High-Frequency Circuits

High-frequency circuits are specialized electronic circuits designed to process, transmit, or receive electrical signals operating at frequencies typically above 3 MHz. In radio technology, the range between 3 MHz and 30 MHz is defined as "high frequency (HF)", while in modern applications such as microwaves and 5G, this definition has expanded to include frequencies of 1 GHz and higher. The key distinction from low-frequency circuits lies in the deviation of circuit components from ideal behavior as signal frequency increases, and the critical importance of previously negligible parasitic effects such as distributed capacitance and inductance, along with signal propagation delay. These circuits perform functions such as signal transmission, frequency selection, and impedance transformation through passive components, active devices, and passive networks composed of these elements. With advances in science and technology, the importance of high-frequency circuits is steadily increasing in fields such as wireless communication, satellite systems, radar, and high-speed data transmission.

Basic Components of High-Frequency Circuits

High-frequency circuits include not only standard electronic components but also specialized elements and structures that exhibit behavior unique to this frequency range. The selection and placement of components directly affect circuit performance.

Passive Components and Filters

Resistors, capacitors, and inductors are the fundamental passive components of high-frequency circuits. However, their behavior at these frequencies differs from that at low frequencies. For example, the parasitic inductance of a resistor or the inductance formed by the leads of a capacitor can alter the overall impedance and frequency response of the circuit. Therefore, Surface Mount Technology (SMT) components are frequently preferred due to their small size and low parasitic inductance, which minimize signal loss and reflections. Material selection is also critical; metal film resistors are favored for their low temperature coefficients and stability, while carbon composite resistors offer a cost advantage.

Electronic filters are used to suppress unwanted frequency components (noise) and allow desired frequencies to pass. They are classified into two types based on structure: passive (composed of resistors, capacitors, and inductors) and active (containing power-dependent elements such as op-amps). Based on operating principles, they are divided into four main categories:

• Low-Pass Filters: Allow signals below a specific cutoff frequency (fc) to pass while attenuating those above it.
• High-Pass Filters: Allow signals above a specific cutoff frequency to pass while attenuating those below it.
• Band-Pass Filters: Allow signals within a specific frequency range to pass.
• Band-Stop Filters: Suppress signals within a specific frequency range.

In low-frequency circuits (<100 kHz), simple RC (Resistor-Capacitor) filters are often sufficient, whereas in high-frequency circuits (>100 kHz), RLC (Resistor-Inductor-Capacitor) circuits are typically used. The order of the filter (the number of reactive components it contains) determines the steepness of the transition region. For example, a second-order filter provides a sharper frequency transition than a first-order filter.

Active Components and Amplifiers

Active components such as transistors and integrated circuits are used to amplify, switch, or process signals. Amplifiers are among the most important active circuits in this domain. High-frequency amplifiers are designed to boost radio frequency (RF) signals in the range of 3 MHz to 30 MHz or higher microwave frequencies. They exhibit different characteristics depending on their operating class:

• Class A: Processes the entire waveform of the signal, offering the lowest distortion but with very low efficiency and significant heat generation.
• Class B: Increases efficiency by processing the positive and negative halves of the signal using separate transistors, but may introduce "crossover" distortion at transition points.
• Class AB: Combines Class A and Class B. It operates like Class A at low signal levels to minimize distortion and like Class B at higher levels for efficiency. This is the most common design in commercial products.
• Class C: Processes only a small portion of the signal and reconstructs the full waveform using resonant circuits (LC tanks). It offers very high efficiency but high distortion, making it suitable primarily for fixed-frequency transmitters such as in telecommunications and radio.
• Other Classes (D, F, G, H): These modern designs aim to achieve very high efficiency using switching techniques—for example, Class D employs pulse width modulation.

Printed Circuit Board (PCB) Design and Layout

The performance of high-frequency circuits depends heavily on the design of the Printed Circuit Board (PCB). Poor design can cause a circuit to fail even when using the highest quality components. Therefore, specific design rules must be strictly followed.

Material Selection

Standard FR-4 PCB materials can cause increased signal loss at high frequencies. For this reason, specialized substrate materials such as Arlon, Rogers, Taconic, and PTFE (Teflon) are used. The key properties of these materials include:

• Low and Stable Dielectric Constant (Dk) and Dissipation Factor (Df): A low Dk increases signal propagation speed, while a low Df reduces signal loss.
• Coefficient of Thermal Expansion: A coefficient close to that of copper foil prevents damage to solder joints and traces during temperature changes.
• Low Moisture Absorption Rate: Moisture can negatively affect the Dk and Df values of the material.

Cabling and Layout Rules

In high-frequency PCB design, strict rules are applied to maintain signal integrity and minimize parasitic effects.

• Multilayer Boards: Using multilayer PCBs in highly integrated circuits is an effective method to separate signal and power layers, create wide ground planes, and shorten signal traces. It has been noted that a four-layer board can have a noise level 20 dB lower than a two-layer board made from the same material.
• Short and Direct Traces: Signal traces must be kept as short as possible. Signal radiation intensity is directly proportional to trace length.
• Avoid Sharp Corners: Instead of 90-degree sharp turns, 45-degree angles or circular arcs should be used. Sharp corners cause impedance discontinuities and signal reflections.
• Crosstalk Prevention: Electromagnetic coupling between parallel signal traces can cause unwanted signal transfer (crosstalk). To reduce this, the distance between parallel traces should be increased, a ground trace should be placed between them, or traces on adjacent layers should be routed perpendicularly.
• Grounding: Wide and continuous ground planes provide a low-impedance return path for signals and reduce noise. Analog and digital grounds should be separated and connected at only a single point.
• Decoupling Capacitors: High-frequency decoupling capacitors placed as close as possible to the power pins of integrated circuits suppress noise on the power rail and ensure a stable supply voltage.

Application Areas

High-frequency circuits are indispensable in many areas of modern technology:

• Communication Systems: Mobile phones, Wi-Fi, Bluetooth, satellite communication, shortwave radio broadcasting, and all receiver-transmitter systems requiring frequency demodulation.
• Radar and Remote Sensing: Collision avoidance systems in the automotive sector, military and civilian radar systems, weather radar.
• High-Speed Digital Systems: DDR memory buses in computers, and high-speed data interfaces such as USB and HDMI, which require high-frequency design techniques to maintain signal integrity.
• Industrial and Medical Applications: High-power ultrasonic generators for welding, cutting, and cleaning; medical imaging systems such as MRI.
• Defense and Aerospace: Electronic warfare systems, avionics, and guidance systems.

Testing and Measurement

Testing and measuring high-frequency circuits require specialized equipment and techniques, as important parameters such as resistance, capacitance, and inductance vary with frequency. Devices such as oscilloscopes, spectrum analyzers, and vector network analyzers are used to verify circuit performance. Quality control tests applied at every stage of production ensure that circuits operate reliably, durably, and at the desired performance level.