Correlators

A correlator is passive linear SAW tapped delay line with impulse response being a sequence of RF pulses (taps) of arbitrary amplitude and phase. The pulses are all at the Center Frequency (F0) and their repetition rate is called the Chip Rate (B). The number of pulses is called the Code Length (N). Most commonly, the tap amplitudes are equal and the tap phases are a pseudo-random sequence of 0 and 180 degrees. A Correlator is a matched filter to an input signal which is the time inverse of its impulse response. In this case the Correlator output is a spike of width 1/B with sidelobes <-10*log10(N) dB extending +-N/B on each side of the spike. The peak signal to noise ratio is improved by the Processing Gain 10*log10(N) dB. A Perfect sequence has all sidelobes –20*log10(N) dB. The longest Perfect sequence is the Barker Code with N=13.

Applications

Radar pulse compression: Digital phase modulation of an RF carrier generates the transmitted pulse of length T=N/B, and the SAW Correlator in the receiver compresses the return signal from length T to 1/B. The received signal to noise ratio is improved by the processing gain 10*log10(N) dB. The process is quite Doppler sensitive: processing gain will degrade ~1dB for .2/B Doppler.

Communication channel multiplexing: Correlators offer instantaneous code acquisition at the expense of having a fixed non-programmable code.

Correlator Performance

Practical Maximum Values
Chip Rate B MHz 100
Length N 500
Length T=N/B us 20
Center Frequency MHz 500