Center-Frequency-Stabilized Frequency-Modulation System

A system comparing the phase between two frequencies in a balanced phase detector utilizes the output to maintain synchronization between two oscillators. The phase detector operates between ±90 degrees. By dividing the frequency of a frequency-modulated oscillator, the frequency swing is reduced to less than ±24 degrees. The mean divided modulated oscillation is compared in the phase detector to the same frequency obtained by dividing the frequency of a crystal oscillator. The correction voltage resulting from a change in phase between the two oscillators changes the capacitance injected into the tank of the modulated oscillator, and thus maintains synchronization. A modulator utilizing the linear relationship between the input capacitance and the transconductance of a vacuum-tube amplifier is used to control the total capacitance of the frequency-determining circuit of a master oscillator. Part of the output of the master oscillator is multiplied in frequency and power to produce the transmitter output, and part goes to a frequency divider having a ratio of 1 to 256. This practically eliminates the frequency swing resulting from modulation and permits the output frequency to be synchronized with that of a similarly frequency-divided crystal oscillator. Any shift in phase between these two synchronized frequencies produces a correcting voltage which is applied to the modulator of the master oscillator. This correcting voltage acts to maintain the synchronization of the two divided frequencies.