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The core of the SR530 is a precision analog sinewave multiplier. Lock-ins use a multiplier (demodulator) to translate the input signal (at the reference frequency) down to DC where it can be filtered and amplified. Many lock-ins use square wave multipliers which introduce spurious harmonic responses. The SR530 uses clean sine-wave multipliers which are inherently free of unwanted harmonics.
Unit is missing two connectors on the front (Photograph 4 and 7) and one foot on the bottom better two feet than one. Equipment is offered here powering on, but severely damage with missing connectors at the very least some board work will be required. This equipment is offered here for parts only with no warranty.