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: http://www.naic.edu/alfa/ealfa/meeting1/minutes/wapp.html
Дата изменения: Mon May 8 23:01:36 2006 Дата индексирования: Sun Dec 23 01:27:13 2007 Кодировка: |
WAPP stands for "Wide Angle Arecibo Pulsar Processer" (but don't be fooled by the acronym!). Further information on the WAPP and its status can be found at Jeff's release notes page.
A WAPP provides 100 MHz BW from 200 MHz dual 8 bit digitizer. Bill Sisk has designed a lag demultiplexer card. There are now 4 of these machines. The spectral line mode is in test phase but has been demonstrated. The current plan is to make 3 more for 7 feeds. This actually requires a lot of effort, but we know it will work.
Max. no of lags per output product (or channel) =
16*1024
------------------------------------------------------
{log_3(N_level)}^2 * N_interleave * N_output_products]
N_level is 3 or 9 [i.e. log_3(N_level) is 1 or 2 respectively];
N_interleave is 1 (for BW <= 50 MHz) or 2 for 100 MHz BW;
N_output_products is 1 or 2 (channel autocorrs) or 4 (Stokes);
& log_3() indicates log to the base 3.
The best resolution for a 100MHz bands, using 16 bits/lag, and summing the polarizations are given by the formulas,
#lags = (i - 1)* 6.25, where i = integration time in usecs
Data rate to disk = 12MBytes/s; Resolution = 100MHz/#lags
| Integration time | Max lags | Resolution |
|---|---|---|
| 100us | 616 | 162kHz |
| 64us | 392 | 255kHz |
| 32us | 192 | 520kHz |
2 channels,continuous sampling
| Bandwidth(MHz) | Data Rate Mbyte/s | Bit Packing |
|---|---|---|
| 50 | 25 | 1 |
| 25 | 25 | 2 |
| 12.5 | 25 | 4 |
| 6.25 | 12.5 | 4 |
| 3.125 | 6.25 | 4 |
| 1.56 | 3.12 | 4 |
| 0.78 | 1.56 | 4 |
| 0.390 | 0.78 | 4 |
| 0.195 | 0.39 | 4 |
Note that the WAPP capabilities include dumping all lags for a mode, a dump time of 0.1 sec, and 3 or 9 level digitization.
| Bob B. | Does it dump the spectrum or the ACF at 0.1 sec? |
| Jeff | Either one; you can choose./td> |
| Eli B. | What is the ultimate limitation? |
| Jeff | The data rate out of correlator into computer. For pulsars, there is a limit in the data rate in wideband mode (100 MHz). Tn 50 MHz mode, it's the disk drive. |
| Riccardo | What about adding more WAPPs to get more BW? |
| Jeff | A proposed enhancement would be to add another correlator board and at 2 100 MHz bands that you place somewhere within the front-end 300 MHz band. |
| Riccardo | What is cost for this in hardware and manpower? Is it feasible? |
| Bill S. | $2K/board plus I am not sure how much manpower. We are now trying to figure out this is a practical option. We have the chips but would have to manufacture boards. A technician has to build the board and chassis. It is possible that we could try to get such an enhancement ready by the time ALFA is ready. A second enhancement would be to provide 200 MHz WAPP LO Control. The question was asked "How about overlapping bands? Must there be a gap in between?". With single control, you could overlap the bands, but then total BW would be somewhat less (say 180 MHz instead of 200 MHz) |
| Bob B. | Is there a software issue here? |
| Jeff | Yes, to develop all the modes, there is a lot of s/w that has to be written. Spectral line mode requires a data formatter that produces SDFits. There is also a CLASS religion. I think data will be writted as SDFits but there will also be a CLASS output. |
| Karen O. | SDFits can be converted to IDL (Carl Heiles has converter). |
| Lister | Quite a bit of progress in RFI excision can be achieved by doing cross-correlation between pixels. |
| Jeff | That might be accomplished by building an 8th WAPP. |
| Desh | Cross correlation done later in processing. Build a pair of feeds, one looks at horizon, one looks at rest of solid angle, that could detect RFI and hence used for crosscorrelation. Not in voltages but in time sequences. |