Exploring the Key High-Frequency Components in PCB Design

High-frequency circuit design powers much of today's advanced technology, from communication systems to sophisticated radars and military grade equipment. High frequency systems are not fundamentally different from other types of electric circuits. The same laws of physics apply, and the basics of electronics remain the same at that point also.

But what makes these circuits able to handle such rapid signals? It’s the specialized components, each with unique roles in stabilizing and amplifying signals while minimizing distortion. HF design involves a unique set of challenges and objectives and some special considerations should be taken while working with them. We often categorize components as either active or passive, and this approach is equally valid in the realm of RF. Here is a reference guide for the type of high frequency that can be identified in circuit design.

Capacitors at High Frequency:

As we know from the basics of circuit element session, the capacitor is basically a small energy storage unit. It blocks the DC and lets the AC signal pass through. So ideally it should behave the same for all types of AC signals, but that’s not correct. The function changes as the AC frequency applied to it changes. Here is the model representing the real electrical behavior of a capacitor.

“C” corresponds to the ideal capacitor that is buried among so many parasitic elements. We have non-infinite resistance between the plates (RD), series resistance (RS), series inductance (LS), and parallel capacitance (CP) between the PCB pads and the ground plane

Inductors at High Frequency:

Inductor on the other hand, has the opposite properties to capacitor. From basics we know it is basically a copper coil that works on faraday law of EMI, stores energy in the form of a magnetic field. It blocks the AC signal and lets the DC out easily, we can say it opposes the change of signal. An inductor provides impedance that steadily increases as frequency of AC signal increases. Here is an equivalent circuit for an inductor, A model representing the real electrical behavior of an inductor.

Parasitic behavior of Capacitors and Inductors: when you’re working with high frequencies, parasitic circuit elements are everywhere. No matter how simple or ideal a resistive element is, it still needs to be packaged and soldered to a PCB, and the result is parasitics.

We expect the impedance of a capacitor to endlessly decrease as frequency increases, but the presence of the parasitic inductance causes the impedance to dip down at the self-resonant frequency and then begin to increase. Similarly the wire to wire capacitance in case of inductors pushes it towards the same phenomenon of resonance.

Diodes: Detectors and Modulators at High Frequencies:

Diodes in high-frequency circuits operate as modulators and detectors. Schottky diodes, known for low forward voltage and fast switching, are commonly used in RF applications for signal demodulation and mixing, critical in radio receivers and transmitters.

Varactor diodes also play a role in frequency tuning by adjusting capacitance, allowing dynamic control of the oscillation frequency. The internal capacitance of a diode is the function of bias voltage across it. And frequency is the function of capacitance hence these can be used to generate different signals by changing voltage across them.

Crystals Oscillators and Filters:

Resonant circuits, including oscillators and filters, are the backbone of signal tuning in high-frequency designs. Crystals are a fundamental means of generating a stable frequency reference. This specified frequency is used as essential for clocks in wireless systems, while filters select and block frequencies, enabling clear signal paths and isolating noise.

Crystal oscillators are the time keepers circuits, used to provide stable clocks in digital designs. It consists of a piezoelectric crystal inside, which resonates at a rate when voltage is applied to it. The oscillation frequency of an ordinary crystal is sensitive to temperature variations. The resulting frequency instability creates problems for RF systems, especially systems that will be exposed to large variations in ambient temperature.

FIlters can be of two types: Active or passive, used to suppress a particular frequency band according to the circuit requirements. Filter as per frequency suppress has following types:

• Low Pass Filter
• High Pass Filter
• Band Pass Filter
• Band Reject Filter
• Notch Filter

Through Hole Components in HF:

We have seen the high frequency equivalent diagrams of the different components. Parasitics plays an important role as the operating frequency of the circuit increases.  Through-hole components are by no means unsuitable at HF, on the other hand surface-mount packaging is inherently superior when you’re working with high-frequency signals. With SMT small components and short pin lengths parasitic inductance at HF is minimized. Longer leads have more inductance, and consequently surface-mount packaging is preferred.

Amplifiers at HF:

Amplifiers are either built with a discrete transistor based system or with OP-AMP. Amplifiers have a mid band, which allows only some of the frequencies to pass through and amplify. In RF systems, there are two fundamental types of amplifiers: power amplifiers and low-noise amplifiers. In HF amplifiers have the main application to boost the signal which is going to be transmitted through the antenna.

Power Amplifiers

It increases the power level of a signal, similar situation is found in audio circuits. A power amplifier is needed to supply large amounts of current to the speaker coil to produce more sound. Similar to that some circuits require a power amplification stage while transmitting/ receiving a signal.

Low-Noise Amplifiers

As the name suggests, Low noise amplifiers work at higher frequencies to boost the signal with minimum distortion. LNA block is used in communication systems while receiving the low magnitude signal from an antenna which also carries a lot of environmental noise. Thus, a low-noise amplifier is designed to provide high voltage gain while contributing minimal noise. The noise performance is given by “noise figure” (NF), which helps to determine the maximum SNR of the system. Ideally we need NF=0 and SNR= infinity.

Mixers in High Frequency Circuits:

It is not an audio kind of mixer which is used to produce modulated signals from base, instead a HF mixer takes two input frequencies and generates a third output frequency via multiplication. In other words, a mixer performs frequency translation.

Mixers shift signals to higher or lower frequencies while preserving the original details. For example, a baseband signal carrying information (modulated signal) can be shifted to a higher frequency, making it suitable for wireless transmission. This frequency shift keeps all critical modulation details from the original signal, allowing accurate transmission and reception.

Baluns:

One component that is common in HF systems, one can be found in antenna TV systems. The name comes from “balanced to unbalanced,” a phrase which helps us to remember that baluns are used to convert differential signals to single ended or vice versa. They have main applications in impedance matching of the transmission line and the receiver unit.

In lower-frequency circuits amplifiers are used to convert between single-ended and differential. But a common HF decision is to use simple passive components when you’re dealing with very high frequencies.

Antennas:

Antenna’s are the passive reciprocal devices which are used to convert an RF electrical signal into electromagnetic radiation (EMR), or vice versa. Whereas in the HF design we are suppressing the leakage of EM waves here in the antenna part we do the opposite. The ideal antenna should emit all the electrical power as electromagnetic waves into space.

At higher frequencies the size of the components and antenna is reduced. Nowadays it is common to use either a “chip antenna,” which is soldered to a PCB like typical surface-mount components, or a PCB antenna, which is created by incorporating a specially designed trace into the PCB layout.

Phase-Locked Loops:

A phase-locked loop (PLL) contains a phase detector, a low-pass filter, a voltage-controlled oscillator (VCO), and a frequency divider. It can generate a wide range of frequencies from one frequency by minimizing the phase error between the signals. An oscillator combined with a PLL is referred to as a synthesizer, i.e., a component that can generate a range of frequencies. This ability to adjust the oscillator frequency is very important in HF design. Furthermore, the frequency spacing between adjacent channels may be relatively small, and thus the adjustments must be precise.

RFIC:

Like in low-frequency circuits, transistors are the essential active components in RF integrated circuits. However, the term "component" here often refers to complex devices made up of multiple transistors. Practical RF engineering thus focuses on integrating these ready-made components into functional circuits and resolving the unique complexities that arise in high-frequency design.

Because at integrated circuit level the size of components are rather small, and suitable for the high frequency applications. Due to which a push toward a very high frequency can be made which increases the overall computational power keeping the power requirements minimum.

Conclusion:

At fabrication level silicon is still the dominant material in semiconductor manufacturing. However, other materials are more compatible with the high signal frequencies. Gallium nitride (GaN), gallium arsenide (GaAs), and silicon germanium (SiGe) are common alternatives in RF semiconductors, each supporting efficient operation at these higher frequencies. Advanced semiconductor technologies enable the creation of devices that perform well even above 100 GHz, allowing for robust functionality in high-frequency applications.

Modern high-frequency circuit design depends on a combination of carefully selected components, each fulfilling a role that helps optimize signal stability, efficiency, and clarity. From passive elements like capacitors and inductors to advanced ICs, these components bring stability and precision, allowing RF circuits to meet the demands of cutting-edge technology.