Frequency domain cross-correlation fluorometry with phase-locked loop frequency synthesizers

Frequency domain cross-correlation fluorometers may be improved over present designs by providing a coherent signal at one frequency to means for detecting luminescence from a sample, which is elicited by light modulated at another frequency, in which the two frequencies, obtained from a pair of phase-locked loop frequency synthesizers, are different from each other by at least 100 hertz, which is a significantly greater difference than used in corresponding prior art fluorometers. This permits the use of the phase-locked loop frequency synthesizers, rather than the currently used direct synthesis synthesizers, providing significant advantages.