USB-5684 Measurement Uncertainty
- Updated2024-01-05
- 3 minute(s) read
USB-5684 Measurement Uncertainty
Power measurements have many components that affect overall measurement uncertainty when using the USB-5684. 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-5684 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-5684 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-5684 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 signal (CW) at +12.0 dBm and –55.0 dBm from a source with 1.5:1 VSWR, +20 °C to +30 °C. The example is based on an aperture time of 20 ms and 64 measurement averages.
| Specification | Uncertainty Specification at +12 dBm | Uncertainty Specification at ‑55 dBm | Probability Distribution | Divisor | Standard Uncertainty at +12 dBm | Standard Uncertainty at ‑55 dBm |
|---|---|---|---|---|---|---|
| Linearity and Calibration Factor | 2.33% | 3.16% | Normal at 2σ | 2 | 1.16% | 1.58% |
| Noise | 0.02% | 7.63% | Normal at 2σ | 2 | 0.01% | 3.82% |
| Zero Set | 0.01% | 10.51% | Rectangular | √3 | 0.00% | 6.07% |
| Zero Drift | 0.01% | 10.86% | Normal at 2σ | 2 | 0.00% | 5.43% |
| Mismatch Uncertainty | 4.00% | 4.00% | Rectangular | √3 | 2.31% | 2.31% |
| Combined Standard Uncertainty | — | — | — | — | ±2.59% | ±9.42% |
| Coverage Factor | — | — | — | — | 2 | 2 |
| Expanded Uncertainty | — | — | — | — | ±5.18% (+0.22 dB, -0.23dB) |
±18.84% (+0.75 dB, -0.91 dB) |