Abstract: The mix-lookin detection technique for high frequency signal measurement with low frequency look-in amplifier is presented. This technique being used for junction capacitance measurements of low resistance semicondnetor deoices. satisfied results are obtained. The operatinal frequency of this apparatus is continuously tunable within 400 kHz-4 MHz nange. The junction capacitance of devices, which junction resistance is as low as 50 ohm, can be measured. The measurement results for Hg Cd Te photodiodes are presented.