From Phase-Locked Loop Engineering Handbook for Integrated Circuits
3.6 Spurious Signals
Any non harmonically related signal is a spurious signal. These signals limit the electronic performance of many devices. Consequently, minimizing these signals and their amplitude levels is the goal of any good system design. Studying the generation mechanism of these signals helps us understand how to minimize their generation.
Figure 3.60 shows a spectrum analyzer that contains many interfering signals in a PLL. This figure shows harmonics of the carrier, additional phase noise from loop instability, reference sideband feed-through, and an intermodulation product. Harmonics of the carrier do not present a significant problem in many PLL designs. We have already studied loop instability and its causes. Intermodulation products are the next spurious signals that we will study. In Section 3.6.2, we will study the effects of reference sideband feed-through.
Figure 3.60: Typical intermodulation spectrum for a PLL
3.6.1 Intermodulation Products
In this section, equations are shown that compute the frequencies where intermodulation products will occur and the power levels that they will have. Next, some relationships between product-level and design parameters are discussed. Then, examples of up conversion and down conversion are shown.
Nonlinear circuits (e.g., diodes, I/O buffers, amplitude limiters, frequency dividers, analog multipliers) cause intermodulation products. Studying the intermodulation products of a mixer helps one understand how these products are generated. Equation (3.190) describes the mathematical relationship between the mixing of two signals, which are called the local oscillator (LO) and radio frequency (RF) signals [21, p. 62]:
The desired output is at M rf
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