Allowed "multiple regions modes" for the ALMA bilateral correlator

Date: Thu, 09 Dec 2010 10:05:11 -0500
From: Rich Lacasse <rlacasse@nrao.edu>

Hi Todd,

The table you have below is correct. I'd like to point out two addition things (that you probably already know)

1 - Multi-region modes are a complete no-brainer for the correlator. The high level software just provides DDS frequencies that set the center of each of the 62.5 MHz bands. We download those and the the multi-resolution modes just happen. At the output end, the high level software also has the burden of dealing with the 62.5 MHz bandwidth outputs as groups of disjoint filters or as sets that need to be joined together.

2 - In practice, when reconstructing a spectrum out of 62.5 MHz chunks, the 62.5 MHz filters are made to overlap, resulting in less bandwidth for all modes with bandwidth > 62.5 MHz.

Rich

• Multiple region mode allows one to define multiple windows of equal bandwidth that can be placed anywhere in the 2 GHz baseband. See ALMA Memo 556. These are the ten supported combinations:

Total BW	Number of regions * Bandwidth of region
2 GHz	not supported
1 GHz	16*62.5 MHz	8*125	4*250	2*500
500 MHz	8*62.5	4*125	2*250
250 MHz	4*62.5	2*125
125 MHz	2*62.5
<125 MHz	not supported

• The corresponding velocity widths in the different bands are given below. This takes into account the 6.25% overlap required (which reduces the total velocity width) when multiple 62.5 MHz windows are combined to produce a broader window.
• All the filters are individually tunable, so you could play "mix-and-match". For instance, in the 1-GHz bandwidth modes, you could use one quadrant to observe 8 * 62.5 MHz regions and 1 * 500 MHz.

Number of windows per baseband:		2	4	8	16
Number of windows in 4 basebands:		8	16	32	64
Band	Frequency (GHz)	500 MHz	250 MHz	125 MHz	62.5 MHz
3	100	1406 km/s	703	351	187
4	150	938	468	234	125
5	183	769	384	192	102
6	230	611	306	153	82
7	345	408	204	102	54
8	460	306	153	76	41
9	690	204	102	51	27
10	810	173	87	43	23

-- ToddHunter - 2010-12-09