Internal block diagram and explanation of vector network analyzer

1. What is a vector network analyzer?

The vector network analyzer is a vector network analyzer with high performance, large dynamic range and low noise. The frequency range covers the entire mobile communication frequency band, full dual-port S-parameter measurement, high measurement accuracy, good test stability, and fast measurement speed.

Usage: It can be widely used in the R&D and production testing of radio frequency devices and components in the fields of mobile communications, military industry, semiconductors, radio and television, scientific research and education.

High degree of integration, digitization and automation coupled with high-resolution frequency synthesizer make fast measurement and application possible. The measurement results can be printed into charts through the instrument's various output configurations and storage memory systems or connected to a computer to form an automatic test and management system, and the operator's measurement waveforms stored inside the instrument can be recalled at any time for comparison measurements.

2. Composition of vector network analyzer

Vector network analyzer components

The figure above shows the internal block diagram of a typical dual-port vector network analyzer. The network analyzer contains the following four parts:

1. Signal source: Provides excitation input signal for the device under test;

2. Signal separation device: Contains power splitter and directional coupling device to complete the extraction of input and reflection signals of the device under test respectively;

3. Receiver: (R1, R2, A, B) tests, compares and analyzes the reflection, transmission and input signals of the device under test;

4. Processing and display unit: Complete processing and display of 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 relationship between the transmission and reflection characteristics of the device under test and the 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 sweeping the signal source. Since the test structure needs to consider the impact of many different signal source parameters on the system, we generally use Synthesize sweep signal source.

When the frequency range of the sweep is set to zero (Span=0Hz), the network analyzer output signal is a point frequency CW signal.

The output power control of the network analyzer relies on two parts: ALC (AutomaTIc Level Control) and the attenuator. ALC ensures the stability of the input signal power and power sweep control. Since the ALC control range is limited, the attenuator is required to complete the large-scale control. Range power.

2.2 Signal separation device

The power divider and directional coupler inside the network analyzer complete the extraction of the input signal and reflected signal of the device under test respectively. These two parts are collectively called the signal separation device. This part of the hardware is usually tested as a "test socket". In some special test occasions (high power testing, etc.), the built-in test socket integrated with the network analysis instrument can be used instead of the built-in test socket. equipment.

The power divider inside the network analyzer distributes the output power of the signal source to the two reference receivers R1, R2 and as the input signal to the two ports. The directional coupler is directly connected to the test port to extract the reflected signal and measure the reflection characteristics.

The directivity (DirecTIvity) of a directional coupler is used to reflect the ability of a directional coupler to separate two signals in opposite transmission directions.

The directionality is calculated as shown in the following formula.

Vector network analyzer directivity calculation formula

Directivity (dB) = Isolation (dB) - Coupling coefficient (dB) - Attenuation (dB)

2.3 Receiver

The receiver completes testing and analysis of the amplitude and phase parameters of the reference signal, reflected signal, and transmitted signal.

Receiver performance affects the test accuracy, dynamic range and test speed of the network analyzer. In order to have good test sensitivity and dynamic range, a tuned receiver is used, which can also suppress harmonics and spurious signals.

The picture above shows the working principle diagram of the tuning receiver. The tuning receiver uses a local oscillator signal (LO) to mix the radio frequency signal to obtain a relatively low-frequency intermediate frequency signal (IF). After the IF signal is band-pass filtered, the receiver bandwidth can be narrowed and the sensitivity and dynamic range can be significantly improved. Finally, the network subdivision uses ADC (analog-to-digital conversion) and DSP (digital signal processing) to extract amplitude and phase information from the intermediate frequency signal. Tuned receivers are commonly used in vector network analyzers and spectrum analyzers.

Dynamic range = maximum received power - received noise level

As shown in the figure below, the maximum received power in the left and right figures is the same, both around 0dB. However, the received noise level in the left picture is less than -120dB, while the noise level in the right picture is about -90dB. Therefore, the dynamic range of the left picture is greater than 120dB, while the dynamic range of the right picture is only about 90dB.

Figure 2-1 The impact of network analyzer receiver bandwidth on test dynamic range

Figure 2-1 The impact of network analyzer receiver bandwidth on test dynamic range

The receiver frequency sweep test process uses a phase-locked loop to ensure synchronous scanning with the frequency of the excitation source, and synchronous control of the RF processing and baseband processing of the four-channel receiver to maintain phase coherence.

2.4 Handling display units

The display processing part of the network analyzer completes the processing of the test results and displays the test results in the required manner. The display function is very powerful and flexible, such as passing test results, limit line judgment (limit line), marking test results (marker), file processing (normalization, storage and reading, etc.), built-in VBA programming and other functions of test data Processing (embedding processing, de-embedding processing, differential parameter conversion, impedance conversion, time domain conversion, etc.), etc.