Sidebar
There are two loops of interest in the DAS :
- the GAIN loop : a voltage-controlled amplifier ahead of the sampler is servod to maintain constant 8-bit sampled amplitude.
- the THRESHOLD loop operates on the digitally filtered data to keep the statistics of the 2-bit output data at 34.6%.
At Narrabri there is no GAIN loop in the DAS. Before CABB 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 seems to do the most damage to pulsars - presumably this means it runs on a faster timescale than the threshold loop.
Turning off both GAIN and THRESHOLD loops is a bit of a pain if you're relying on the sampling statistics being maintained at 34.6% - they drift proportionally with input level. Unless particularly slow pulsars are being observed I'd recommend turning off only the GAIN loops.
For observing Vela I would recommend disabling the GAIN loops. This is obviously much more important if you're intending the use the single-dish data but probably still worth doing for VLBI since it's a pretty simple procedure.
Here's the procedure, using the GUI version of the DAS software. (DOS version available on request!).
- 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.
- Select “Detail” under “Monitor view”.
For IFP #1, observe and note the average value of the two numbers
"Level" and "Counter" as shown below. If you're disabling the GAIN loops but not the THRESHOLD loops ignore the references to 'cntX', 'cntY' etc.
Level : lvl Auto ⇐= lvl
X-Th BS: xth Auto Counter: cnt <== cntX X-Th FT: ... Auto Counter: ...
Round the average (fluctuating) value of “cntX” the the nearest
EVEN integer CNT2X (e.g. 70, 72 etc). If the value is less than 64, round it to the nearest multiple of FOUR.
For “lvl”, just estimate the nearest integer value LVL.
Make a note of CNT2X and LVLX.
If using a band*2 filter (VSOP-style profile), note cntY and round
to CNT2Y. (If using a band*1 filter the X and Y numbers should be identical).
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)
If you want to lock the threshold loops as well, set threshold
control to MANUAL, and;
Set the X counter value to CNT2X
Set the Y counter value to CNT2Y
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 [if you've disabled THRESHOLD too] Counter: cntx X-Th FT: ... Auto Counter: ...
(the numbers 128 and 72 might be varying by +/-1)
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 the top bargraphs cented, 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.
