What are S-parameters?
S-parameters are the electrical characteristics of a signal in a complex network. It is also known as scattering parameters and mostly used in RF network and applications. Vector Network Analyzer is used to measure and analyze signal behavior of S-parameters. The signal behavior can be observed as gain, return loss, VSWR, reflection coefficient and group delay etc.
Modern VNAs are capable of performing multiple tasks than just measurement of S-parameters. Basic VNA will have two ports and expandable up to 24 ports depends on the model and applications.
We will take a band pass filter as an example for this analysis. Band pass filters are tuned to a particular bandwidth which is called the passband. It gives maximum suppression to lower and higher frequencies (outside the passband).
Let’s take a look on each formats of VNA (vector network analyzer) and find out what are the uses of each format.
A. Cartesian Display formats (rectangular display formats)
Cartesian diagram is the most common rectangular diagram to represent transmission and reflection parameters of S-parameters. Linearly scaled stimulus data will be displayed on the X-axis and measured value on Y-axis. It is further subdivided into other formats.
1. Logarithmic Magnitude (Log Mag /dB Mag)
In Log Mag format, magnitude is displayed on the Y-axis and frequency will be displayed on X-axis. Log mag format is suitable to analyze below parameters:
- Insertion loss
- Return loss
2. Phase angle
It displays phase deviation of the signal related to the reference calibration line. Phase displayed on Y-axis and frequency on X-axis.
3. Group Delay
Group delay used to display the propagation time delay of the signals in the network. Group delay is frequency dependant and has different delay time for transmission (at different frequencies). Delay time will be displayed on Y-axis (in seconds) and frequency on X-axis.
- Group delay
- Propagation delay analysis
4. Linear Magnitude
Linear magnitude displays the positive values of the magnitude measurement. Normalized measurement displayed on Y-axis and frequency on X axis.
- Reflection coefficient
- Transmission coefficient
- Time domain transfer function
5. Voltage Standing Wave Ration (VSWR)
VSWR format displays reflection coefficient parameter determined by the formula VSWR = (1 + |Γ|)/(1 – |Γ|)where Γ is reflection coefficient.
- Standing wave ratio
6. Real Format
Real format only displays real part of the complex measurement data.
- Time domain measurements
7. Imaginary Format
Imaginary format only displays imaginary part of the complex data.
- Impedance analysis
B. Smith chart format
Smith chart format is a representation of real (r) and imaginary part (r+jX) of the complex measurement. Horizontal line at the middle separates the inductive and capacitive region.
Center of the smith chart is perfect 50 Ohm impedance.
Moving towards the left side of the horizontal line reaches zero impedance which represents a perfect short condition.
Towards right side of the horizontal line, impedance gets higher and higher and until infinity represents perfect open condition.
- Impedance Matching and analysis
Inverse smith chart used to display admittance (Y-chart).
C. Polar Format
Polar format displays both magnitude and phase angle of the reflection coefficient of parameters S11 and S22 etc. It represents linear magnitude or logarithmic magnitude (dB Mag) and phase angle in degree.
- Phase angle
S-parameters are one of the significant information for engineers to design, analyze and simulate complex networks and resolve an issue. Modern network analyzers can be configured to perform even complicated real time small signal and large signal analysis with sophisticated analysis software. The scope of RF has been widening with technology advancements and it demands efficiency improvements.