How to test the PCB impedance value?

                                                            How to test the PCB impedance value?

At present, the main test used for PCB (printed circuit board) signal lines and component impedance is TDR test.

There are many factors affecting the accuracy of TDR impedance test, including reflection, calibration, reading selection, etc. Reflection will cause serious deviations in the test impedance value of the short signal line of the PCB, especially in the case of using TIP (probe) to test the impedance. The deviation is more obviously because the TIP and signal line contact points cause large impedance discontinuities, causing reflections and causing fluctuations in the impedance curve of the PCB signal lines in the vicinity of three or four inches.

There is a requirement about the single-ended signal impedance line, differential signal impedance line and connector impedance test, which is combined with the actual application conditions. For example, if the rising edge of the signal line is about 300ps, then the rising edge of the TDR output pulse signal should be set at about 300ps instead of using the rising edge of 30ps. Otherwise, the impedance result of the test may be quite different from the actual impedance value.