# RF Building Blocks Wireless communication systems are basically composed of a sequence of blocks chained together from transmitter to receiver (including the free space between them) that mathematically process and modifies the signal end-to-end. Historically, modulating/demodulating, mixing and filtering signals has been done on discrete—active and passive—devices. With the progress of semiconductor technology in areas such as analog-to-digital converters and [[Semiconductors#Digital Signal Processing|digital signal processing]], now it is possible to move such things into software. How so? Well, processing signals is ultimately a mathematical problem. Filtering, mathematically speaking, is done by computing the [[Semiconductors#Digital Signal Processing#Linear Time-Invariant Systems and the Convolution Sum|convolution]] of a signal (or, in the discrete jargon, a [[Semiconductors#Digital Signal Processing#Discrete-time Systems|sequence]]) with the step response of the [[Semiconductors#Digital Signal Processing#Digital Filtering and Discrete-Time Fourier Transform (DTFT)|filter]]. Or, equivalently, multiplying their frequency responses in the frequency domain. This means, a [superheterodyne receiver](https://en.wikipedia.org/wiki/Superheterodyne_receiver) is nothing else but a sequence of mathematical manipulations of the input signal. So, if you manage to digitize the signal as soon as possible in the chain (this is, as close to the antenna as possible), then you can do the math operations fully on the digital domain. ## Switch Circuits that switch signals such as RF and microwave signals are very useful, especially for applications such as phased-array radar and instrumentation. The best integrated technology for switches is the FET because of the inherent isolation between the gate contact and the source and drain contacts, and because the gate draws virtually zero current in both control states (on and off). The advantages of FET switches over other alternatives such as PIN diode switches are that they have negligible DC power consumption and require much simpler bias and driver circuits. When used as a switch, the FET is in a passive mode (i.e., there is no DC bias voltage on the drain contact), and it functions as a voltage-controlled resistor; the resistance between the source and drain contacts is controlled by the voltage applied to the gate contact through a large value resistor. This resistor completely decouples the DC control circuitry from the high-frequency signals and consumes negligible DC power because of the near-zero gate current. ![[Pasted image 20250107214427.png]] > [!Figure] > Three different switch topologies (source: #ref/Marsh ) ![[Pasted image 20250107214646.png]] > [!Figure] > Series and shunt configuration FETs operating as single-pole single-throw (SPST) switches. (source: #ref/Marsh ) ## Filter A filter is any device used to control the frequency response in a signal chain by providing transmission at frequencies within the passband of the filter and attenuation in the stopband of the filter. Typical frequency responses include low-pass (blocking frequencies higher than the cutoff frequency), high-pass (blocking frequencies below the cutoff frequency), bandpass (blocking frequencies outside a given range), and band-reject (opposite to bandpass, rejecting frequencies within a band) characteristics. Applications can be found in virtually any type of analog or digital signal processing or conditioning application such as audio, microwave communication, radar, or test and measurement system. ## Oscillator In the most general sense, an oscillator is a nonlinear circuit that converts DC power to an AC waveform. Most oscillators provide sinusoidal outputs, which minimizes undesired harmonics and noise sidebands, although there is always distortion present in an oscillator's output. ## Frequency Multiplier As frequency increases into the millimeter wave range it becomes increasingly difficult to build fundamental frequency oscillators with good power, stability, and noise characteristics. An alternative approach is to produce a harmonic of a lower frequency oscillator through the use of a frequency multiplier. A nonlinear element may generate many harmonics of an input sinusoidal signal, so frequency multiplication is a natural occurrence in circuits containing diodes and transistors. Designing a good-quality frequency multiplier, however, is a difficult task that generally requires nonlinear analysis, matching at multiple frequencies, stability analysis, and thermal considerations. ## Mixer A mixer is a three-port device that uses a nonlinear or time-varying element to achieve frequency conversion. An ideal mixer produces an output consisting of the sum and difference frequencies of its two input signals. Operation of practical mixers is usually based on the nonlinearity provided by either a diode or a transistor. A nonlinear component can generate a wide variety of harmonics and other products of input frequencies, so filtering must be used to select the desired frequency components. Modern communication systems typically use several mixers and filters to perform the functions of frequency up-conversion and down-conversion between baseband signal frequencies and RF carrier frequencies. ## Modulator Information may be impressed upon a sinusoidal carrier using amplitude, frequency, or phase modulation. Modulation means altering a fundamental characteristic of a signal (carrier) with a baseband signal (the modulating signal) which contains the information to be transmitted. If the modulating signal is analog, as in the case of AM or FM radio, the amplitude, frequency, or phase of the carrier will undergo a continuous variation. If the modulating signal represents digital data in binary form, the variation in the amplitude, frequency, or phase of the carrier will be limited to discrete values. In the frequency domain, modulation is the process of shifting the spectral content of the modulating signal to a different frequency range (the carrier's). This shift is necessary because the baseband signal often occupies low frequencies—therefore long wavelengths—that cannot propagate through transmission media such as [[Antennas|antennas]]. When modulation occurs, the spectrum of the information signal is translated to a higher frequency band centered around the carrier frequency. Many different modulation schemes exist, from very simple and spectrally inefficient to highly complex to achieve very high spectral efficiency. In fact, most digital modulation techniques possess an intermediary step where collections of $b$ bits forming a binary message $m_b$ are mapped to a symbol, which is then used to define the physical characteristics of a continuous waveform in terms of amplitude and phase. ## Amplifier Signal amplification is one of the most basic and prevalent circuit building blocks in modern RF and communication systems. Early amplifiers relied on tubes, but due to the dramatic improvements and innovations in solid-state technology that have occurred in the last 50 years, most amplifiers today use transistor devices such as Si [[Semiconductors#The Transistor Drama|BJTs]], GaAs or SiGe HBTs, Si MOSFETs, GaAs MESFETs, or GaAs or GaN HEMTs. Transistor amplifiers are low-cost, reliable and can be easily integrated in both hybrid and monolithic integrated circuitry. ## Couplers, Splitters, and Combiners Power splitters separate a signal at their input port into two or more parts with equal or unequal magnitude in each output port. Couplers perform the same function, usually with only two output ports, and tend to couple only small amounts of power into the second output port. Couplers are typically used to sample small amounts of power from a signal path where their coupled output is either connected to power meters to monitor the main signal level or fed into mixers to phase-lock the signal generator. Power splitters are used to split the power between several transistors in power amplifiers or to send the signal down more than one path in a more complex system. As only passive elements are being considered here, the splitters and couplers are reciprocal devices, so when they are used in the opposite direction, they act as power combiners and combine one or more signals together into one output port. ## Phase Shifter Phase shifters are used to delay or shift the phase of an RF signal by a fixed or variable amount. The phase shift can be continuously variable, as in an analog phase shifter, or it can be switched in discrete steps, as in a digital phase shifter. Analog phase-shifter designs include the reflective and the loaded-line styles, and digital phase-shifter designs include the switched-delay-line and switched-filter styles. ## Attenuator An attenuator is a passive device used to reduce the power level of a signal without significantly distorting its waveform. It is typically used to manage signal levels, prevent overloading of sensitive components, or match impedance within an signal chain. Attenuators achieve this by introducing a controlled amount of loss, often measured in decibels (dB), through resistive elements or other mechanisms. In practical applications, attenuators are critical for ensuring that signals stay within the dynamic range of devices such as amplifiers, receivers, or test equipment. They come in various forms, including fixed, variable, and step attenuators, and are designed to operate across specific frequency ranges. By controlling signal strength while preserving the signal's characteristics, attenuators play an importnt role in maintaining the integrity, stability, and efficiency of RF systems in communication, testing, and measurement applications. > [!warning] > This section is under #development