What is the ALV-LinCorr ?

The ALV-LinCorr is a real-time digital Linear Tau Correlator based on the ALV-MultiCorr platform. It implements a very fast correlator block with 9 x 9 bit input data resolution and 32 bit store (higher bit store accumulation to 64 bit is performed on the PC) and is capable of computing up to nearly 10 GigaCorr/s. It allows the computation of either a single (auto or cross) correlation function with 2048 channels or two independent (auto or cross) correlation functions with 1024 linear correlation channels each at a fastest sampling time of 250 ns at 100% efficiency and fully real-time.

Faster sampling times can be used at 100% real-time if the number of correlation channels to be computed is reduced, in detail, a single (auto or cross) correlation function can be computed at 137,5 ns sampling time for 1024 correlation channels, 75 ns sampling time for 512 correlation channels, 37,5 ns sampling time for 256 correlation channels and 25 ns sampling time for 128 correlation channels.

For all modes, but the 2048 channel mode, additional 32 correlation channels are computed at lag times 2015 ... 2047, which can be used to establish a “measured baseline”. Furthermore, a full set of monitor channels and sample counter is computed which allow the usual normalisation schemes to be applied.

Four LVTTL/TTL inputs can be applied, allowing the computation of even two independent cross correlation functions in the DUAL 1024 channel mode, the pulse pair resolution of the inputs is better than 5 ns.

The ALV-LinCorr is interfaced to the PC via a fast EPP interface and features correlation data update rates of ~5 full correlation data transfers per second. It has maximum power consumption of slightly more than 1 W (usually less than 0.8W), and thus shows a remarkably high Corrs/mW ratio of ~10 Million Corrs/s per mW.

Why a Linear Correlator ?

Multiple Tau Digital Correlation has proven to obtain optimum statistics for decaying correlation functions (exponentials, hyperbolics ...). However, some experiments show additional oscillating structures in the correlation function. Multiple Tau, integrating over these fluctuations quite quickly, may no longer process optimum statistics in these cases (although has been proven to still work well) and a linear correlator should be used instead. Since most of these experiments show quite fast oscillations (either due to high frequency modulation of the light in the MHz regime, or simply by the underlying physical process), a high speed linear correlator with “enough” correlation channels becomes necessary. The ALV-LinCorr, using sampling times below 40 ns @ 256 linear correlation channels, as well as offering up to 2048 linear channels at still “fast” 225 ns sampling time is perfectly suited for such experiments.