What is VNA?

矢量网络分析仪（VNA）是测量电气网络参数的测试仪器。 它们对于各种无源和有源器件（包括滤波器、天线和功率放大器）的射频（RF）和微波元器件分析至关重要。

网络分析仪是在设计和生产过程中进行传输、反射和阻抗测量以及 s 参数测量的理想仪器。

网络分析仪可用于表征射频（RF）器件。尽管最初只是测量 S 参数，但为了优于被测器件，现在的网络分析仪已经高度集成，并且非常先进。

我们先来看一下网络分析的基础知识，再来看看网络分析仪还可以执行哪些高级测量。

什么是网络分析仪？

网络分析仪可用于表征射频（RF）器件。尽管最初只是测量 S 参数，但为了优于被测器件，现在的network analyzer已经高度集成，并且非常先进。

射频电路需要独特的测试方法。在高频内很难直接测量电压和电流，因此在测量高频器件时，必须通过它们对射频信号的响应情况来对其进行表征。网络分析仪可将已知信号发送到器件、然后对输入信号和输出信号进行定比测量，以此来实现对器件的表征。

早期的网络分析仪只测量幅度。这些标量网络分析仪可以测量回波损耗、增益、驻波比，以及执行其他一些基于幅度的测量。

现如今，大多数网络分析仪都是矢量网络分析仪——可以同时测量幅度和相位。矢量网络分析仪是用途极广的一类仪器，它们可以表征 S 参数、匹配复数阻抗、以及进行时域测量等。

这个测量示例的高级框图显示，有一个正向发送的信号通过被测器件的输入端到达了输出端。从器件的输入端到输出端的测量被称为正向测量The

网络分析仪的接收端可以测量入射、反射和传输的信号，以便计算正向 S 参数。

Key Specifications of Vector Network Analyzers

Vector network analyzers are both signal generators and receivers, so they have a large number of very necessary technical specifications. In this section, you will learn about some of the key technical specifications of network analyzers.

Maximum frequency

The maximum frequency of a VNA is the highest frequency it can measure. The receiver side of a network analyzer has an analog-to-digital converter (ADC) that converts the input signals into digital format. These signals can then be analyzed and displayed. However, the ADC does not have the ability to convert signals in the RF range, so the incident signal must be downconverted to its operating frequency. This operating frequency is called the intermediate frequency (IF).

dynamic range

Dynamic range is the range of power over which the response of a component can be measured.

This figure shows two different ways of defining dynamic range. The system dynamic range is the value used for instrument specifications.

• The system dynamic range is the function of the instrument when no boost amplifier is used and the gain of the device under test is not taken into account. The maximum source power of an instrument is its maximum power level, Pref
• The receiver dynamic range is the dynamic range of the instrument when power amplification is used. Unlike using the source power as the maximum power level, this specification is based on the maximum power Pmax that can be measured at the receiving end of the instrument.

A trace measured by the bandpass filter S21, which shows the dynamic range of the instrument, is shown in the lower left-hand plot. The upper limit of the trace is relatively flat and the lower limit contains some noise. Let's look at what factors determine the shape of these boundaries.

The maximum power level of the dynamic range is determined by the upper limit of the source power level and the compression point of the receiver.

The mixers and amplifiers that make up a receiver can only handle so much power before they reach saturation, or before they reach maximum output. When a device is in the saturation region, there is no longer a linear relationship between its input and output.

The saturation of the amplifier can be seen in the right-hand diagram below. At input powers higher than 1W, the actual output (red) will deviate from the desired output (green). This phenomenon is called compression. The receiver cannot capture the signal from any device output above its compression point. This limitation of the input power constitutes the upper limit of the dynamic range.

output power

Output power reflects how much power the VNA's signal generator and tester can transmit into the device under test. It is expressed in dBm and is referenced to 50Ω impedance to match the characteristic impedance of most RF transmission lines.

High output power is useful for improving the signal-to-noise ratio of a measurement or determining the compression limits of the device under test.

Many active devices, such as amplifiers, require extremely challenging linear and nonlinear high power measurements beyond the power limits of the network analyzer.

trace noise

Trace noise is the superimposed noise you see forming on the response of the device under test due to random noise in the system. It can make the signal look less smooth and even a little jittery.

Trace noise can be eliminated by increasing the test power, decreasing the bandwidth of the receiver, or averaging.