Composition and principle of vector network analyzer (vector network)

One 、 summary
Vector network analyzer is a common RF measuring instrument , It is mainly used to measure high-frequency devices 、 Performance parameters of circuit and system , Such as linear parameter 、 Nonlinear parameters 、 Frequency conversion parameters, etc .
1.1 classification
Vector network analyzer is generally divided by frequency , The higher the cut-off frequency , The more expensive the price is ;
According to the number of test ports, it can be divided into ： Dual port 、 3 port 、 4 port 、 6 port ;
Common manufacturers are ： Agilent of the United States （ Agilent）、 Roeder and Schwartz of Germany （ R&S） , Domestic ones include Nanjing Pune and China power 41 the .
1.2 Common functions
Common test devices are ： Power divider 、 Combiner 、 filter 、 Attenuator 、 The antenna 、 cable 、 amplifier 、 Mixers 、 Duplexer 、 coupler 、 Isolators 、 Circulator 、 Adapter 、 Waveguide 、 Differential devices, etc
Common test functions include ： In Bobbie 、 Return loss 、 Insertion loss 、 Flatness 、 Out of band suppression 、 attenuation 、 gain 、 Isolation 、 Characteristic impedance 、 I / O impedance 、 phase 、 Time delay 、 1dB Compression point 、 Noise factor 、 Differential parameters 、 Common mode parameters 、 Common mode rejection ratio
CMRR etc. .
Two 、 Composition and working principle of vector network analyzer

The figure above shows the internal composition block diagram of a typical dual port vector network analyzer , The network analyzer consists of the following four parts ：
1、 Signal source ： Provide the excitation input signal of the tested device ;
2、 Signal separation device ： Including power divider and directional coupling device , Complete the input and reflection signal extraction of the tested device respectively ;
3、 Receiver ： (R1、 R2、 A、 B) Reflection of the device under test 、 Transmit and input signals for testing 、 Compare and analyze ;
4、 Process the display unit ： Finish processing and displaying the test results .
2.1 Signal source
The signal source provides the excitation signal of the device under test , Because the network analyzer needs to test the transmission of the tested device 、 The relationship between reflection characteristics and operating frequency and power . therefore , The signal source in the network analyzer needs to have frequency scanning and power scanning functions .
The device under test responds to the excitation wave through transmission and reflection , The frequency response of the device under test can be obtained by scanning the frequency of the signal source , Because the test structure needs to consider the impact of many different signal source parameters on the system , Therefore, we generally use a synthetic swept signal source .
When the frequency range of scanning is set to zero （ Span=0Hz） when , The output signal of network analyzer is point frequency CW The signal .

The output power control of network analyzer depends on ALC（ Automatic Level Control Automatic level control ） And attenuator , ALC Ensure the stability of input signal power and power scanning control , because ALC Limited scope of control , Therefore, the attenuator is required to complete a large range of power .
2.2 Signal separation device
The internal power divider and directional coupler of the network analyzer complete the extraction of the input signal and reflected signal of the tested part respectively . These two parts are collectively referred to as signal separation devices , This part of hardware is usually tested as “ Test seat ” , In some special testing occasions ( High power test, etc ) It is not necessary to use the integrated built-in test seat of network analyzer , And use external test stand equipment .
The internal power divider of the network analyzer distributes the output power of the signal source to two reference receivers R1、 R2 And as the input signal of the two ports . The directional coupler is directly connected to the test port , Used to extract reflected signals , Measure the reflection characteristics .

Directivity of directional coupler （ Directivity）, It is used to reflect the ability of directional coupler to separate signals in two opposite transmission directions .
The calculation of directivity is as follows .

Directional （ dB） = Isolation （ dB） - Coupling coefficient （ dB） - attenuation （ dB）

2.3 Receiver
The receiver completes the reference signal 、 Reflected signal 、 Test and analysis of amplitude and phase parameters of transmission signal .
The performance of the receiver affects the test accuracy of the network analyzer 、 Dynamic range and test speed . In order to have good test sensitivity and dynamic range , Using a tuned receiver , It can also suppress harmonics and parasitic signals .

The above figure shows the working principle of the tuning receiver , The tuning receiver uses a LO signal （ LO） To mix RF signals , Get a low-frequency IF signal （ IF） . If signal is bandpass filtered , It can narrow the bandwidth of the receiver and significantly improve the sensitivity and dynamic range . Finally, the network is used ADC（ A / D conversion ） and DSP（ Digital signal processing ） Extract amplitude and phase information from intermediate frequency signal . Tuning receiver is widely used in vector network analyzer and spectrum analyzer .
Dynamic range = Maximum received power - Receive noise level
As shown in the figure below , The maximum received power of the left and right figures are consistent , All in 0dB near . But the received noise level in the left figure is less than -120dB, The noise level in the right figure is -90dB about , Therefore, the dynamic range of the left figure is larger than 120dB, The dynamic range of the right figure is only 90dB about .

chart 2-1 Influence of network analyzer receiver bandwidth on test dynamic range
The frequency scanning test process of the receiver is synchronized with the frequency of the excitation source through the phase-locked environmental protection certificate , 4 Synchronous control of RF processing and baseband processing of channel receivers , Maintain phase coherence .
2.4 Process the display unit
The display processing part of the network analyzer completes the processing of the test results and displays the test results in the required way . The display function is very powerful and flexible , If the test results are qualified 、 Limit judgment （ limit line） 、 Identify test results （ marker） 、 Document processing （ normalization 、 Storage, reading, etc ） 、 built-in VBA Processing of function test data such as programming （ Embedded processing 、 To embed processing 、 Differential parameter conversion 、 Impedance conversion 、 Time domain conversion, etc ） etc. .
3、 ... and 、 Test principle analysis
1、 Traditional vector network analyzer VNA It includes a device under test (DUT) And a RF signal generator with multiple measurement receivers , The measurement signal is incident during forward transmission and reverse transmission 、 Reflect and transmit signals . The signal source sweeps the frequency at a fixed power level to measure S Parameters , And scan its power at a fixed frequency , It can measure the gain compression and AM-PM transformation . These measurements can measure the performance of linear and simple nonlinear devices .
2、 For basic S Parameters and compression tests , The signal source and receiver are tuned to the same frequency . however , By shifting the frequency of the signal source and the receiver , Tune the receiver to an integral multiple of the excitation frequency , It can also measure the harmonic performance of the amplifier . The ability to offset the frequency of the signal source and receiver can also measure the frequency converter ( Such as mixer and frequency converter ) Range of 、 Phase and group delay performance .
3、 VNA Only one RF source is needed to measure the S Parameters 、 Compression and harmonics , But adding a second internal signal source can deal with more complex nonlinear characteristics , Such as IMD, Take measurements , This is especially true when these two sources are used together with the signal combiner inside the network instrument . about IMD measurement , Use the signal combiner to combine the two signals , Then it is sent to the amplifier under test (AUT) The input end of the . AUT The nonlinearity of will cause intermodulation components that appear with the amplified input signal . In a communication system , These redundant components will enter the working frequency band and cannot be removed by filtering . In practice , Only third-order components are measured , Because they are the most important factors causing system performance degradation .
4、 Test principle of balance circuit ： It can reduce the sensitivity to electromagnetic interference and reduce the generation of electromagnetic interference . The balance element can be a balance with three ports - Single ended device or balance with four ports - Balance device . Use four ports VNA It is easy to test these components , Differential mode response, common mode response and mode transformation term can be measured . These tests can be done with single ended excitation or real mode excitation . Single ended method is to test only one at a time DUT Port and calculate the differential mode response, common mode response and cross mode characteristics . This is the fastest and most accurate technology , The condition is that the applied power level should make AUT Keep in a linear or moderately compressed workspace .