A Linear Phase Modulator for Large Baseband Bandwidths

A linear phase modulator with a stable carrier frequency would be a useful component for the following three applications. (i) As a replacement modulator for the reflex Klystron in an otherwise all solid-state repeater of the TL System. 1 («) For frequency-division frequency-modulation multiplex systems with baseband bandwidths of the order of 100 MHz. 2 (Hi) For multi-level coherent phase-shift-keyed P C M systems with baud rates of the order of 100 megabauds per second. J The modulator described in this paper appears adequate for these applications. It is based upon the original Armstrong circuit which is well suited to large baseband bandwidths and is reasonably linear follow modulation indexes. 4 An important feature of this method of modulation is that the carrier frequency can be stable with respect 1893 1894 T I I E BELL SYSTEM T E C H N I C A L J O U R N A L , OCTOBER 1970 to ambient effects since it can be derived from a temperature-stabilized quartz crystal oscillator. The baseband bandwidths which may be achieved are those for which low-index double sideband amplitude modulators can be built. An analysis of distortion is presented for the types of baseband signals used in the three applications discussed above, and a circuit is described in which the phase deviation can be increased to any desired value. I I . CIRCUIT D E S C R I P T I O N