No Fringe, or Fringe vanished? Try This!

This document is based on the experience from control room. More points can be added by individuals if they have, which are not covered in this document.

No Self?

• Make sure that RF, LO, IF & BB has been set properly, if not set it again.

• Make sure the bandshapes, from correlator o/p to see the baseband is properly set and the channel used (in matmon) to see fringe has proper power in it. Some time baseband fails to set one set of antenna and all such antennas remains at old settings. But this happens for 4 antennas (one pol[130 or 175] only) at a given time.

• If self at corr o/p is zero, and BB power is OK, try noise in correlator and make sure correlator is giving right o/p power. If it is, there is likely to be a problem with connecting cables from BB to CORR. If correlator is not being used for observation, self test can be done.

Self ok but no fringe?

• Set LO and confirm LO LOCK.(LO monitoring need to make robust!)

• Check if existing pointing offsets are not loaded.

• Find out whether feed is not rotated or is in acceptable range of counts given by theodolite experiment.

• If it is just start of new Frequency after long time. make sure swap/unswap has been taken into account, try swapping RF in such antenna and update the RF-swap table accordingly.

• If calibrator is being observed, a small pointing can be done to see if the there is pointing offset by loading different offsets w.r.t. the existing one, rather than taking antenna out of subarry. Same test can be done in case antenna is giving low fringe amplitude. User need to load pointing offsets in steps of the beamwdth of current Freq. in observation, which is usefull to save time. This happens mostly if feed is rotated for some reason after the last pointing was done.

• Make sure antenna/ABC/servo takes the proper source co-ordinate (target) and track accordingly. 'sacw' window in ondisp(ONLINE) should always show zero error below that antenna for elevation and azimuth while antenna is in tracking.

• If simultaneously five antennas are failed, and they are from same FFT control card, try corr_config, or let correlator expert attend it.

• If only CSQ antennas are showing fringes and not arm antennas, it means either 'fstop' is not running or das has been given wrong source or somehow 'fringe stop' has failed.

• If self power keeps on fluctuating, or showing constant high power, it is likely to be a RFI in that channel only and in that antenna only (in case the RFI is generated in system of same antenna). check the bandshape, and matmon in other channel.

If fringe amplitude is low

• Check phases on calibrator. If phases are winding, find out if any other antenna which shows similar winding. If such a pair exists, check the connectivity of cables in OF/BB/corr or mapping in corrsys.hdr, for whether they have got swapped. If there is such a pair for which there is no fringe amplitude and phase for no clear reason, and the pair form longer baseline, the problem is most likely to be of this sort.

• If fringe amplitude is low and phase is showing slightly more or no scatter, then check if the RF noise has left on.

• Same, if fringe amplitude is low and phase also showing somewhat more scatter, check if the antenna need RF swap and that antenna has more polarization leakage, so that the fringe amplitude user get is not real but leaked.

If fringe vanished Suddenly:

• Check if servo has applied the brakes, OR ABC has got reset.

• Check if that antenna is showing 'flat bandshape' at correlator output.

• If fringe falls gradually, it's most likely that, ONLY ABC got reset and other electronic, viz. RF, LO, IF, are still ok, but servo stopped tracking as effect of ABC reset.

• Check if RF switch has got operated, in that case, ABC starts giving ABC T.O. and self power in matmon falls to very small value and 30-1 monitoring also show the fall in power leve.

• Check if feed has got rotated, few antennas had problem of feed rotation on power on reset.(this test can be ignored if such problem does not exists.)

• Make sure the antenna setting was not got changed (by mistake) with some other antenna under maintainance.

• If fringe in all antennas has gone down by some extent, it's more likely that, while loading pointing offset in one antenna, the same offset has got loaded in all antenna by mistake.

• If fringe in all antennas suddenly stopped, check the master LO is OK, or correlator is getting STA as well as GPS clock (using irq_seq)

• Check if LO1 has been become unlock. (more technical clarification needed here, for can this really happen)

If all antennas showing no (or unstable) fringe:

• check if proper CLK_SEL was selected in 'init.hdr.new' file and the same is given to 'dlytrk' with a option '-clk_sel'. Also make sure appropriate BB bandwidth selected.

• Make sure, by using 'showband' that the channel selected for seeing fringe, in matmon is not affected by any unstable features like, RFI or band oscillations.

• Watch if the error in 'acq30' o/p is showing 'processing lagged behind'. As effect of this record does not print the 'number of STA cycles' (under title "wt") same as given (e.g. while in no error condition, if LTA1=16 & LTA2=8, the wt should be equal to 128, a product of LTA1 & LTA2, in error condition this number will be lesser than that.)

• For individual antennas, if fringe started showing unstability, check if that antenna has started showing EDO.

• If all antennas showing fluctuation in phases as well as fringe amplitude, it's well known ionosperic scintillation.