autocorrelator

An autocorrelator is an electronic instrument that compares a signal against a time-shifted version of itself to reveal periodic patterns buried beneath random noise. The device produces a correlation function by multiplying the incoming signal by delayed copies of itself, then integrating the result over time. Where noise is random, these products average toward zero; where the signal repeats, they reinforce, producing a peak in the output.

The physical implementation varies by application and signal bandwidth. A basic autocorrelator contains a delay line (a length of coaxial cable, a charge-coupled device shift register, or a digital memory buffer), a multiplier circuit, and an integrator. Radio frequency autocorrelators use diode mixers and video detection; microwave versions employ directional couplers. Digital implementations perform the correlation in software after sampling, which allows arbitrary precision and multiple delay values simultaneously.

Finding signals hidden in noise

Autocorrelators excel where conventional detection fails. A weak radar return scattered by rain, a modulation scheme buried under thermal noise in a receiver, or periodic vibration masked by acoustic clutter all become visible when the signal's own structure is used as a matched filter. The output peaks at delay values equal to the signal's period, its harmonic periods, or the pulse repetition interval in radar and sonar. The peak height depends on signal strength and the integration time; longer integration times increase sensitivity at the cost of slower response.

Practical limitations come from the integrator's finite time constant and the delay line's maximum length. A signal with period longer than the available delay line's span will not produce a peak. Nonlinearities in the multiplier and integrator degrade the correlation function at small signal levels. Environmental temperature changes shift the delay line's electrical length, particularly in RF designs, requiring careful calibration.

Autocorrelators remain standard in radio astronomy for detecting pulsar signals, in fiber optic coherent receivers for measuring laser linewidth, and in test equipment for characterizing phase noise in oscillators. The term derives from the mathematical operation: correlation of a signal with itself, as opposed to cross-correlation between two different signals.