The digital controller for GMRT antennas, is built around Intel's 8086 processor running at 8 MHz and is called as the ``Station Servo Computer" The 8086 is a bus master, controlling two slave processors 8031, for analog and encoder interface. The position loop of both the axes of the GMRT servo system is implemented digitally in this servo computer. The elevation and azimuth axes angles along with time, are fed to the servo computer by the antenna base computer (ABC, see Section 24.2.4). The servo computer computes the error of both the axes and performs necessary filtering (compensation). The compensator output is converted into analog signal by using 16 bit DAC and then applied to the rate loop.
For the digital implementation of a position loop, the sampling rate must be large enough. The ``S" domain transfer function of the compensator is converted into a ``Z'' domain transfer function, by using the ``Tustins approximations''. The Z-domain transfer function is further converted into a difference equation, to be solve recursively at a regular interval. Tustin proposes that the sampling frequency must be greater than 10 times the compensator bandwidth. With 1.5 Hz as a structural resonant frequency of the GMRT antennas, the position loop bandwidth can be around 0.4 Hz to 0.5 Hz . For a 0.5 Hz loop bandwidth the sampling rate should be more than 5 Hz. This sets the lower limit of the sampling rate. The upper limit of the sampling rate is determined by the processor speed, other tasks of the processor, the transport lag etc. We have chosen 10 Hz as a sampling rate. The processor is interrupted at regular interval of 100 ms to run the real time programme.