High Frequency Board Design Basics

 

BASIC DEFINITIONS

Impedance can be defined as some kind of obstruction (resistance) to the flow of alternating current in systems operating with alternating voltage. Thus, the impedance is a complex quantity.

The spectral characteristic of the signal can be compared with the spectrum of light radiation with many colors and is a set of sinusoidal signals of certain frequencies with certain amplitudes.

The frequency of a signal is defined as the number of complete current or voltage cycles of that signal per second.

Wavelength is the distance traveled by a wave in one complete cycle. It must be understood that this distance does not always refer to the air environment. In air, radio waves propagate at the speed of light, but in electronic devices (in particular, printed circuit by CAMTECH PCB ), the propagation speed decreases due to dielectric effects. In air or vacuum, the speed of light is very close to 300 mm per nanosecond, in typical printed circuit boards it drops to 150 mm per nanosecond. Thus, a 300 MHz signal, for example, in a printed circuit board will have a wavelength of about 500 mm. Therefore, it is often necessary to take into account the influence of dielectric effects in calculations.

Time and frequency analysis are two methods for investigating the behavior of an alternating current or voltage. The first method refers to the behavior of the signal in time, the second – to the behavior of the frequency response.

A transmission line is some kind of structure designed to transmit high frequency energy from one place to another. The characteristic impedance (terminal resistance of an electrically homogeneous transmission line) is a very important value in terms of transmitting maximum signal energy, as well as for matching the input and output impedances of various circuit elements. Transmission lines come in many varieties, but the most commonly used in PCB design are asymmetrical microstrip, embedded microstrip, and stripline transmission lines (Figure 1). Each of these types has its own parameters. Most often, printed circuit boards have embedded microstrip transmission lines located on the surface of the board and covered with a layer of varnish (mask) on top.
The carrier is traditionally understood as the center frequency that carries the radio signal. The carrier can be modulated in various ways to transmit information.

Modulation is the process of changing the behavior of a carrier frequency to transmit information.

Bandwidth – The range of  high frequency pcb around the frequency of interest.

Skin effect (skin effect) – a phenomenon in which an alternating current tends to flow along the surface of a conductor with increasing frequency. The skin effect causes the conductor impedance to increase as the frequency of the signal increases.

Resonant frequency – the frequency at which the reactances of a series or parallel connected inductor and capacitor are equal. A series resonant circuit has a low impedance at the resonant frequency, while a parallel resonant circuit has a high impedance (Fig. 2).

Power measurement is a measurement method (in the context of RF applications) in which the characteristics of a circuit or device as a whole are preferably defined in terms of power ratios rather than in absolute units of voltage or current. This value is usually expressed as the logarithm of the ratio and describes the gain or loss of amplifiers or other circuits. Ratio values ​​expressed in logarithmic units are more convenient to add and subtract than to multiply and divide absolute values.

Dielectric constant is a measure of the effect of the properties of an insulating material on a transmitted RF wave or adjacent material, relative to the effect of the same wave passing through air.

PLACEMENT OF COMPONENTS

As with the vast majority of things, careful and careful placement of components is the key to good circuit performance.
Optimal component placement reduces the length of high-frequency signal lines, reduces or eliminates signal interference, and minimizes coupling between powerful and sensitive parts of the circuit.

DETERMINING THE BEST SIGNAL PATH

Before general wiring, it is necessary to carefully consider the paths of high-frequency signals, which involves minimizing the length of high-frequency traces, reducing the interference of signals, except for special cases, spacing the input and output busses of amplifiers and filters.

 

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