Teardown & Analysis of an Anritsu MN9610B Variable Optical Attenuator

In this short episode Shahriar takes a detailed look at an Anritsu variable optical attenuator operating in the L- and C-Bands. The mechanical attenuator is defective and provides an opportunity for complete disassembly and examination of the free-space optics, mechanical components and electronics.

The electronics drive a pair of DC motors with potentiometer decoders in feedback along with operational amplifiers and a DAC. This allows the microprocessor to set the exact position of the motors based on calibrated values stored on EPROM. The free-space optics consists of two back-to-back disks with radial metal coatings. Depending on the position of each wheel, the coating limits the transmission of the light through the wheel causing attenuation. The entire housing is hermetically sealed to prevent degradation of the disks.

One comment

  1. Max Hadley says:


    It looks as if the motor servos have both position feedback from the pot, and velocity feedback from what looks like a tachogenerator at the end of the shaft. Perhaps they are using a nested feedback loop system: the outer, position feedback loop senses the position error and gives a target velocity setpoint to the inner, velocity feedback loop. This loop varies the motor voltage to reach the required velocity. The reason for this is to give a servo with fast response and good stability under varying load conditions. In turn, this might be necessary because the rotating shaft seals can have significant ‘stiction’ varying with temperature (and possibly also with barometric pressure – there doesn’t look to be a breather for the optics enclosure).

    If you haven’t junked it already, an analysis of the servo system might make a good tutorial video!


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