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Microwave Tech

Microwave Tech

Microwave Tech

Microwave Tech

Decrypting the RF signal chain: features and performance indicators
2021-09-14
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Author:Frank

By focusing on the outstanding characteristics of RF, including phase shift, reactance, dissipation, noise, radiation, reflection, and nonlinearity, a consistent definition basis can be established, covering multiple meanings. 1 This foundation represents a modern all-encompassing definition that does not rely on a single aspect or specific value to distinguish RF from other terms. The term RF applies to any circuit or component that has the characteristics that make up this definition.

We have set the background of this discussion, and now we can start to get into the topic and analyze the general RF signal chain. Among them, the distributed component circuit model is used to reflect the phase shift in the circuit. This shift cannot be ignored at a shorter RF wavelength. Therefore, the approximate representation of the set pcb total circuit is not suitable for these types of systems. The RF signal chain may include a variety of discrete components, such as attenuators, switches, amplifiers, detectors, synthesizers, and other RF analog devices, as well as high-speed pcb and DACs. Combining all these components for a specific application, the overall nominal performance will depend on the combined performance of these discrete components.

Therefore, in order to design a specific system that can meet the target application, RF system engineers must be able to truly consider from a system-level perspective, and have a consistent understanding of the basic key concepts and principles. These knowledge reserves are very important. For this reason, we have compiled this discussion article, which contains two parts. The goal of the first part is to briefly introduce the main characteristics and indicators used to determine the characteristics of the RF device and quantify its performance. The goal of the second part is to provide an in-depth introduction to the various individual components and their types that can be used to develop the RF signal chain for the desired application. In this article, we will focus on the first part, and consider the main characteristics and performance indicators related to the RF system.

Introduction to RF terminology
There are a variety of parameters currently used to describe the characteristics of the entire RF system and its discrete modules. Depending on the application or use case, some of these features may be extremely important, while others are less important or irrelevant. With this article alone, it is certainly impossible to conduct a comprehensive analysis of such a complex topic. However, we will try to follow a common idea, which is to transform a series of complex related content into a balanced, easy-to-understand RF system properties and characteristics guide, so as to concisely and comprehensively summarize the most common RF performance.

In the case of network matching, S21 is equivalent to the transmission coefficient from port 1 to port 2 (S12 can also be defined in a similar way). The amplitude |S21| expressed on a logarithmic scale represents the ratio of output power to input power, which is called gain or scalar logarithmic gain. This parameter is an important indicator of amplifiers and other RF systems, and it can also take a negative value. Negative gain represents inherent loss or mismatch loss, and is usually represented by its reciprocal, namely insertion loss (IL), which is a typical indicator of attenuators and filters.

If we now consider the incident wave and reflected wave at the same port, we can define S11 and S22 as shown in Figure 2. When other ports are terminated with matching loads, these terms are equivalent to the reflection coefficient |Γ| of the corresponding port. According to formula 1, we can correlate the size of the reflection coefficient with the return loss (RL):

Return loss refers to the ratio of the incident power of the port to the reflected power of the source. Based on the port we use to estimate this ratio, we can distinguish between input and output return loss. The return loss is always a non-negative value, indicating how well the input or output impedance of the network matches the impedance of the port toward the source.
It should be noted that this simple relationship between IL and RL and S parameters is valid only when all ports are matched. This is a prerequisite for defining the S matrix of the network itself. If the network does not match, it will not change its inherent S parameters, but it may change the reflection coefficient of its ports and the transmission coefficient between ports. 2
Frequency range and bandwidth
All these basic quantities that we describe will constantly change in the frequency range, which is the common basic characteristic of all RF systems. It defines the frequency range supported by these systems and provides us with a more critical performance metric-bandwidth (BW).

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Non-linear
It should be pointed out that the characteristics of the RF system will not only change with the frequency, but also with the signal power level. The basic characteristics we described at the beginning of this article are usually represented by small signal S-parameters, and non-linear effects are not considered. However, in general, the continuous increase of the power level through the RF network will usually bring more obvious non-linear effects, and ultimately lead to its performance degradation.

When we talk about RF systems or components with good linearity, we usually mean that the key indicators used to describe their nonlinear performance meet the target application requirements. Let's take a look at these key indicators that are commonly used to quantify the nonlinear behavior of RF systems.

The first parameter we need to consider is the output 1 dB compression point (OP1dB), which defines the inflection point for the general device to switch from linear mode to non-linear mode, that is, the output power level when the system gain is reduced by 1 dB. This is the basic characteristic of the power amplifier, which is used to set the working level of the device to the saturation level defined by the output power towards saturation (PSAT). The power amplifier is usually located at the last stage of the signal chain, so these parameters usually define the output power range of the RF system.

Once the system is in the non-linear mode, it will distort the signal and produce spurious frequency components, or spurs. Spurious is measured relative to the level of the carrier signal (unit: dBc) and can be divided into harmonics and intermodulation products (see Figure 3). Harmonics are signals at integer multiples of the fundamental frequency (for example, H1, H2, H3 harmonics), and intermodulation products are signals that appear when two or more fundamental signals are present in a nonlinear system. If the first fundamental signal is at frequency f1 and the second is at f2, the second-order intermodulation products appear at the sum and difference frequency positions of the two signals, namely f1 + f2 and f2 – f1, and f1 + f1 and f2 + f2 (the latter is also called H2 harmonic). The combination of the second-order intermodulation product and the fundamental signal will produce the third-order intermodulation product, two of which (2f1 – f2 and 2f2 – f1) are particularly important, because they are close to the original signal, so it is difficult to filter out. The output spectrum of a nonlinear RF system containing spurious frequency components represents intermodulation distortion (IMD), which is an important term to describe the nonlinearity of the system. 2



The spurious components related to second-order intermodulation distortion (IMD2) and third-order intermodulation distortion (IMD3) can cause interference to the target signal. An important indicator used to quantify the severity of interference is the Intermodulation Point (IP). We can distinguish second-order (IP2) and third-order (IP3) intermodulation points. As shown in Figure 4, they define the hypothetical points of input (IIP2, IIP3) and output (OIP2, OIP3) signal power levels. At these points, the power of the corresponding spurious components will reach the same electrical level as the fundamental component. flat. Although the intermodulation point is a purely mathematical concept, it is an important indicator to measure the tolerance of the RF system to nonlinearity.



noise
Now let's take a look at another important characteristic inherent in each RF system-noise. Noise refers to the fluctuation of electrical signals and contains many different aspects. According to its frequency spectrum, the way it affects the signal, and the mechanism by which it generates noise, noise can be divided into many different types and forms. However, despite the existence of many different noise sources, we do not need to delve into their physical characteristics in order to describe their ultimate impact on system performance. We can study based on a simplified system noise model, which uses a single theoretical noise generator and is described by the important indicator of noise figure (NF). It can quantify the decrease in signal-to-noise ratio (SNR) caused by the system, which is defined as the logarithmic ratio of the output signal-to-noise ratio to the input signal-to-noise ratio. The noise figure expressed on a linear scale is called the noise factor. This is the main feature of the RF system and can control its overall performance.

For a simple linear passive device, the noise figure is equal to the insertion loss defined by |S21|. In a more complex RF system composed of multiple active and passive components, the noise is described by the respective noise factor Fi and power gain Gi. According to the Friis formula (assuming that the impedance of each stage is matched), the influence of noise is on the signal Decrease step by step in the chain:


It can be concluded that the first two stages of the RF signal chain are the main source of the overall noise figure of the system. This is why the components with the lowest noise figure (such as low noise amplifiers) are placed at the front end of the receiver signal chain.

If we now consider a dedicated device or system that generates a signal, when it comes to its noise performance characteristics, it generally refers to the signal characteristics affected by the noise source. These characteristics are phase jitter and phase noise, which are used to represent signal stability in the time domain (jitter) and frequency domain (phase noise). The specific choice generally depends on the application. For example, in RF communication applications, phase noise is generally used, while in digital systems, jitter is generally used. Phase jitter refers to small fluctuations in the phase of a signal, and phase noise is its spectral representation. It is defined as the noise power within a 1Hz bandwidth at different frequency offsets relative to the carrier frequency. It is considered that the power is balanced in this bandwidth (
in conclusion
We can use a variety of characteristics and performance indicators to characterize the RF signal chain. They involve different system aspects, and their importance and relevance may vary from application to application. Although we cannot fully explain all these factors in an article, if RF engineers can deeply understand the basic characteristics discussed in this article, they can easily be transformed into target applications such as radar, communications, measurement, or other RF systems. Key requirements and technical specifications.
ADI relies on the industry's extensive combination of RF, microwave and millimeter wave solutions, as well as deep system design expertise, to meet a variety of demanding RF application requirements. These wide-ranging discrete and fully integrated ADI solutions from antennas to bits help open the entire spectrum from DC to over 100 GHz and provide outstanding performance, supporting communications, test and measurement instruments, industrial, aerospace, and A variety of RF and microwave designs are implemented for defense and other applications.