0 Hints for Making Successful Noise Figure Measurements Application Note 57-3
Table of Contents Introduction ● HINT 1: Select the appropriate noise source ● HINT 2: Minimize extraneous signals ● HINT 3: Minimize mismatch uncertainties ● HINT 4: Use averaging to minimize display jitter ● HINT 5: Avoid non-linearities ● HINT 6: Account for mixer characteristics ● HINT 7: Use proper measurement correction ● HINT 8: Choose the optimal measurement bandwidth ● HINT 9: Account for path losses ● HINT 10: Account for the temperature of the measurement components Appendix A: Checklist Appendix
Introduction To achieve accurate and repeatable results at RF or microwave frequencies, measurement uncertainties and barriers to measurement repeatability must be minimized. The performance of a device can be obscured if errors are allowed to accumulate. For the most accurate measurement, it is important to understand the nature of the error contributors and identify which of these can be influenced or changed to improve the quality of the results.
● HINT 1: Select the appropriate noise source ENR Frequency range The output of a noise source is defined in terms of its frequency range and excess noise ratio (ENR). Nominal ENR values of 15 dB and 6 dB are commonly available. ENR values are calibrated at specific spot frequencies. The uncertainties of these calibrations vary over the frequency range of the noise source and add to the uncertainty of the measurement. This uncertainty is typically limited to approximately 0.
● HINT 2: Minimize extraneous signals A noise figure analyzer measures the noise power from the noise source as affected by the DUT. It uses the power ratio at two detected noise levels to measure the noise figure of whatever is between the noise source and the instrument’s detector. Any interference, airborne or otherwise, is measured as noise power from the DUT and can cause an error of any magnitude.
● HINT 3: Minimize mismatch uncertainties Mismatch at connection planes will create multiple reflections of the noise signal in the measurement and calibration paths (as shown in Figure 3-1). Mismatch uncertainties at these planes will combine vectorially and will contribute to the total measurement uncertainty. Alternately, insert a well matched attenuator (pad) between the noise source and the DUT to attenuate multiple reflections.
● HINT 4: Use averaging to minimize display jitter Noise measurement inherently displays variability or jitter because of the random nature of the noise being measured. Averaging many readings can minimize displayed jitter and bring the measurement closer to the true mean of the noise’s gaussian distribution. If time constraints limit the number of averages during DUT measurement (e.g. in manufacturing), use more averages during calibration.
● HINT 5: Avoid non-linearities Avoid all predictable sources of nonlinearities: A Y-Factor noise figure analyzer assumes a linear change in the detected noise power as the noise source is switched between Thot and Tcold. Any variations from linearity in either the DUT or in the detector directly produce an error in the Y value and hence in the noise figure that is displayed. The instrumentation uncertainty specification accounts for the linearity of the analyzer’s detector.
● HINT 6: Account for mixer characteristics If the device under test is a mixer: • Measure the same sideband(s) that will be used in the application of the mixer. FLO LSB Noise Power avera • For double sideband measurements, select a LO frequency close to the RF band of interest. ge USB FIF • For single sideband measurements, select a LO far from the RF band of interest, if possible. Freq • Choose the LO to suit the mixer. Figure 6-1 • Filter the RF signal (i.e.
To determine if sideband-averaging error is a problem, set up the noise figure measurement for a mixer with swept LO and fixed IF (modern noise figure analyzers allow this). Monitor the noise figure reading. If the noise figure values change dramatically as the LO sweeps, then SSB measurement is recommended (See reference 5 for further information.) Balanced and double-balanced mixers have more than one diode to perform the frequency conversion.
f) Filter the IF signal (ie, the DUT output) if necessary. FLO LSB USB Noise Power Keep the LO outside of the frequency range of the instrument if possible. LO power will almost certainly leak through to the IF port of the mixer. Assume that LO to IF isolation will be insufficient. If this leakage is within the band of the measurement, it will add to the noise figure measured.
● HINT 7: Use proper measurement correction Take the following steps to ensure the measurement system itself does not add error to the measurement. The cascade equation shows how F12 is very sensitive to uncertainty margins in the second stage term [(F2 - 1) / G1]. (To see how F12 would vary with marginal changes in F2 or G1, see reference 6.) If the DUT has insertion loss (e.g. a mixer, attenuator, etc.), use a low noise pre-amplifier before the instrument to reduce the uncertainty margin.
● HINT 8: Choose the optimal measurement bandwidth Select a measurement bandwidth no larger than the pass band of the DUT. Modern noise figure analyzers provide a selection of various measurement bandwidths to enable measurements that are more relevant to current practical applications (e.g. individual wireless GSM channels). 4 MHz bandwidths were common in past generations of noise figure instruments; modern analyzers can measure down to at least 100 kHz bandwidth.
● HINT 9: Account for path losses Adapters must be used if the connectors between the noise source, DUT, and measurement system do not mate, as in Figure 9-1. It is most important to avoid adapters where the signal is smallest in the measurement setup. For devices with gain, avoid adapters before the DUT. For devices with loss, avoid adapters after the DUT.
● HINT 10: Account for the temperature of the measurement components Y-Factor noise figure analyzers assume that the surface temperatures of all components in the measurement (noise source, DUT, connectors, cables, etc.) are the default value for Tcold, 290K (16.8°C, 62.2°F). If this is not the case, enter the correct temperature of each component into the analyzer and monitor them regularly.
Appendix A: Checklist ❑ Account for mixer characteristics. See Hint # 6. ❑ Measure the same sideband(s) that will be used in the application. ❑ For double-sideband measurements, select a LO frequency close to the RF band of interest. ❑ For single-sideband measurements, select a LO far from the RF band of interest, if possible. ❑ Choose the LO to suit the mixer. ❑ Filter the LO if necessary to diminish spurious signals and broadband noise. ❑ Keep the LO outside of the measurement bandwidth if possible.
Appendix B: Total uncertainty calculations The error model in the spreadsheet shown in Figure B-1 is obtained from the derivative of the Cascade equation (F1 = F12 - [(F2-1) / G1]). It takes into account the individual mismatch uncertainty calculations at each reference or incident plane of the DUT, noise source and measurement system. This example represents the total noise figure measurement uncertainty, RSS analysis, for a microwave transistor with an S11 of 0.5, S22 of 0.8 and S21 of 5 (14dB).
Appendix C: References 1. Agilent 346A, 346B, and 346C Noise Sources (10 MHz to 26.5 GHz), Technical Specification Sheet, literature number 5953-6452 2. Fundamentals of RF and Microwave Noise Figure Measurements, Application Note 57-1, literature number 5952-8255E 3. Noise Parameter Measurement Using the Agilent 8970B Noise Figure Meter and the ATN NP4 Noise Parameter Test Set, Product Note 8970B/S-3, literature number 5952-6639 4.
Appendix D: Appendix E: Abbreviations Glossary and definitions AGC DUT DSB ENR F LSB NF RSS SSB Tc Tcold Th Thot USB URL WG 1. Excess Noise Ratio (ENR): ENR is the measure of how much more noise power is output from a noise source when “ON” (i.e. operating at virtual temperature Thot) than is output when “OFF” (i.e. operating at ambient temperature Tcold), normalized by its output power at the standard temperature 290K.
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