Generally speaking, the network analyzer is the most basic and the most widely used instrument in the measurement of radio frequency and microwave components. It can provide measurement parameters of linear and nonlinear characteristic components. Therefore, almost all the simulation, process and test of all RF main and passive components will be used. In measuring parameters, it can not only provide reflection coefficient, but also change the size of impedance from reflection coefficient, measure penetration coefficient, and push important S parameters and other important parameters, such as phase, group velocity delay (Group Delay), insertion loss (Insertion Loss), gain (Gain) and even 1dB of amplifier. The compression point (Compression point) and so on.

Basic principle

When electronic circuit components work at high frequency, many characteristics are different from those of low frequency. At high frequency, their wavelengths are relatively smaller compared to the physical scale of the actual circuit components. For example, the speed of the electromagnetic wave in the vacuum is the speed of light, then c= lambda x F, of which C is 3 * 108m/sec of light speed, if operating in 2.4GHz In frequency, if the dielectric coefficient of the air is not taken into account, the wavelength lambda =12.5cm, that is, in a short number of centimeters, the size of the voltage varies greatly because of the phase shift. Therefore, at high frequency, we use the concept of energy and impedance to replace the expression of low frequency voltage and current. At this time, we will introduce the concept of "wave" mentioned in the previous article.

Light waves are a kind of electromagnetic wave. When we use light to analyze a component, we use a known incident light source to measure unknown objects. When the light waves from the air to the other medium, the reflection and partial penetration will be produced by the difference of the refractive index, such as the penetration and inverse of the chemical composition analysis. Radiation spectrum. For radiofrequency, which is the same as electromagnetic wave, the truth is interlinked, the refraction of light is better than the concept of microwave to the impedance. When an electromagnetic wave reaches another discontinuous impedance interface, it will also have the behavior of penetrating and reflecting, from these reflections and the changes in the size and phase of the reflection and penetration. The characteristics of the component are analyzed.

There are many kinds of parameters used to describe components, some of which contain only amplitude messages, such as R.L. Return Loss, Bobbi (SWR Standing Wave Ratio) or insertion loss (I.L. Insertion Loss), which we call pure quantity, and can be obtained as reflection coefficient (gamma Reflection coefficient) and penetration coefficient. Coefficient, etc., we call it vector, where vector can deduce scalar behavior, but pure quantity can not deduce vector properties without phase information.

Important vector coefficients

Reflection characteristics

Here, we focus on several important vector coefficients: first, we define from the reflection coefficient, in which Vrefect is a reflection wave and a Vinc is an incident wave. Both are vectors, that is, the information containing the amplitude and phase, and the reflection coefficient represents the ratio of the incident and reflected energy. Through the theoretical calculus, it can be transmitted from the transmission. The characteristic impedance ZO (Characteristic Impedance) of the line gets the load impedance ZL of the component to be measured, that is, in the network analysis, the Smith diagram (Smith Chart) is generally used to indicate the impedance values at different frequencies. In addition, the reflection coefficient can also be expressed in polar coordinates: the magnitude of the reflection coefficient and the phi mean the phase difference between incident and reflected waves.

Next, we introduce two pure parameters, Bobbi and return loss, in which the meaning of the standing wave is the voltage or current standing wave effect on the transmission line caused by the reflected wave reflected by the incident wave and the device to be measured, and the definition of the Bobbi (SWR) is the ratio of the maximum to the minimum energy in the standing wave, and we can be from the pure quantity. The reflection coefficient is obtained.

In the same way, we can also define the return loss (R.L.) from the value of Rho, which is the value of the ratio of the reflected energy to the incident energy, and the greater the value, the less the reflected energy is.

As for the relative pure parameters derived from the reflection coefficient, we collate it into table 1. Basically, they are a conversion process and tend to use a certain parameter because of different industries and applications.

REMARK:

The standing wave coefficient is also called Bobbi. If there is a reflection wave on the cable line, it produces a standing wave with the interaction of the traveling wave. At this time, the amplitude of the voltage of some points on the line is Vmax, the amplitude of the voltage of some points is Vmin, the ratio of the maximum amplitude to the minimum amplitude is called the resident wave coefficient. The greater the standing wave coefficient, the reflection wave on the line is expressed. In order to control the inhomogeneity of the cable, the input standing wave coefficient S of the cable in the frequency band of a certain length is not more than a specified value. The size of the inhomogeneity in the cable can also be represented by the reflection attenuation. The logarithm of the value is called the reflection attenuation. The larger the reflection attenuation, the smaller the reflection coefficient, that is, the smaller the standing wave ratio, that is, the smaller the internal inhomogeneity.

Penetration characteristics

For the characteristics of penetration, the same is divided into two kinds of pure and vector. For vector coefficients, the most important is the penetration coefficient, in which the Vtrans is the penetrating wave after the object to be measured and the Vinc is the incident wave, and the tau is the pure quantity of the penetration coefficient, and theta represents the phase difference between the incident and the penetrating wave.

For the definition of the pure quantity, the most common use of the passive component is the insertion loss (I.L. Insertion Loss), that is, the parameters associated with the above tau value, defined as. If active components such as amplifiers are used, the amplification effect of the transmitted signal is gain (Gain).