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lbaops:daspulsarprocedures [2014/09/19 16:18]
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lbaops:daspulsarprocedures [2016/01/07 13:44] (current)
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 +===== Procedure for disabling DAS gain and threshold loops for pulsar observing =====
 +-------------------------------------------------------------------------
 +
 +Version 19-Sep-2014:​ John Reynolds
 +
 +Updated and expanded notes including a correction for the "​DOS"​ version ​
 +of the DAS interface, which neglected some steps for "​band*2"​ filters ​
 +(e.g. VSOP-type profiles).
 +
 +These procedures apply to observations of pulsars recorded with an LBA 
 +DAS system where the recorded data from an individual station is to be 
 +analysed separately, as "​single-dish"​ data. Where the data are to be 
 +used only for cross-correlation with other stations (VLBI data) these 
 +procedures may not be necessary, at least on smaller antennas, though ​
 +it may be prudent in some cases to follow them anyway, just in case. 
 +The PI may change their mind and decide they need the single-dish data, 
 +or subtle effects may appear in the correlated data between the 
 +large antennas.
 +
 +There are two internal loops in the DAS that try their best to cancel ​
 +any pulsar signal;
 +
 +<​file>​
 +        -the GAIN loop : a voltage-controlled amplifier ahead of the
 +         ​sampler,​ servo'​d to maintain constant 8-bit sampled amplitude.
 +
 +        -the THRESHOLD loop operating on the digitally filtered data
 +         to keep the statistics of the 2-bit output data fixed (usually ​
 +         at 34.6%). This loop operates in a stepwise fashion, resetting ​
 +         the thresholds on a fixed cycle which depends on filter ​
 +         ​bandwidth. For 16MHz filters the cycle is 2^19 samples = 33ms.
 +</​file>​
 +
 +(At Narrabri there is no GAIN loop in the DAS. In the Pre-CABB era the 
 +equivalent loop was located with the samplers in each antenna but could 
 +be disabled from CAOBS. With CABB there'​s no active loop at all).
 +
 +The GAIN loop evidently runs on a faster timescale than the threshold ​
 +loop and was initially considered to be the more damaging effect. For 
 +the Vela pulsar with a period of 89ms (f=11Hz) the dilution and 
 +distortion of the pulse profile is severe. For longer periods the effect ​
 +gets worse.
 +
 +The threshold loop is slower but introduces sharp level-steps on 
 +boundaries of 2^19 samples, which can be very nasty for single-dish ​
 +purposes (e.g. Tid and Parkes data in RadioAstron expt re70a in 2012).
 +
 +So if you're going to turn one of these loops off you should probably ​
 +disable both of them. This is a bit of a pain if you're relying on 
 +keeping the sampling statistics fixed, which will almost certainly be 
 +the case (usually at 34.6%). With both loops disabled the stats will 
 +drift proportionally with input level, requiring analogue level 
 +adjustment ahead of the DAS (e.g. step attenuators),​ or resetting the 
 +digital gain, as described below, when the stats drift out of an 
 +acceptable band.
 +
 +There are two procedures, depending on whether you run the original DOS 
 +engineering version of the DAS control s/w (LBA_DAS.EXE,​ written by Martin ​
 +Osterreich and Dick Ferris), or the GUI version (adapted from the DOS version ​
 +by Simone Magri and Mark Leach).
 +
 +=== Procedure for GUI Version of control software: ===
 +
 +----------------------------------------------
 +<​file>​
 + 1. Wait till you're on source (or levels are stable), then adjust the
 +    pre-DAS attenuation to centre the "​LEVEL"​ bargraph/​slider on each 
 +    IFP.
 +
 + 2. Select "​Detail"​ under "​Monitor view".
 +
 +    For IFP #1, observe and note the average value of the three arrowed
 +    "​Level"​ and "​Counter"​ numbers as shown below. ​
 +
 +    Level  : lvl Auto    <== lvl
 +    X-Th BS: xth Auto                   Y-Th BS: yth Auto
 +    Counter: cnt         <​== cntX       ​Counter:​ cnt       <​== cntY
 +    X-Th FT: ... Auto                   Y-Th FT: ...
 +    Counter: ...                        Counter: ...
 +
 +   Round the average (fluctuating) value of "​cntX"​ by eye to the 
 +   ​nearest EVEN integer CNTX (e.g. 70, 72 etc). If the value is less 
 +   than 64, round it to the nearest multiple of FOUR.
 +
 +   If using a band*2 filter (e.g. VSOP-style profile), also note the 
 +   "​Counter:"​ value cntY, which is displayed to the immediate right 
 +   of cntX. Eyeball-average and round this value cntY to CNTY, as above.
 +
 +   If using a band*1 filter the cntX and cntY numbers should be 
 +   ​identical so use CNTY = CNTX.
 +
 +   For "​lvl",​ just estimate the nearest integer value LVL.
 +
 +   Make a note of CNTX, CNTY and LVL.
 +
 + 3. Do the same for the 2nd IFP.
 +
 + 4. Under "​Config_Sftw",​ select "​Filter BSB"
 +
 +      Set gain control to MANUAL for both IFPs
 +
 +      Set the GAIN value to 4*(255-LVL)
 +
 +      To lock the threshold loops set threshold control to MANUAL, and;
 +
 +      set the X counter value to CNTX
 +      set the Y counter value to CNTY
 +
 + ​5. ​ Repeat for the 2nd IFP.
 +
 + ​6. ​ Hit "​Config_Hdw"​ to upload the revised configuration to the DAS.
 +
 + ​7. ​ On the "​Detail Monitor view" display you should now see for each 
 +     IFP:
 +
 +     ​Level ​ : 128 Manual
 +     X-Th BS:  72 Manual ​      Y-Th BS: 72 Manual
 +     ​Counter:​ cntX             ​Counter:​ cntY
 +     X-Th FT: ... Auto
 +     ​Counter:​ ...
 +
 +     where the values 128 and 72 might be varying by +/- a few.
 +
 + 8. Check that the stats on your VLBI recorder look normal (34.6% of
 +    samples in the two outer bins combined).
 +
 +    Throughout the observation,​ monitor the "​level"​ numbers for each IFP
 +    on the detailed monitor display. Adjust the step attenuators to keep
 +    the top bargraphs centred, or the levels as close as possible to
 +    128 (on the alternate display), if you want to keep the stats as
 +    close as possible to 34.6%. These numbers are in ~0.1dB
 +    steps, so if the lvl drops to say 123, remove 0.5dB to centre it.
 +</​file>​
 +
 +=== Procedure for DOS version of control software ===
 +
 +---------------------------------------------
 +<​file>​
 + 1. Wait till you're on source (or levels are stable), then adjust the
 +    pre-DAS levels/​attenuators to centre the "​LEVL"​ bargraph on each IFP.
 +
 + 2. Hit the space bar to view the alternate monitor display.
 +    For IFP #1, observe and note the average value of the three numbers
 +    "​lvl"​ and "​cntX"​ and "​cntY"​ as shown below;
 +
 +    LEVL: lvl C    <== lvl       ​OFFS: ​  0 C
 +
 +    X-TH: xth C                  Y-TH:  yth C
 +    CNTR: cntX     <​== cntX      CNTR:  cntY     <​== cntY
 +    X-TH: ... C
 +    CNTR: ...
 +
 +    Round the average (fluctuating) value of "​cntX"​ to the nearest
 +    EVEN integer CNTX (e.g. 70, 72 etc). If CNTX is less then 64 round
 +    it to the nearest multiple of FOUR.
 +
 +    For band*1 filters the values of cntX and cntY should be identical,
 +    in which case use CNTY = CNTX.
 +
 +    For band*2 filters the number will in general be different so
 +    take the mean value of cntY and round, as above, to get CNTY.
 +
 +    For "​lvl",​ just estimate the average integer value LVL.
 +    Make a note of CNTX, CNTY and LVL.
 +
 + 3. Type "​1"​ to get into the first page of the IFP1 config.
 +
 +      Set parameter 23 to 1
 +                    24 to CNTX
 +                    25 to CNTY
 +                    26 to 1
 +                    27 to 4*(255-LVL) ​    <= enter number
 +                                             given by this expression.
 +
 +
 + 4. Repeat steps 2 and 3 for IFP2 (type "​4"​ instead of "​1"​ in step 3.).
 +
 + 5. Type "​S"​ to "​Setup"​ the DAS (i.e. to upload changed config to DAS)
 +
 + 6. On the monitor display you should now see for each IFP:
 +
 +      LEVL: 128 M
 +
 +      X-TH:  72 M             ​Y-TH: ​  72 M
 +      CNTR: CNTX               CNTR: CNTY
 +      X-TH: ... C
 +      CNTR: ...
 +
 +      where the actual values 128 and 72 might be varying by +/- a few.
 +
 + 7. Hitting the space bar to return to the default display you should now
 +    see "​Monitoring Only" for "​IF"​ on both IFPs
 +    The associated bargraphs should all be centred.
 +
 + 8. Check, monitor and adjust sampling stats, as for GUI version above.
 +
 +</​file>​
 +
 +JER
 +19-Sep-2014
 +
 +
    
  
lbaops/daspulsarprocedures.1411107524.txt.gz · Last modified: 2015/12/18 16:38 (external edit)