As the demand increases for faster and denser PCB designs, controlling the impedance of signal traces has become crucial to ensure reliable signal integrity. Two popular methods to measure and control impedance are through Time Domain Reflectometry (TDR) and Network Analyzer (NA). In this blog post, we'll compare the two methods and highlight the advantages and disadvantages of each.
Time Domain Reflectometry (TDR)
TDR works by sending a fast pulse down a trace and measuring the reflected pulse as it returns. The reflected pulse varies depending on the impedance of the trace. A TDR can measure the length, impedance, and attenuation of a trace. It's easy to use, and the results are straightforward to understand.
TDR is useful for impedance control during the design phase, but it has limitations when it comes to verifying the impedance of manufactured boards. For instance, a TDR can't measure losses associated with vias or other non-transmission line elements accurately.
Network Analyzer (NA)
In contrast, a network analyzer measures the insertion and return loss of a trace using frequency domain analysis. It can also evaluate the performance of a trace over a wide frequency range. This makes an NA an excellent tool for checking signal characteristics, such as S-parameters, as well as verifying that a board design meets specific standards.
Unlike TDR, the complexity of an NA setup and the results obtained from its analysis can make it hard to interpret the results accurately. However, NAs can capture more data than TDR in manufacturing environments, including losses due to vias and other structures that could impact impedance matching.
Comparison
Overall, both TDR and NA have their strengths and weaknesses when it comes to measuring impedance on PCBs. Below, we’ve compared the two methods based on essential parameters:
| Parameter | TDR | Network Analyzer |
|---|---|---|
| Accuracy | Excellent | Good |
| Frequency range | Limited to narrow bandwidth | Wide bandwidth |
| Ability to measure additional factors like loss | Limited | Yes |
| Ease of use during design phase | Simple | Complex |
| Ability to analyze manufactured boards | Limited | Extensive |
Conclusion
In general, TDR is a less expensive tool when compared to an NA. It is an excellent tool to verify the impedance during the design phase. However, it has limitations when it comes to testing implementation due to the limitation in measurement accuracy for non-transmission line elements. Meanwhile, NA can measure both the transmission, and non-transmission line structures accurately - this makes it ideal for testing during the manufacturing process. However, NAs can be expensive and difficult to set up and use effectively without proper training. In conclusion, a combination of TDR during the design phase and NA during the testing phase may provide a comprehensive way of verifying the impedance quality of the PCBs.
References
- S. S. Kumar, S. Sriram and S. Sridhar, "A comparative study of TDR and Network Analyser for estimating Printed Circuit Board (PCB) trace impedance," 2020 IEEE International Conference on Engineering, Technology and Education (TALE), 2020, pp. 1-5, doi: 10.1109/TALE48848.2020.9368380.