Data Acquisition Sub-System

The ZY laser ranging system uses a 780 nanometer (nm) wavelength laser modulated at 1500 MHz to determine the exact distance to a retroreflector in the field. The reflected signal is received by a PIN silicon photodiode, mixed, digitized by the 16 bit 100 kHz A/D system, and processed to extract the phase. Ideally, the phase of the received 1500 MHz signal would then be compared against a 1500 MHz reference signal to determine the phase difference, and thus the exact location of the retroreflector. However, The A/D sub-system is incapable of sampling a signal of this frequency. To get around this problem, the returning 1500 MHz signal is first mixed with a (1500 MHz + IF reference) signal to produce an IF signal of much lower frequency, which can then be easily sampled. The phase of the IF signal is then compared to the IF reference. To make a measurement, the system samples a number of IF cycles at a predetermined sampling rate. The total number of samples taken is then the product of the number of cycles integrated and the sampling rate, expressed in samples per cycle. The number of cycles to measure, the sampling rate, and the frequency of the IF reference are all programmable.