Number of Antennas

The number of antennas is hard-wired in from the beginning, and would be very difficult to change after the correlator is built. With the chip design proposed in Memo 166 the correlator works most naturally in multiples of 8, e.g. 40 and 80 antenna designs are simple scalings of one another, while using the same design for 75 antennas would be inefficient. The design is probably optimal for 64 antennas, with 72 and 80 following in that order. Handling a bandwidth of 16 GHz per antenna for more than 80 antennas would present significant challenges for this design:

• The number of inherently unreliable high-power power supplies becomes even more of a worry with a larger array: the power requirement goes up faster than linear, perhaps as the number of antennas to the 1.75 power.
• The number of signal wires increases with the increase in the number of antennas: a 100-antenna array would require over one hundred thousand 125 MHz interfaces, i.e. 12,800 cables (at 8 interfaces per cable).
• When the number of antennas gets so big that an array of correlators must be split between two rack bins or even two racks, the number of cables will suddenly jump by a factor of two (because every signal that drives the correlator array must go two places). For this reason the current conceptual design will probably not extend gracefully beyond a 100-antenna array. A 128-antenna array would require a matrix of chips to fit on one card, which is not very attractive simply from a physical standpoint, and would probably have a power dissipation that would be difficult to live with. A 100-antenna array would require a matrix and is also not pleasant to contemplate.
• As more racks are required, the length of each cable increases, making all cables more difficult to control to ensure proper data capture - the signals from all the antennas must be maintained to within a few nanoseconds, everywhere within the correlator.

While a 100 or 125 antenna correlator may not be impossible, the likelihood of an unreliable system or even an outright failure will increase as the array gets bigger (at a higher than linear rate).

Unfortunately one cannot put off the decision on the array size very long. One would not like to work on the design of a correlator system for more than a few months without knowing the final array size. The number of antennas in an array is fundamental to the design of a correlator. A lot of the very early work on the systems aspect of the correlator design has to do with geometric considerations as to how many what per who (how many antennas per chip, how many chips per card, how many cards per bin, how many bins per rack, how many racks per system). All of these considerations are to some extent interdependent and a good design tries to optimize all of them at the same time to the extent possible. For example, the array size might indicate an advantage of a matrix of correlators in the custom correlator chip over a matrix. Thus without the final array size, the best custom chip configuration will have to be guessed at. Putting some restrictions on the array size, like 64, 72, 80 or 88 antennas, would help but the exact size would be much better.