A New Reference Frequency Standard for the L Multiplex System

The L multiplex system is a single-sideband suppressed carrier system in which the carrier frequency supplies in the various offices are synchronized by the transmission of pilot frequencies over a treelike network. At each office the carrier frequencies necessary to perform the modulation and demodulation steps are derived from the outputs of an "office master" supply which is phase-locked to the incoming synchronizing pilot tone. This "office master" supply, named the primary frequency supply in L multiplex terminology, has been described in previous literature. 1 The tree-like synchronization network originates at a single primary frequency supply called the "system master." While relative frequency accuracy between offices depends on the phase-locked 2183 2198 TI-IE BELL SYSTEM TECHNICAL JOURNAL, DECEMBER 1968 synchronization network, the absolute frequency accuracy of the pilot tones is dependent upon the absolute frequency accuracy of the system master. Although the free-running accuracy of a primary frequency supply is such that fairly appreciable periods of pilot outage can be tolerated, a highly accurate reference frequency standard is still required to ultimately control the system master. In the past this frequency standardization service has been provided by Bell Telephone Laboratories, Murray Hill, New Jersey, where the Bell System Primary Standard of Frequency has been maintained to an absolute frequency accuracy of one part in 10° by periodic corrections. In recent years the need has arisen to replace the Murray Hill frequency standard with a more reliable and rugged solid-state version which would be located and maintained in a hardened telephone office.