Analysis of Adjacent Channel Power Ratio Performance in Two-Point Modulated Systems

In this study, the degradation of adjacent channel power ratio (ACPR) performance in personal mobile radio (PMR) systems, which use two-point modulation in their phase locked loops (PLLs) is investigated. First an expression for the widths of charge pump pulses, which arise due to imbalanced modulation are found by also taking into account the low pass filtering of the loop. Then the frequency domain representation of charge pump pulses is obtained by Fourier transform. Using this representation and the loop filter response, the spurious signals in the spectrum of the voltage controlled oscillator (VCO) is calculated by using narrowband FM assumption. Evaluating the power of the spectrum within the bandwidth of the adjacent channel, ACPR performance is obtained. A practical setup is constructed to validate the proposed method. Coherence is observed between the measurements of the test setup and expressions obtained in this study.