Atec Agilent-8757D - Manual (Page 7)

7
Precision detector vs. power sensor absolute
power measurement accuracy
Accuracy terms differ depending on the test equip-
ment used to make absolute power measurements.
The following table simplifies and compares the
accuracy terms of an Agilent 8757 system (using
an 85037 series precision detector) and a power
meter (using a power sensor). A measurement
accuracy example is also provided.
Scalar terms Equivalent power meter/sensor terms
Absolute power Power reference uncertainty
accuracy at 50 MHz Instrument linearity
Zero set
Noise
Frequency response Sensor calibration factor uncertainty
Mismatch Mismatch
Scalar analyzer vs. power meter
Absolute power measurement uncertainty examples
Assumptions:
• Measurement frequency = 10 GHz
• DUT input/output SWR = 1.5
• Power measurement range = +10 to –20 dBm
Uncertainty component
(see above table for
8757D Opt. 002/ EPM-4418B
equivalent power meter terms) 85037B 8485A
Absolute power accuracy
at 50 MHz (±dB)
0.11 0.09
Frequency response (±dB) 0.18 0.09
Mismatch (±dB) 0.18 0.12
Total (±dB) 0.47 0.30
Reflection measurement accuracy
Uncertainties due to calibration error and the
frequency response of the source, detectors, and
bridges are removed via open/short averaging.
The remaining uncertainties are primarily the sum
of directivity uncertainty, effective source match
uncertainty, and dynamic power accuracy. As shown
in the graphs below, directivity is the dominant error
term when measuring small reflected signals (high
return loss) and source match is dominant when
measuring large reflected signals (low return loss).
Effective source match vs. reflection uncertainty
Directivity vs. reflection uncertainty
Example calculation
The following example shows how to find the
uncertainty (excluding dynamic accuracy) in
measuring a 14-dB return loss (SWR = 1.5)
with an 85027A directional bridge at 10 GHz
(directivity = 40 dB, test port match = 1.25 SWR).
Uncertainty component Uncertainty
Source match error approximately ±0.2 dB
Directivity error approximately ±0.4 dB
Total uncertainty approximately ±0.6 dB