Design and Implementation of Frequency Measuring Circuit Based on Countdown Counter

Abstract

After down-conversion and shaping, the transmission pulse signal of radar turns to intermediate frequency TTL square signal. This signal, on the basis of the frequency measuring principle of countdown counter, is counted by three pieces cascading of binary synchronous counter 74LS161. Three hundred pulses of the signal are chosen to form the counting section, in which the 100MHz standard square wave is counted by 74F161 with three pieces cascading. Then the result is loaded into MCU. The MCU works out current IF-RE and controls the DDS local frequency synthesizer to adjust the local output, in order that the frequency of IF signal is stabilized in the specified scope. The digital automatic frequency control system of radar is constituted. Integrated circuit TTL has the longest history and highest production at home and abroad. Its inherent structural features, good electrical property, constant constitutional improvement and performance improvement help the “aged” TTL circuit maintain its popularity after several changes[1]. The integrated circuit TTL possesses high reliability, broad range of operating speed, low power dissipation and favorable antijamming capability. In this essay, with TTL integrated circuit and 51 MCU[2], the frequency measuring circuit based on countdown counter is realized, and with the assist of DDS frequency synthesizer the automatic frequency control system of radar[3] is constituted and has successfully been applied into some domestic warning radar.