Table 10

Table 10.3 Selected correlator modes

# of Digitizers	Bandwidth/ Digitizer	Cross-pol Products?	Channels/ Product 2 bit X 2 bit	Channels/ Product 2 bit X 4 bit	Channels/ Product 4 bit X 4 bit
8	2 GHz	Yes	64	--	--
8	2 GHz	No	128	--	--
8	1 GHz	No	256	128	64
8	500 MHz	Yes	256	128	--
8	250 MHz	No	1024	512	256
4	2 GHz	Yes	128	64	--
4	1 GHz	No	512	256	128
4	500 MHz	Yes	512	256	128
4	250 MHz	No	2048	1024	512
2	2 GHz	Yes	256	128	64
2	1 GHz	No	1024	512	256
2	500 MHz	Yes	1024	512	256
2	250 MHz	No	4096	2048	1024

This table is meant to update that in Chapter 10 of the Project Book for the specific case of operation of the ALMA Baseline Correlator in a 4 bit 16 level mode. The default personality of the correlator is as defined in Table 10.1, reproduced above. The 4bit mode could be available through the downloading of a new ‘personality' into the correlator chip, an operation which could be accomplished very rapidly (seconds) in the array. Under this new personality, the correlation format line above would be changed to 4 bit 16 level and operation would be restricted to bandwidth per digitizer of 1 Ghz or less, and at each mode half the number of channels would be available as would be available using the default personality. For many scientific applications, a gain in sensitivity is achieved with very little cost in throughput. John Webber explains further:

Normal multiplication = multiplier is 4-level X 4-level

2 X 4 multiplication = multiplier is 4-level X 16-level

4 X 4 multiplication = multiplier is 16-level X 16-level

This is accomplished in the following manner. In normal mode, the multipliers in every chip are used to multiply the two most significant bits coming out of each FIR filter (which carries 7-bit precision in its final stage). In 2 X 4 mode, half the elemental multipliers work on normal data, and half work on the two most significant bits from one antenna and the two next most significant bits from the other antenna, but at the same delay values as the normal multipliers for that particular channel. In 4 X 4 mode, 4 different cross- products are calculated for the same delay values:

"high" bits X "high" bits

"high" bits 1 X "next highest" bits 2

"next highest" bits 1 X "high" bits 1

"next highest" bits 1 X "next highest" bits 2

In either 2 X 4 or 4 X 4 modes, it is up to software to reassemble the various products from the accumulators which service them in order to get the full-precision product.

It is because some multipliers must be assigned to the extra cross-products that the frequency resolution decreases.