be increased to find the FFO Vstart again.

That stepsize for the FFO CL is 0.02 mA. This amount covers a resulting range from Vzero to Vstart of roughly 0.8 mV. (The point Vzero is the FFO voltage that results from setting the last measured FFO CL). Each decrease of the FFO Bias causes a decrease in Vzero that is on average 0.001236 mV. The area thus increases with 0.1235 percent. Equally the stepsize should be increased with this percentage as well. Alternatively, we increase the number of steps that the macro does -- but since that makes the routine slower, I prefer the former solution.

After making the changes, the routine still can't find Vstart after a couple of FFO bias decrements. Analyzing the area that's covered from Vzero to Vstart, but this time in raw values, it seems that it increases by about 3 bitsteps every FFO Bias decrement. This is a lot since the actual area between Vstart and Vstop is only 3 bitsteps in and of itself! Since it's all too easy to make the step up size so large it passes over Vstop, I increase the maximum number of steps in the macro. It's now 32 and I increase it to 128.

The first test I run, this number becomes irrelevant. The procedure bumps into two Fiske steps. The first is bridged by the increasing FFO CL, the second isn't. The macro thus returns that the scan steps are too small, when in actuality, we just can't cross the gap over the second Fiske step and thus the target Vstop is never reached. Should we increase the scan step? I don't think so -- that'll make the macro give less results between Vstart and Vstop. Should we increase the number of scan steps? I don't think so either -- it's not a goal in itself to get to Vstop.

The answer lies in the analysis: the result of the macro should be returned not as an error. This is a situation where there's enough data and we should analyze it instead. However, this could also be an error situation when we're in the first scan after the setup.

After fixing this came the case where a programming error kept causing the analysis to fail and say that every detected point was to the left. Then another where the voltage range around the center was a factor of ten too small.

What's still happening is that with every FFO bias decrement, the Vzero-Vstart range gets larger and larger. The FFO CL needs to be looked at, something like where the previous nice value is copied and passed to the macro.