L5 System: Jumbogroup Frequency Supply

It is the goal of the Bell System to maintain frequency offset in any transcontinental circuit to within ±2 Hz. This requirement is imposed by the most sensitive message signals, such as those in programming, telegraph signals, and data signals. In an analog amplitudemodulated system, such a frequency offset can be introduced into the message signal if the transmitting and receiving multiplex carriers are not synchronized. In a single-sideband suppressed carrier multiplex format as used in jumbogroup multiplex1 terminals ( J M X ) , the multiplexing carriers are not transmitted. Thus, precision frequencies must be provided for the multiplex terminal at each geographical location. The top channel frequency of the long-haul, analog, frequencydivision multiplex system has been steadily increasing. With the increase in top channel frequency (F), the frequency precision required of the multiplex carriers increases. This is because the frequency offset 2109 (AF) is limited to 2 Hz, whereas the ratio A F / F decreases as F increases. In the L5 coaxial system, the top channel frequency is at 60 MHz, and the 2-Hz requirement results in a ratio of 3.3 parts in 10s (3.3/10 8 ). Because a typical signal may traverse through several JMX terminals, carrier frequencies must be synchronized with an accuracy of about 1 part in 108. The frequency precision demanded by the L5 system could not be provided by the synchronizing network existing at the time L5 design work was initiated. Several alternatives were studied.