the photocell is used to aim the target, so you need to get the sync signal before you to integration.
the photocell is a photo-resistor, so the signal may be DC+AC, but the interesting part is the time that the "slit" passby and return, you can always have a SIN+DC signal from your ADC, but you dont know the time that the movment direction changes. The meter part of this collimator is not used to directly read the result of angle, but only to tell you if the collimator is perfectly aimed at the same position (by adjusting the drum wheel). so we are not measuring the voltage of photocell, but trying to measure the time of the vibrating slit whenever change direction. the sync signal is necessary and vital.
the original circuit does have a phase sensitive rectifier, the reference phase signal is supplied by the 3-15V winding from the mains transformer while the vibrating slit is also powered my the same transformer, so the reference phase from the mains transformer is synchronized with the amplified photocell signal
i'll post the test result later
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