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How do I measure the Zo characteristic impedance of my PCB trace or my cable with a network analyzer?

You may use an 871XC, ET or ES with option 100 Structural Return Loss and Fault Location. This algorithm finds the average impedance of a trace or cable, rather than a discrete point impedance at a discrete frequency. It then measures the variations from that average impedance.


You may use network analyzer time domain, option 010 on the 8752C, 8753E, 8720D family and 8510C. However the result is the impedance at a point in the trace or cable, not the average impedance. Here is the procedure:

To calibrate, perform a full 2-port error correction if both ends of the DUT may be connected to the analyzer ports 1 and 2. Of only one end is accessible, perform a full 1-port calibration. For accurate time and distance data, enter the DUT velocity factor/2 in the calibration menu (CAL -> MORE -> VELOCITY FACTOR).

Connect the DUT. If only one end is accessible, terminate the other end in a Zo impedance.

Display S11 log magnitude (MEAS -> S11, FORMAT -> LOG MAG).

In the transform menu, select low-pass step (SYSTEM -> TRANSFORM MENU -> LOW PASS STEP). This algorithm extrapolates the DC response necessary for an impedance measurement. This mode assumes that Zo is all real SQRT L/C and so there is no imaginary component in the result. Turn the transform on in the same menu (TRANSFORM on).

Reset the start time to zero seconds (START -> enter 0 and x1 from the keypad). Adjust the start time as necessary to the input launch point, and the stop time to the output adapter or connector or termination. Use markers to accurately identify these components in distance. Then place a gate around a section of trace or cable such that these components are eliminated from the display (SPECIFY GATE in the same menu).

Place a marker near the beginning of the time response to minimize the effects of masking loss on your measurment. Masking loss is the combined effect of IR or dissipative loss and mismatch loss that results in a sloped response.

Turn on smoothing to 20% (AVG -> SMOOTHING APERATURE -> enter 20 and x1 from the keypad -> SMOOTHING on).

Format the display to Smith Chart (FORMAT -> SMITH CHART). The marker now displays all real Zo for that point selected in step F.

!-- To obtain an average value over some span, the point impedance result above may be entered into equation 3 in white paper "Cable Impedance and Structural Return Loss Measurement Methodologies" by Joe Rowell, Joel Dunsmore and Les Brabitz. Keep in mind that the accuracy of time domain measurements is not NIST-traceable. --