R&S®ZRB2 SWR Bridge
Broadband impedance and reflection coefficient measurements
Measurement accuracy
Measurement accuracy is determined by two effects:
- The finite directivity of the SWR Bridge; the error signal is coupled directly to the bridge output (dotted signal path D).
- Multiple reflections between the device under test and the test port; it is enough for the estimation of the error to consider one reflection only (dashed signal path s).
The approximate relationship between the input signal a and the output signal b is:
b = T x (D + r + S x r2) x a
where T = insertion loss, D = directivity, S = return loss at the test port of the SWR Bridge and r = reflection coefficient of the device under test.
This equation shows that measurement of small reflection coefficients will be detrimentally affected by the finite directivity D of the SWR Bridge. The relative measurement error increases with decreasing reflection coefficient. Reflection coefficients that are smaller than the directivity of the bridge cannot be measured directly. With measurements of large reflection coefficients, this error can be neglected. The error of the measurement depends on the mismatch of the test port of the SWR Bridge. With a directivity of 46 dB and a return loss at the test port of 26 dB, the maximum absolute error as a function of the reflection coefficient to be measured will be 0.005 + 0.05|r|2.
Maximum measurement errors with an assumed return loss at the test port of 22 and 26 dB (SWR = 1.17 and 1.1) and a directivity of 34, 40 and 46 dB
Precision model
The diagram above shows the maximum measurement error to be expected with respect to the measured return
loss and allows a quantitativ determination of this relationship. The plotted curves represent the highest possible positive and negative deviations of the measured value from the true value of the return
loss. It is to be noted that these values are the specified limit values of the
