LASER ANEMOMETER FREQUENCY TO D.C. CONVERTER

A method and apparatus for converting frequency information to a DC voltage for use particularly with a laser velocity measuring system whereby a laser beam is passed through a flowing media, and a portion of the Doppler shifted beam is optically combined with the non-shifted beam to produce a fixed homodyne signal which is then applied to a photodetector to produce a modulated electrical signal. This modulated electrical signal is then amplified and delayed by a fixed or variable time interval. Next, the delayed and undelayed signals are mixed together and the output integrated or applied to a low frequency filter to produce a DC signal having an amplitude proportional to the cosine of a constant times the velocity of the flowing media. The amplitude of the coefficient of the cosine function can be readily ascertained by adjusting the variable time delay line until the minimum or maximum value on the DC meter is observed, and then the velocity can be determined by adjusting the time delay until a DC meter has an average fluctuation of zero at which point the speed can be quickly determined from the value of the time delay. Alternatively, the delay line can be fixed and the signal from the flowing media can be mixed with a variable oscillator output signal to produce a signal at any given frequency which will give a zero reading on the DC meter. In a further embodiment a hold and track circuit is employed to cause the output to retain the previous tracked value whenever an interval occurs in which the signal is predominantly noise. In another embodiment a feed back loop is employed to vary the frequency of an oscillator, whose output is mixed with the velocity signal, in order to maintain a null at the output. In yet another embodiment, a digital system is employed to compare the frequency of the velocity signal with the frequency of the signal from a variable frequency oscillator.