How does a VNA measure the amplitude and phase response of a device under test?
1 Answer
A Vector Network Analyzer (VNA) is a sophisticated electronic test instrument used to measure the amplitude and phase response of a device under test (DUT) across a range of frequencies. It is commonly used in the fields of RF (Radio Frequency) and microwave engineering. The primary function of a VNA is to analyze the behavior of high-frequency components and systems.
The VNA operates by sending a known test signal through one port of the DUT and measuring the response at other ports. The VNA measures both the magnitude (amplitude) and phase of the transmitted and reflected signals to characterize the DUT's performance. Here's how the process works:
Test Setup: The VNA is connected to the DUT via coaxial cables or waveguides. The VNA typically has multiple ports, with two of them being the most common (referred to as "Port 1" and "Port 2"). For two-port VNAs, one port is used for signal generation (output) while the other is used for signal reception (input). For VNAs with more ports, the concept is extended to measure more complex devices.
Calibration: Before measuring the DUT, the VNA must be calibrated. Calibration is necessary to remove the systematic errors introduced by the test setup and the VNA itself. Calibration standards, such as short, open, and load standards, are connected to the VNA's test ports. The VNA records the known characteristics of these standards to later compensate for their effects during measurements.
Sweeping Frequencies: The VNA operates by sweeping through a range of frequencies to measure the DUT's response across that frequency span. The VNA sends a continuous wave (CW) signal at each frequency point and measures the reflected and transmitted signals at the output port.
S-parameters Measurement: The VNA uses scattering parameters (S-parameters) to characterize the DUT. For a two-port VNA, there are four S-parameters: S11, S12, S21, and S22. These parameters represent the magnitude and phase relationships between the incident and reflected waves at each port.
S11: Reflection coefficient at Port 1 (incident at Port 1 and reflected back).
S12: Transmission coefficient from Port 1 to Port 2.
S21: Transmission coefficient from Port 2 to Port 1.
S22: Reflection coefficient at Port 2 (incident at Port 2 and reflected back).
Calculating Amplitude and Phase: From the measured S-parameters, the VNA calculates the amplitude and phase response of the DUT at each frequency point. The amplitude response is obtained from the magnitude of S-parameters, and the phase response is obtained from the phase angle of S-parameters.
Display and Analysis: The VNA displays the amplitude and phase data on its screen or outputs it to a computer for further analysis. Engineers and researchers can use this data to understand the behavior of the DUT, design matching networks, characterize filters, measure impedance, etc.
By repeating this process over a frequency range, the VNA builds up a comprehensive picture of the DUT's behavior, allowing engineers to understand its performance and make informed decisions in high-frequency system design.
The VNA operates by sending a known test signal through one port of the DUT and measuring the response at other ports. The VNA measures both the magnitude (amplitude) and phase of the transmitted and reflected signals to characterize the DUT's performance. Here's how the process works:
Test Setup: The VNA is connected to the DUT via coaxial cables or waveguides. The VNA typically has multiple ports, with two of them being the most common (referred to as "Port 1" and "Port 2"). For two-port VNAs, one port is used for signal generation (output) while the other is used for signal reception (input). For VNAs with more ports, the concept is extended to measure more complex devices.
Calibration: Before measuring the DUT, the VNA must be calibrated. Calibration is necessary to remove the systematic errors introduced by the test setup and the VNA itself. Calibration standards, such as short, open, and load standards, are connected to the VNA's test ports. The VNA records the known characteristics of these standards to later compensate for their effects during measurements.
Sweeping Frequencies: The VNA operates by sweeping through a range of frequencies to measure the DUT's response across that frequency span. The VNA sends a continuous wave (CW) signal at each frequency point and measures the reflected and transmitted signals at the output port.
S-parameters Measurement: The VNA uses scattering parameters (S-parameters) to characterize the DUT. For a two-port VNA, there are four S-parameters: S11, S12, S21, and S22. These parameters represent the magnitude and phase relationships between the incident and reflected waves at each port.
S11: Reflection coefficient at Port 1 (incident at Port 1 and reflected back).
S12: Transmission coefficient from Port 1 to Port 2.
S21: Transmission coefficient from Port 2 to Port 1.
S22: Reflection coefficient at Port 2 (incident at Port 2 and reflected back).
Calculating Amplitude and Phase: From the measured S-parameters, the VNA calculates the amplitude and phase response of the DUT at each frequency point. The amplitude response is obtained from the magnitude of S-parameters, and the phase response is obtained from the phase angle of S-parameters.
Display and Analysis: The VNA displays the amplitude and phase data on its screen or outputs it to a computer for further analysis. Engineers and researchers can use this data to understand the behavior of the DUT, design matching networks, characterize filters, measure impedance, etc.
By repeating this process over a frequency range, the VNA builds up a comprehensive picture of the DUT's behavior, allowing engineers to understand its performance and make informed decisions in high-frequency system design.