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Using the Power Sensor Uncertainty of a Measurement
MA24104A UG PN: 10585-00016, Rev. A 3-17
3-6 Uncertainty of a Measurement
Power measurements have many component parts that affect overall measurement uncertainty when
measuring power with the MA24104A sensor:
• Measurement Uncertainty: Measurement uncertainty includes the uncertainty associated with the
correction of frequency and the linearity response of the sensor over the entire dynamic range. Anritsu
follows the industry standard condition of calibrating the power-sensing element at a reference power of
0 dBm (1 mW) and an ambient temperature of 25° C.
• Temperature Compensation: Sensor Temperature Compensation describes the relative power level
response over the dynamic range of the sensor. Temperature Compensation should be considered when
operating the sensor at other than room temperature.
• Noise, Zero Set, and Zero Drift: These are factors within the sensor that impact measurement
accuracy at the bottom of the power sensor’s dynamic range.
• Mismatch Uncertainty: Mismatch uncertainty is typically the largest component of measurement
uncertainty. The error is caused by the differing impedances between the power sensor and the devices
to which the power sensor is connected. Mismatch uncertainty can be calculated as follows:
• Source Mismatch:
% Source Mismatch Uncertainty = 100
[|
1 +
Γ
1
Γ
2
| 2
– 1
]
dB Mismatch Uncertainty = 20log
|
1 +
Γ
1
Γ
2
|
• Load Mismatch (not considering inline power sensor insertion loss):
% Load Mismatch Uncertainty = 100
[|
1 +
Γ
2
Γ
3
| 2
– 1
]
dB Load Mismatch Uncertainty = 20log
|
1 +
Γ
2
Γ
3
|
• Load Mismatch (considering inline power sensor insertion loss):
% Load Mismatch Uncertainty = 100
[|
1 + t
2 Γ
2
Γ
3
| 2
– 1
]
dB Load Mismatch Uncertainty = 20log
|
1 + t
2 Γ
2
Γ
3
|
• Directivity Uncertainty:
% Uncertainty due to Finite Directivity = 100
[
(1 +
Γ
3
/D)
2
– 1
]
where
D is the directivity of the inline power sensor expressed in linear units
Γ
1
is the reflection coefficient of the inline power sensor
Γ
2
is the reflection coefficient of the source
Γ
3
is the reflection coefficient of the load
t is the inline power sensor’s transmission coefficient
t = 10
(IL/20)
IL = Insertion Loss of the inline power sensor
Uncertainty Examples
Two measurement uncertainty calculations for Low Aperture Time mode are shown for the MA24104A in
Table 3-3. The MA24104A is used to measure the power of a 1 GHz, +50.0 dBm and +10 dBm CW signal from a
signal source with a 1.5:1 VSWR and a load having a 1.2:1 VSWR. The example is based on 128 measurement
averages.