How to test the circuit impedance of the PCB board with TDR

The next-door King has already talked about the principle of TDR and how to determine the TDR resolution. So, we have to correctly measure the circuit impedance on the PCB board, and what needs to be noted?

1, the impedance Test industry standards

Have posted a lot of impedance test pictures, re-paste a picture for everyone to see. Impedance is not an imaginary straight line, but a downs. In the front and back end will be affected by the probe or open circuit, in the middle due to the production process of the relationship, there will be fluctuations.

So how do we judge the test results? How to determine whether the PCB impedance of the production meet the requirements? First look at the IPC specification, IPC2557A recommended measurement range is the 30%~70% interval of the DUT.

Looking at the test habits of Intel and now the mainstream board factory, to avoid the effects of the launch area and the reflection area, the test interval is recommended to be 50% to 70% of the Dut area.

To test the line impedance with TDR, we first need to understand the requirements of the test interval to accurately understand the results of the measurement.

2, the impact of the probe on the impedance test results

Typically, the TDR test uses the following probes:

Among them, the plate factory will usually use hand-held probe point measurement, probe influence of the emotional. Si labs commonly use SMA to connect test cables, and the SMA may exhibit a capacitive resistance test. The effect of the two probes on the test results is as follows:

Due to the perceptual or capacitive effects of probe, the final DUT test results will be slightly biased.

How to test the circuit impedance of the PCB board with TDR