[evla-sw-discuss] pulsar data

[Brent Carlson]

Some comments on Tim's 2 cents:

Tim Hankins wrote:

>My 2 cents:
>   I've hardly been in the loop on the correlator discussions, but  
>I'd like to offer a couple of comments on today's discussion.
>   1. Gating: Multiple gates per period, e.g., on-pulse and off- 
>pulse, are desirable for HI obs or other observations where one is  
>looking for weak emission near the pulsar. Obviously separate  
>accumulators are required. Does phase binning take care of this?
>  
>
Gating works to only accumulate data when the gate is on. When the gate 
is off, the data is ignored. Thus, phase binning is far superior. Gating 
is like phase binning, but with keeping only one bin and tossing out the 
rest. The only time when gating has the edge, is for very narrow gate 
widths that can't be handle by phase binning.

>   2. Specification of pulsar ephemeris: on my own data acquisition I  
>specify the pulsar name and frequency, and the "system" computes the  
>polynomial at the start of the observation (takes about 2 seconds),  
>based on pulsar ephemeris PARAMETERS obtained from  people who  
>generate these things for a living. This has proven to be far more  
>reliable than independent polynomial generation and subsequent  
>transfer to the observing system. And it allows flexibility in case  
>of interference, receiver problems, etc.
>   3. Realtime pulse phase adjustment: For many pulsars the predicted  
>phases are pretty good, even a year ahead. But I suspect that there  
>will be many interesting pulsars for which they are not so good. The  
>ability to refine a gate or sampling window in real time would be  
>very nice, (I do it with mouse driven cursors; credit to Jeff Kern)  
>and may lead to improvement in efficient array utilization. Going  
>back to the same pulsar two weeks later may just not work.
>  
>
With evenly distributed phase binning there is no need for feedback to 
adjust the pulse phase. With phase bin bunching, the feedback allows 
more bins to be placed in the part of the pulse period of greatest 
interest. Phase bin bunching is entirely a real-time s/w feature so 
could be added afterwards.

>   4. Binning: The observer will want as much time resolution as he  
>can get, so the ability to "bunch" bins would be nice. If not, can  
>the number of bins per pulse period be a power of 2 for more  
>efficient post-processing FFTs?
>  
>
Sorry...but two banks of 1000 bins is it. The rest of the phase bin 
memory (24 locations) is required hold auxillary data (timestamp etc.). 
You could just use 512 bins of the 1000, though :-)

>   5. Multiple subarrays: I would guess that the most often use of  
>this capability is for multiple frequency observations of one pulsar.  
>Sounds like this is no problem.
>   6. I don't recall if there is a (digital) output "spigot" like the  
>current "analog sum" for phased array use. VLBI? Real fast, single  
>pixel, backends?
>  
>
There will be a digital phased output, with minimum 1 GHz phased 
bandwidth. A requirements doc for the board to do it is available if you 
are interested.

Brent

-- 
Brent R. Carlson
Brent.Carlson at nrc-cnrc.gc.ca
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