We consider the selfhyphen;beating of incoherent light from an extended source at a widehyphen;band photomixer. The intermediate frequency mean squared current is found to be proportional to the product of the direct detected source power and the power within the i.f. bandpass about the optical frequency and within the heterodyne field of view. Thus the ratio of the heterodyne signal to selfhyphen;beating power is proportional to the ratio of local oscillator power to direct detected power. This ratio is usually large for systems employing efficient local oscillators that generate shothyphen;noisehyphen;limited operation. In less efficient systems, such as those employing tunable diode lasers, the effects of selfhyphen;mixing may be significant.
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