To take advantage of the algorithm at any time, enable the Calibration mode and pass over 40 fringes.  The digital processing also provides an accessible troubleshooting mode. Parameters such as signal  amplitude, laser current, and signal speed are exportable.   To maintain a constant input signal level to the A/D converters, an automatic gain control (AGC) loop  varies the VCSEL current. This loop uses the vector magnitude of the main track as the control. If the required magnitude cannot be obtained (to within a predetermined percent of the target amplitude), the  circuit will switch to the redundant VCSEL.   Operation of the index circuitry allows measurement of the index VCSEL power at all times except at the  index. This measurement determines whether to select the back-up VCSEL. Due to the large operating signal range of the index circuitry, it is not necessary to use AGC. The fringe co unting and index circuitry  are robust (see the Fringe Counting section below).   Formatting the output signals is perfor med within the FPGA. A low voltage differential signal (LVDS)  driver/receiver translates the input and output signals from the device. Internal timing synchronizes to a  68 MHz oscillator.   Upset Effects.   As previously mentioned, the circuitry and algorithms in the design are either not effected  by upsets or quickly recover from upsets with no permanent change. All digital circuitry uses integrated circuits fabricated using radiation-hard processing and all critical registers use triple redundant voting  logic to eliminating SEU concerns. Thu s, it is only necessary to conside r the following three effects on the  analog signals: fringe interpolation, fring e counting, and index homing.  Fringe Interpolation.   MicroE utilizes an interpolation method that is absolute in nature. It is essentially an  arctan method of interpolation. Therefore, if the sine or cosine signal is disturbed, the interpolated value  will become incorrect (as it will for any encoder, incremental or absolute), but will recover with the correct  position word by the next interrogation as long as the encoder has not moved beyond one optical fringe; i.e., one analog signal cycle.   Fringe Counting.   The algorithm utilized for fringe counting only allows the count to change when the sine  transitions from the fourth to first quadrant (increase count) or the first to fourth quadrant (decrease count)  and ignores the all other quadrant changes. In order to miss a count, the disturbance would have to be larger than the peak amplitude of the normal signal, effectively eliminating one of the quadrants.  Radiation analysis indicates that the amplitude of the upset signals is more than an order of magnitude  lower than this.  Index Homing.   The index circuit uses the index window to determine which fringe is home . The algorithm  that determines this home  position requires that the index window be a minimum of one-half fringe wide.  Under normal rate conditions, this is a minimum of 100 /g151s. Since any upset signal will decay with a time constant of 1.7 /g151s (due to the system bandwidth), it is not possible for an upset to be present for 100 /g151s.  Accuracy.   The Aerospace sensor has several improvements over older encoder models. In particular, the  detector is larger for better averaging and robustness and the array pattern modified for better signal  filtering as mentioned earlier; these all contribute to enhance intrafringe accuracy.   Note the Aerospace Encoder’s enhanced harmonic suppression and the low error of 18-nm peak—a  dramatic reduction of approximately 4x down from the typical 70 – 80 nm peak error of previous models.  In addition to these short-range accuracy improvements, the Aerospace Encoder’s long-range  (interfringe) accuracy improved due to the larger sampling area and a shorter optical path length between the grating and the sensor head.  197