An autodyne frequency tracker for laser Doppler anemometry

A frequency tracker has been developed that produces a running average omega av of the frequency omega s of the laser doppler signal which is weighed according to the relation omega av=a2 omega s/a2, where a is the signal amplitude. It is shown that this mode of processing most effectively averages noise and ambiguity fluctuations. A practical system is described which operates at a Doppler frequency of up to 10 MHz. Measurements support predictions of response time (2 mu s at 1 MHz), accuracy and response to noise and ambiguity fluctuations. Measured long term accuracy is 0.2%.