A Linear Phase Modulator for a Short-Hop Microwave Radio System
01 October 1972
A linear phase modulator is a useful component in a short-hop radio system using digital modulation. 1,2 A possible application of this modulator in a digital radio system is shown in a block diagram in Fig. 1. A shared delta modulator multiplex operating at a line rate of 20 megabits is an example of a digital multiplex which could be used in such a system. 3 The output binary signal of this multiplex can be converted into a baseband pulse sequence by a 4-level block coder.4 The modulator described in this paper satisfies the requirements for the above application. It is based upon the original Armstrong circuit, which is well suited to large baseband bandwidths and is reasonably linear for low modulation indexes.5 An analysis of distortion for the type of baseband signal used in this application is discussed by C. L. Ruthroff and W. F. Bodtmann in Ref. 1. The output of the phase modulator is at an IF frequency of 300 MHz. It can be converted in a linear mixer to any desired RF frequency in the microwave or millimeter-wave range, and can be amplified for transmission by an injection-locked oscillator amplifier.6