[evla-sw-discuss] pulsar data

[Brent Carlson]

Michael,

Some comments.

Brent

Michael Rupen wrote:

> Hi Brent,
> nice to have you back!
>
>> 1. There is indeed one pulsar timer per baseband pair, meaning 4 
>> timers in total. Each of these can be set for a different pulsar 
>> ephemeris. The restriction is that all of the correlations happening 
>> within one sub-band correlator *must* use the same ephemeris. With 16 
>> sub-band correlators, this leaves plenty of flexibility to track 4 
>> pulsars simultaneously.
>
>
> So the four timers implies a maximum of 4 pulsars, each of which would 
> have
> to be observed with a different BB pair. The additional restriction on
> sub-bands implies that in this mode one would NOT obtain a full BB pair's
> worth of bandwidth on each pulsar, but would have to split the 16 
> sub-bands
> amongst them, e.g., 4 sub-bands on each of the four pulsars, or a 
> 8-4-2-2 split, or some such. Is that right?

That is correct.

>
> I fear I confused the issue with all that talk of phasing the array. 
> Since
> you get all the correlations in all the bins, there's no need to phase 
> the array. I was thinking of too many things at once :}
>
> Does one get an "untimed" bin at the same time as all these other bins --
> i.e., the regular correlator output? or can one set things up that way?

No. If you want an untimed bin, you would have to give up one of the 4 
for that purpose. More than 4 is possible, indeed the original design 
had 8, but going to 8 depends on how much FPGA logic is available, and 
on CMIB processing speed...so let's not think of more than 4 for now.

> I'm thinking of doing a time-binned pulsar observation, while 
> simultaneously making as deep an image as possible of the unpulsed sky 
> in that direction.
> I know one could just sum up all the "boring" time bins, but that costs
> a lot in terms of output data rate.
>
>> 2. Pulsar gating and pulsar phase binning are indeed two different 
>> modes, and operate as Michael describes. Given the pulsar phasing 
>> capability, I can't imagine why one would want to use gating...but it 
>> is there with the ability to generate different gates on different 
>> sub-bands to track dispersion.
>
>
> I imagine this will be used mostly to avoid humongous data volumes. 
> Gating
> gives one output data stream, time bins gives many, and if you want 
> lots of channels the data rate restriction might kill you, esp. in the 
> larger configurations and/or lower frequency bands (where you might 
> want high
> time resolution to avoid time-averaging losses and/or RFI).

The data rate with binning shouldn't kill you if you integrate bins long 
enough. Gating effectively gives you one bin, and the rest are thrown 
into the dust-"bin" (ha ha). If bin bunching is used, it is entirely 
possible to have the correlator throw away those bins that are not of 
use so the backend doesn't see them...its a matter of the "hardware 
executable dump script" that is developed.


-- 
Brent R. Carlson
Brent.Carlson at nrc-cnrc.gc.ca
Tel/Tél: (250) 493-2277 (ext. 346) |  Fax: (250) 493-7767
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