R&S®ZRA SWR Bridge
Reflection coefficient measurements
Measurement accuracy
The accuracy of the SWR bridge is limited by its directivity D and the return loss S at its test port. The finite directivity D causes an error signal (dotted signal path D), which passes from the input of the SWR bridge directly to its output without reaching the DUT. The finite return loss S causes multiple reflections between the test port and the DUT. For estimating the error, it is sufficient to investigate a single reflection only (dashed signal path S).
Taking into account the insertion loss T of the SWR bridge, the approximate relationship between input signal a and output signal b is:
b = T × (r+D+S × r2) × a
This equation shows that the measurement of small reflection coefficients r is impaired by the finite directivity D of the SWR bridge. The relative measurement uncertainty increases with decreasing reflection coefficient. Reflection coefficients that are smaller than the directivity of the bridge cannot be measured directly. When large reflection coefficients are measured, the accuracy mainly depends on the return loss S at the bridge’s test port.
With a directivity of 40 dB and a return loss at the test port of 26 dB for example, the maximum absolute error as a function of the reflection coefficient to be measured is 0.01+0.05 × |r|².
The diagram, which shows the maximum positive and negative measurement uncertainty as a function of the measured return loss, allows a quantitative evaluation of this relationship. lt should be noted that these values are the specified limit values of the
