Tiger Team Week - 13 - 17 April 2015
The goal of the Tiger Team Week is to brainstorm possible causes for the VEGAS bank-to-bank variation problem, and define tests we can do to isolate the origin of the problem. We already have a number of tests defined which we can perform starting on Tuesday morning, but it is still worthwhile discussing other possibilities.
Tsys, the ratio of off/(on-off) times a constant Tcal, as calculated from VEGAS HPC spectral line data is wrong, and can vary (sometimes wildly, depending on setup) from bank to bank and poln to poln.
What I think we know (others, feel free to clarify / add to this list)
- The problem is not present in total power in Mode 1, as measured by the ADCs, or the overall (mean central 80%) Tsys as calculated by GBTIDL. It is present if Tsys is calculated for a narrow portion of the overall Mode 1 passband. It is obvious in Mode 4.
- The problem does not appear to be as obvious in total power, as measured by the VEGAS ADCs.
- Dave MacMahon has evidence it is present (which I have forgotten...)
- They are clearly not as bad as the spectra, when VEGAS is "detuned".
- The problem does not appear to be present when the VEGAS IF modules (or ADCs) are fed with a "well-behaved" lab noise source:
- The ratio appears to be approximately correct (using different input power levels for "ons" and offs");
- The ratio appears to be approximately identical from bank to bank / poln to poln for "identical" lab noise source inputs.
- The problem can be made significantly worse by applying a bad lbw_gain value.
- The problem can be made significantly worse by not applying ADC calibration (we only did this as an all or nothing test, we did not try the individual components.)
- The problem does not appear to be present in the GBT spectrometer. Differences from VEGAS are:
- The Spectrometer is fed by the "Analog Filter Rack" after the Converter Rack, as opposed to the VEGAS IF modules
- The Spectrometer (at least in wide-band mode) has 3-level samplers
- The Spectrometer samples in the second Nyquist zone (I think)
- Anything else?
- The problem does not appear to be GBT receiver specific, implying it is somewhere in the common IF system.
- We have quite thoroughly (but not recently) investigated and ruled out "housekeeping" errors (such as miscalculating exposure times).
- We have not recently looked closely at the behavior of spectra produced by off-line FFTs of the ADC or filtersnap snap blocks.
Hypotheses (speculations?) as to cause
- These is something "nasty" in the GBT IF, which is not seen by the GBT spectrometer, and is not grossly obvious in ADC total power samples.
- This effect either
- varies wildly from IF to IF, or
- the interaction between this effect and VEGAS varies wildly from IF to IF.
- (4) and (5) above might indicate that hypothesis (2b) is more likely than hypothesis (2a).
- Possible causes of this might be:
- Some interaction with the ADC "DC" spikes
- some processing error (e.g. truncation or overflow or something) in the FPGA / HPC code
- Look very carefully at the VEGAS IF Module output and GBT Spectrometer IF inputs, over at least DC to a few GHz, and look for discrepancies.
- split the signal going into the GBT Spectrometer, and feed one copy into:
- VEGAS IF module
- directly into ADCs.
Above may require lab components such as amplifiers / attenuators / filters 3. Look much more closely at spectra derived manually offline from the ADC / filtersnap snapblocks, for existing or new data.
- Do manual (tp_nocal) off/(on-off) observations with VEGAS detuned. Look at results as calculated from the ADCs
- Isolate different aspects of ADC calibration (e.g. just MMCM calibration, just OGP calibration).
- What else?
Monday 13th April 2015
Tiger Team Meeting 2:30pm
- Agree on list of "knowns", add other facts if possible
- Brainstorm above and additional hypotheses
- Define additional tests / data reduction steps which may help further isolate the problem, or distinguish between hypotheses
- Define needed test equipment: GBT Spectrometer working, cables, filters...
- Fully specify individual tests.
- Agree order and approximate time requirements for planned tests
- Identify personnel to perform tests, and process data
- Decide whether we need Wednesday / Thursday
- Any other business