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| 14 The basic RF filter types are low- pass, high-pass, bandpass, and notch (or band reject). 1 The quality of the filter components and design dictate the filter response, which is generally prioritized to focus on the insertion loss, selectivity, and attenuation performance. Examples of RF filters include bulk acoustic wave (BAW), surface acoustic wave (SAW), and filter modules designed for Wi-Fi and other common wireless communication standards. Mixers, Frequency Multipliers, Frequency Dividers, and Prescalers RF mixers, frequency multipliers and dividers, and prescalers are all frequency translation elements. These elements translate RF signals from one frequency to another, either higher or lower, while introducing minimal distortion and nonlinearities. Mixer circuits can be passive or active, where the passive types present an insertion loss, and the active types may minimize the insertion loss or even provide gain at the output for translated signals. It is common to use an amplifier prior to a passive frequency translation component to ensure that the signals are at an adequate power level at the output of the frequency translation component and to ensure a desired dynamic range figure. As these circuits are intrinsically nonlinear, some spurious content and harmonics are always produced. When selecting or designing mixers, engineers must account for the undesirable frequency content by-products and either filter out these by-products or ensure they exist outside of relevant frequency bands. As these devices can introduce interference and noise content into relevant frequency bands, the input and output of these devices are often filtered and even attenuated to prevent standing waves from developing at the input and output nodes of these circuits. LOs, VCOs, PLLs, and DDS Devices Local oscillators (LOs), voltage- controlled oscillators (VCOs), phase-locked loops (PLLs), and direct digital synthesis (DDS) devices are all used to generate desired RF signals for use as carrier frequencies or LO frequencies for frequency translation circuits. LOs are typically designed for a fixed frequency, with additional circuitry to ensure frequency accuracy and minimal phase variations. VCOs are often used as LOs in situations where it is desirable to adjust the LO frequency, either to different frequency channels or for frequency sweeping. PLLs can be used as LOs to ensure highly accurate frequency generation and can often be adjusted over a narrow range. DDS uses digital electronics and signal processing to generate digital signals that are converted to LO or RF signals using high-frequency digital-to-analog converters (DACs). Amplifiers RF amplifiers increase the desired signal energy while minimizing the added noise, distortion, and nonlinearities of signals passing through the amplifier circuit. There are many different types of RF amplifiers, which are generally designated based on the functions they are optimized for. For instance, power amplifiers (PAs) are designed to prioritize high power output over other performance aspects. Similarly, low-noise amplifiers (LNAs) prioritize minimal added noise while increasing the gain of low-power signals. For these reasons, PAs are most often seen at the output of transmitters, while LNAs are most often used at the input of receivers or within the RF signal chain where the gain is needed for low-power signals but minimal added noise is desired. Other common RF amplifier types, such as gain-block amplifiers, are generally used within an RF signal chain to amplify RF signals so the signal power level is adequate after being attenuated by interconnect or other signal chain components. A broadband amplifier is designed specifically to provide a moderate amount of gain over a very wide bandwidth and can be used like a gain block amplifier. RF Limiters, Receiver Protectors, and Digital Step Attenuators RF limiters are devices that either shunt excessive RF energy at the input to a load or reflect the signal back toward the original path. RF limiters mainly protect sensitive components from signal energy beyond what they can handle or beyond a desired limit. They are commonly placed in the receiver signal path prior to an LNA. This prevents the LNA from being exposed to high RF power signals that could damage it or desensitize the receiver circuitry if amplified. Limiters may also be used within a signal chain if there is a chance that a failed high-power component could damage sensitive monitoring circuits or cause reflections that could damage components and devices on the same path.

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