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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Spectrum Analyzers and Signal Analyzers
Spectrum analyzers and signal analyzers display raw, unprocessed signal information such as voltage, power, period, wave shape, sidebands, and frequency. They can provide the user with a clear and precise window into the frequency spectrum.
Audio Analyzers
Audio analyzers measure the noise and audio spectrum of output from an amplifier or a stereo.
RF Upconverters and RF Downconverters
RF upconverters and RF downconverters are integrated assemblies that convert microwave signals to another frequency range for further processing. Generally, they are designed to produce an output signal frequency for a particular frequency band.
RF Frequency Converters
RF frequency converters are integrated component assemblies required for converting microwave signals to lower (or intermediate) or higher frequency range for further processing.  
Radio Receiver ICs
Radio receiver ICs support worldwide radio band requirements, including frequency modulation (FM), FM radio data system (RDS), amplitude modulation (AM), long wave (LW), short wave (SW), and weather bands.

Topics of Interest

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This chapter discusses system requirements for a PLL. Noise basics, phase noise, time-domain response, acquisition, jitter, and spurious signals are the system requirements that are studied in detail.

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