Propagation velocity is becoming an increasingly important specification in high-speed digital
designs. Modern designs require that the flight times and interconnect skews be controlled to
5 ps per inch of trace or less. Calibrated correctly, the VNA can provide a significantly more
accurate characterization of the propagation delay than can be achieved with the TDR.
The VNA can measure propagation delay in two different ways. The first and less accurate
method is to put the VNA in time-domain mode and perform a normal TDR/TDT
measurement as described earlier in the chapter. For TDT measurements the data are taken
by using a two-port measurement that compares the open S
response to the S
The more accurate method is to measure the phase of the signal using a two-port
measurement. This technique simply looks at the phase difference between ports 1 and 2.
The phase difference is proportional to the electrical delay of the structure. The
measurement is most easily taken by displaying the data in a polar format, as shown in
. With the VNA in polar format, the phase delay between ports 1 and 2 can be
measured, which can be used to calculate the propagation delay as a function of frequency.
This is typically much more accurate than the TDR method because the VNA will output a
sine wave. Losses in a system can significantly degrade the edge rate in a TDR
measurement, which leads to
Figure 11.30: Example of a polar display format (magnitude and phase) versus
frequency. Delay will be a function of phase and loss a function of magnitude.
incorrect delay measurements. As losses will degrade primarily the amplitude of a sinusoidal
signal, they will have a minimal effect on the accuracy of the measurement.
Polar measurements are completed measuring forward S
or reverse S
with the VNA
display in polar format. With the unit in polar format, the phase angle as a function of
frequency is displayed. Adjust the marker for the frequency at which the propagation delay is
desired. Delay can then be calculated using the equation
where F is the frequency and is the measured phase angle. The propagation velocity is
calculated with the equation
With the VNA in polar format, the phase delay between ports 1 and 2 can be measured, which can be used to calculate the propagation delay as a function of frequency. Losses in a system can significantly degrade the edge rate in a TDR measurement, which leads to Figure 11.30: Example of a polar display format (magnitude and phase) versus frequency.