In Episode 12 of No Dumb Questions, we explain the concept of impedance in transmission lines and why matching it is essential for reliable signal transmission.
Characteristic impedance is a property of a transmission line determined by its physical structure and dielectric materials. In data systems, the characteristic impedance must be matched across the entire chain—the source, the transmission line, and the load. When they are not matched, part of the signal reflects back, causing degradation. The larger the mismatch, the worse the signal integrity.
Engineers choose the impedance standard based on the specific application and ensure everything in the system is designed and terminated to match. Common design targets include:
- Coaxial cables: Typically 50Ω or 75Ω
- Twinax cables: Typically 90Ω or 100Ω
We also demonstrate how characteristic impedance is measured using a vector network analyzer (VNA). The system tests the cable and displays impedance readings, verifying consistency from different ends of the cable. Measurements are typically taken at a defined time point, such as 1 nanosecond, to ensure proper impedance matching and clean, reliable data transmission.
Key Technical Takeaways
- Characteristic Impedance: Quantifies a transmission line’s electromagnetic properties and is determined by its physical structure and dielectric materials.
- Impedance Matching: Crucial across the source, transmission line, and load to prevent signal reflections and maintain data integrity.
- Common Impedance Targets: Coaxial cables are often designed to 50Ω or 75Ω; twinax cables to 90Ω or 100Ω.
- Measurement: Tested using a vector network analyzer (VNA) from both ends of the cable at a standard time point (typically 1 nanosecond).
More great content coming up in 3…2…1…Episode 13: What are cable considerations for harsh environments?
Episode 12 Transcript – What is Impedance and How is it Measured?
Welcome to No Dumb Questions
Hello and welcome to today’s episode of No Dumb Questions, where the only dumb question is the one you don’t ask. Today we’re going to be talking about impedance. So what is the impedance of a transmission line?
Understanding Characteristic Impedance
All right. Before we go any further, let’s talk about impedance. The characteristic impedance is a term used to quantify a transmission line’s electromagnetic properties. The impedance is determined by the transmission line’s physical structure and dielectric materials. In data transmission, it is crucial to have a matched impedance throughout the full system. The source, transmission line, and load must all be matched.
Any mismatch in the system will result in opposition to signal propagation with reflections. The greater the mismatch, the greater the opposition to signal flow. And now back to what we were saying before.
Common Impedance Targets for Coax and Twinax
When we design a twinax or coax, one of the first things we ask is what impedance are we designing to? Coaxes are commonly designed for 50Ω or 75Ω. And twinaxes are commonly designed to 90Ω or 100Ω. In the end use of the application, you’re going to terminate your transmission line to an equal impedance. So that’s why it’s so crucial that we know ahead of time and we can match it in our design.
Measuring Impedance with a Vector Network Analyzer
So as we do the measurement, we’re going to hook our equipment, or our cable, up to the vector network analyzer. And we have it preset to read all of our common testing. So if I hit stop signal, it performs a test and it displays our results.
This top left screen here is where we read impedance. And as you can see, we’re seeing 98.2 and 98.9. So this essentially just looks at our cable from two different ends to make sure it’s consistent. Our impedance measurement is read at the time of one nanosecond. And that is our typical measurement. We do the same thing for coaxes in a single end format. And that is our common testing.
Thank you for joining us for another session of No Dumb Questions. Please reach out if you have any more.