The design and implementation of Biphase Costas Loop

Abstract

Abstract At present, transmitting data or digital message in carrier band via digital modulation such as digital radio has become more and more popular, and the various digital modulation techniques most oftenly employed almost invariably involve changing the carrier phase in some particular way according to the baseband message entering the modulator. These can be grossly termed “phase shift keying” (PSK). Phase shifted keyed carrier band signal must be detected “coherently”, i. e. at the receiver a locally generated carrier whose phase closely tracks the phase of the carrier signal at the transmitter is required. The process of generating such a local, phase coherent carrier is usually called “carrier recovery”. Since it has been well known that the quality of recovered carrier has a profound effect on the whole PSK receiver system performance (e. g. the bit error rate, etc.), carrier recovery circuit must be analyzed and designed carefully. In this paper, the “Costas loop”—one of the most efficient and commonly used carrier recovery methods in suppressed carrier PSK system—is discussed in somewhat detail, both in its theoretical design and practical hardware implementation. A Costas loop circuit for demodulating 10.7 MHz carrier frequency, 3 Kbits/sec data rate BPSK signal is fabricated and tested. Parameter selection and hardware implementation of important blocks in the loop circuit such as the voltage controlled oscillator, 90° phase shifter, phase comparators and loop filters are all given careful consideration. Experimentally measured result has confirmed that the loop can indeed perform satisfactorily.