Estimated reading time: 1 minute

Beyond Design: Predicting and Measuring Impedance

To control the impedance of high-speed signal interconnects, one first needs to predict the impedance of a specific multilayer stackup configuration. A precision field solver is arguably the most accurate way to calculate the single-ended, edge-coupled, and broadside-coupled differential impedance. Once the board is fabricated, the transmission lines need to be physically measured to determine the actual impedance to qualify the board.

The most common method for measuring PCB trace impedance is to use a time-domain reflectometer (TDR). This measures the impedance in the time domain. However, a far more accurate method is to use a vector network analyzer (VNA), which operates in the frequency domain. The VNA sweeps through a range of frequencies determining the impedance and signal losses at particular frequencies.

However, VNAs are expensive, delicate instruments and not as robust as a TDR, which allows unskilled personnel to operate them in a factory environment yielding repeatable results. Hence, from a cost and practicality perspective, the TDR is the de facto standard impedance measurement instrument in the PCB fabrication industry.

A TDR applies a very fast pulse (less than 100 ps) to an impedance test coupon via a controlled impedance cable and matching impedance probe.

Whenever there is a change in impedance, part of the signal energy is reflected back to the TDR and is measured by the instrument. The magnitude of the reflected signal is related to the value of the discontinuity. Changes in the cross-sectional area of the trace, distance to the plane(s), and return path or proximity to other pads and traces will also cause a change in the impedance and cause a reflection. From this data, it is possible to graph the impedance and its variation over distance or time.

To read the rest of this column, which appeared in the April 2020 issue of Design007 Magazine, click here.