Now that we've seemed to landed on a working configuration, I've re-ran the tests first described in ELOG 11347. I've also compared the real filtering of filter modules to their designs.
TL;DR: No adverse, or even observable, differences have been witnessed.
As a reminder: In c1tst, there are three loops, called LOOPA, LOOPB, and LOOPC.
- LOOPA is a filter module feeding back onto its own input, with a unit time delay block
- LOOPB is a FM whose output goes to the DAC. In meatspace, the AI output is hooked up directly to an AA chassis input, and back to the FM in CDS
- LOOPC includes RFM connections to c1rfm and back again.
Here are the loop delay results, which measure the slope of the phase response of the OLTF. For the purely digital loops (A, C), we know the number of cycles we expect to compare the delay to.
At this time, I haven't done the adding up of cycles, zero-order-holds, etc. to get the delay we expect from the analog loop (B), so I've just looked at whether it changed at all.
Anyways, I've attached the code that analyzes data from DTT-exported text files containing the continuous phase data from the loop measurements.
Before:
Single Model loop cycles: 1.0000000+/-0.0000006, disparity: -0.00+/-0.25 nsec
2 Model RFM loop cycles: 1.9999999+/-0.0000013, disparity: 0.0+/-0.5 nsec
ADC->DAC loop time: 338.2+/-0.4 usec
After:
Single Model loop cycles: 0.9999999+/-0.0000008, disparity: 0.02+/-0.29 nsec
2 Model RFM loop cycles: 2.0000001+/-0.0000011, disparity: -0.0+/-0.4 nsec
ADC->DAC loop time: 338.18+/-0.35 usec
So, the digital loops take the number of cycles we expect, and there are no real differences after the upgrade.
Additionally, for all three loops, I created a simple 100:10 filter in foton, and injected broadband noise with awggui, to measure the real TF applied by the FM code. I want to turn this whole process into a single script that will switch the filter on and off, read the foton file, and compare the measured TF to the ideal shape.
In our system, before and after the upgrade, all three loops showed no appreciable difference from the designed filter shape, other than some tiny uptick in phase when approaching the nyquist frequency. This may be due to the fact that I'm comparing to the ideal analog filter, rather than what a 16kHz digital filter looks like.
What I've plotted below is the devitation from the ideal zpk(100Hz, 10Hz, 0.1) frequency response, i.e. Hmeasured / Hideal. The code to do this analysis is also attached, it estimates the TF by dividing the CSD of the filter input and output by the PSD of the input. The single worst coherence in any bin of all the measurements is 0.997, so I didn't really bother to estimate the error of the TF estimate.
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