USB-5681 Measurement Uncertainty
- Updated2024-01-05
- 3 minute(s) read
USB-5681 Measurement Uncertainty
Power measurements have many components that affect overall measurement uncertainty when using the USB-5681. Visit ni.com/info and enter Info Code ex6a5f to use the uncertainty calculator for your RF power sensor device. Use this calculator to determine the USB-5681 measurement uncertainty, or you can manually calculate the uncertainty by following the process outlined in this section.
The following factors contribute to 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. NI calibrates the power-sensing element at a reference power of 0 dBm (1 mW) and an ambient temperature of 25 °C.
- Temperature Compensation—Describes the relative power level response over the dynamic range of the device. Temperature compensation should be considered when operating the device at temperatures other than room temperature.
- Noise, Zero Set, and Zero Drift—Factors within the device that impact measurement accuracy at the bottom of the device dynamic range.
- Mismatch Uncertainty—Typically the largest component of measurement
uncertainty. Differing impedances between the USB-5681 and the DUT cause the
uncertainty. Mismatch uncertainty is calculated the following ways:
% Mismatch Uncertainty = 100 [(1 ± |
1||2|)2 - 1]dB Mismatch Uncertainty = 20 log(1 ± |
1||2|)2where
1 and 2 are the reflection coefficients of the
USB-5681 and the DUT, respectively.
Measurement Uncertainty Example
The values in the following table show an example of the measurement uncertainty of a 3 GHz continuous wave (CW) signal at +12.0 dBm and –35.0 dBm from a source with 1.5:1 VSWR. The example is based on an aperture time of 20 ms and 64 measurement averages.
| Uncertainty Term | Specification | Uncertainty Specification at +12 dBm (%) | Uncertainty Specification at –35 dBm (%) | Probability Distribution | Divisor | Adjusted Uncertainty at +12 dBm (%) | Adjusted Uncertainty at –35 dBm (%) |
|---|---|---|---|---|---|---|---|
| Sensor Linearity (<+18 dBm) | <3% | 3.0 | 3.0 | Rectangular | √3 | 1.7 | 1.7 |
| Noise | Measurement range 1: <8 mW Measurement range 2: <40 nW | 0.0 | 1.6 | Normal at 2σ | 2 | 0.0 | 0.8 |
| Zero Set | Measurement range 1: <1 mW Measurement range 2: <10 nW | 0.0 | 3.2 | Rectangular | √3 | 0.0 | 0.6 |
| Zero Drift | Measurement range 1: <0.5 mW Measurement range 2: <10 nW | 0.0 | 0.9 | Normal at 2σ | 2 | 0.0 | 0.6 |
| Calibration Factor Uncertainty | 10 MHz: <2.3% 50 MHz to 18 GHz: <1.5% | 1.5 | 1.5 | Normal at 2σ | 2 | 0.8 | 0.8 |
| Mismatch Uncertainty | — | 4.0 | 4.0 | Rectangular | √3 | 2.3 | 2.3 |
| Combined Uncertainty (RSS), Room Temperature | — | — | — | — | — | 3.0 | 3.6 |
| Expanded Uncertainty with K=2, Room Temperature | — | — | — | — | — | 6.0 | 7.3 |
| Temperature Compensation | <1.4% | 1.4 | 1.4 | Rectangular | √3 | 0.8 | 0.8 |
| Combined Uncertainty (RSS, 0 to 50 °C) | — | — | — | — | — | 3.1 | 3.6 |
| Expanded Uncertainty with K=2 (RSS, 0 to 50 °C) | — | — | — | — | — | 6.2 | 7.5 |
The following table shows the noise measurement uncertainty calculations.
| Noise Calculations | 12 dBm (16 mW) | –35 dBm (316 nW) |
|---|---|---|
| Noise | 1 µW/16 mW = 0.0% | 5 nW/316 nW = 1.6% |
| Zero Set | 1 µW/16 mW = 0.0% | 10 nW/316 nW = 3.2% |
| Zero Drift | 0.5 µW/16 mW = 0.0% | 3 nW/316 nW = 0.9% |