A digital filter implementation of the deep space transponder phase lock loop integrator

Having suffered in reliability due to malfunctioning capacitors, the phase lock loop integrating filter in the NASA standard deep space transponder is reconsidered. An alternative design, using a digital filter which eliminates these critical components is presented. The analog-to-digital conversion, digital filtering, and digital-to-analog conversion are all performed by an Intel 2920 digital signal processing integrated circuit. The circuit was tested for its step response and frequency response. The output for a 0.75 volt step input was recorded over a 3 hour period at 5 minute intervals and compared with the ideal response to the linear integrator. The maximum error was 1.5 percent. The filter was not tested in a loop.