The calculation of frequency source requirements for digital communications systems

Frequency sources, such as oscillators and synthesizers, are critical to the proper operation of digital communications systems. Frequency sources are used as local oscillators (LO) in RF systems, data clocks in all systems, and sampling clocks in digitally implemented systems. Frequency sources affect critical communications system parameters such as acquisition, loss of lock, and bit error rate (BER) performance. The issues for the first two items are well known to the time and frequency community - those of synchronization, syntonization, and cycle slipping. This paper focuses on item three, how frequency source parameters affect BER performance. The paper is tutorial in nature, rather than an exhaustive treatment, and is meant for those familiar with time and frequency theory but not digital communications theory. A quadrature phase shift keyed (QPSK) RF communication system is for illustrative examples, but the methods outlined in this paper are generally applicable to all types of digital communications systems.