Evaluating the behaviour of phase-modulated modems sensitive to amplitude and phase transients

In practical communication systems, spurious amplitude and phase effects are normally mutually interrelated. Most semiconductor and thermionic amplifier devices, when driven from their quasilinear region into a saturated state by large amplitude signal changes, impress phase and amplitude changes onto other signals operating within the normally linear region. Practical digital modems are sensitive to varying levels of spurious amplitude and phase modulation, but the ability to quantify this for one type of interference without the presence of the other has proved difficult. The paper describes a method of generating test signals which contain one or other of these modulation modes with a high rejection of the other. Test results of measurements on a typical phase-reversed-keyed modem are included and show the effects of these two forms of interference