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Demystify Noise Figure Measurements

Demystify Noise Figure Measurements. 8 Oct 2013 14:00h - 14:40h Presented by: Stefano Balzarini. Agenda. Background Definition and Importance Methods Y-factor (or hot/cold) Cold source Implementations Composite receiver Importance of linearity and gain adequacy Measurement steps

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Demystify Noise Figure Measurements

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  1. Demystify Noise Figure Measurements 8 Oct 2013 14:00h - 14:40h Presented by: Stefano Balzarini EuMW Seminars 2013

  2. Agenda • Background • Definition and Importance • Methods • Y-factor (or hot/cold) • Cold source • Implementations • Composite receiver • Importance of linearity and gain adequacy • Measurement steps • Uncertainties • The basic model, terms of interest • What really matters? • The Uncertainty Calculator • Summary EuMW Seminars 2013

  3. Definition F = Noise Factor Si = Input signal power Ni = Input noise power So = Output signal power No = Output noise power GDUT = Gain of amplifier Nadded = Noise added by amplifier Noise figure (NF) is: NF (dB) = 10 * log (F) Easy enough…a quasi-normalized measure of noise power added by the DUT. However, many there are assumptions made about the impedance environment, the type of signals in the measurement, temperatures, etc. 3 EuMW Seminars 2013

  4. Noise Parameters What are noise parameters? A measured NF value is only valid for a source impedance of 50 ohms (usually). At other impedances, the practical noise figure varies and is DUT dependent (usually). Source reflection coefficient for minimum NF Equivalent noise resistance Concept: present different source reflection coefficients (with tuner) and measure NF; solve for the unknowns 4 EuMW Seminars 2013

  5. Importance • Why is NF important? • Cellular systems  voice quality • Digital communications  bit-error rate • Radar  range • Because of this importance, • NF is still a critical customer-vendor specification • It sometimes has a steep cost-function (especially for ultra-low-noise cases • and at mm-wave frequencies) 5 EuMW Seminars 2013

  6. NF Methods: Y-factor (hot-cold) What is it? Historic method that is based on a ratio of noise powers (avoids absolute power cal). Composite Receiver @ TC and TH Excess Noise Ratio = [(TH - TC) / T0] ENRdB= 10 log10 [(TH- TC) / T0] To = 290o Kelvin Noise receiver + preamps… Noise source DUT Noise power is measured when driven with a source in a cold state (near room temp) and a hot state (often with 10-20 dB more noise power). Y-Factor output input Noise power Hot state Cold state Time 6 EuMW Seminars 2013

  7. NF Methods: Y-factor • Noise Source Issues: • Difficult to calibrate • Significant uncertainty-adder (often > 0.15 dB) • Fragile (will go way out-of-cal if dropped) • Impedances in hot and cold states are not equal (problems both from power delivery and from noise parameters) |Gs| for hot and cold states High gain broadband amps can actually be damaged by the hot state. 7 EuMW Seminars 2013

  8. Methods: Cold Source What is it? An absolute noise power difference (not a relative one like before) Implication: Need an accurate receiver cal/power cal The only input to the DUT is the noise power (kT0B) from a termination at T0, nominally a load Composite receiver Bandwidth (B) is set by the IF system; can be known a priori From the S/N definition: Result for cascaded system: 8 EuMW Seminars 2013

  9. Methods: Cold Source Cold source is the generally accepted methodology used by VNA and applied up to ~70 GHz until the recent announcement from Anritsu … Industry-first! 70 kHz to 125 GHz VNA Noise figure measurement capability Industry-first! Optimized noise receiver for measurements from 30 GHz to 125 GHz 9 EuMW Seminars 2013

  10. Measuring Noise Power Noise power can be measured statistically A deterministic background signal IF time By acquiring many samples and performing an RMS-like calculation, slowly-varying or constant interferers are removed. 10 EuMW Seminars 2013

  11. Noise Power Measurement Trade-offs • Selection of acquisition parameters (IFBW) and # of RMS points: • Trade-off between data jitter and sweep time • 100 kHz/3000 is default • 10 kHz/6000 for ~0.1 dB jitter at reasonable gains The dependence is stronger at parameter extremes and in low-net-gain situations. 11 EuMW Seminars 2013

  12. Methods: Cold Source (cont.) Considerations with the cold source method: Accuracy of the power/receiver calibration: The power calibration can be affected by mismatch, sensor linearity, harmonic contributions,… Tighter linearity constraints: Since it is more of an absolute noise power measurement, receiver linearity problems map directly to NF uncertainty (including linearity between receiver cal level and measurement level). 12 EuMW Seminars 2013

  13. Implementations: Composite Receiver Accurate noise figure measurements require high composite receiver gain and appropriate filtering. • External Component Benefits • Greater flexibility in composite receiver design • Can optimize for best performance with DUT • Easier to service and maintain B Composite receiver The difference is whether the components for the composite receiver are inside or outside. If inside, a lot of switching and IF receiver modifications are needed - which can degrade performance of the standard s-parameter measurements. 13 EuMW Seminars 2013

  14. Implementation: VS mm-wave • A separate receiver module • Based on unique 3743A Freq Ext Mod • 8% the weight and 2% the volume of other solutions • coupling loss deleted, • un-needed multiplier paths removed • receiver input better matched • Still need to incorporate into a composite receiver with external components 14 EuMW Seminars 2013

  15. Implementations: Composite Receiver • Two key components: pre-amplifiers and filter(s): • Select the proper amplification such that the resultant noise power into the VNA receiver is: • Not too low. • Not too high. • For VectorStar™, generally want composite receiver gain + DUT gain between ~40 dB and 70 dB for linearity optimization. • Using multiple pre-amplifiers is ok. • Lower pre-amplifier noise figures are better. But if the DUT gain exceeds ~10 dB, it does not matter much. 15 EuMW Seminars 2013

  16. Implementations: Composite Receiver • Example: What may cause low end linearity issue? • -Discretization floor of flipping a few least significant bits (LSBs) • -Integral nonlinearity (INL) of the ADC – sometimes related to charge storage in the sample and hold circuit Rarely plotted this way by ADC manufacturers, but it can be important for noise signals 16 EuMW Seminars 2013

  17. Implementations: Composite Receiver • Select the proper filtering to minimize unwanted images: • Most VNAs incorporate harmonic downconversion • Any downconverter has responses for at least some harmonics of the LO (commonly 3rd, 5th, 7th… for balanced mixers and all harmonics for some structures). • Isolate the harmonic of interest • Example: Measuring amplifier NF at 18 GHz 9 18 27 36 • The receiver will be “listening” at sub- and super-multiples of 18 GHz. • If the pre-amps have gain there, use filters. • If the preamps only covered 1-20 GHz, a high pass filter might be adequate. 17 EuMW Seminars 2013

  18. Implementations: Composite Receiver Limited The example below highlights the effects of having a limited noise figure measurement range. Too much gain (~40 dB) Too low pre-amp gain Just right 18 EuMW Seminars 2013

  19. Implementation: VS- Measurement Process • Three main steps: • Receiver calibration (may include a power cal) • Noise calibration • DUT measurement • DUT gain (GDUT) should be • measured beforehand. • T0is room temperature, nominally. • B is the IF bandwidth of the VNA. B Depending on the DUT, 30-50 dB gain Need to avoid images 19 EuMW Seminars 2013

  20. Implementation: DUT Gain Measurement A note about the DUT gain measurement … make sure the DUT is nowhere near compression. 20 EuMW Seminars 2013

  21. Implementation: Receiver calibration • Establish absolute power reference • Power calibrations help accuracy considerably • Must be done at a low level (usually about -50 to -60 dBm) due to the use of the composite receiver B Power cal at this plane over the relevant frequency list 21 EuMW Seminars 2013

  22. Implementation: Receiver calibration Comparison between using a noise source or VNA source (with power meter calibration) as the absolute power reference Noise Source Re-calibration complexity Stability of source calibration (vibration) More thermal instability Match variance Higher fundamental uncertainty Need +28V switching and timing control VNA Source with Power Cal Better calibration stability Better fundamental uncertainty Easier handling of different signal levels 22 EuMW Seminars 2013

  23. Implementation: Noise calibration Measurement of the noise power of the receiver with input terminated. B 23 EuMW Seminars 2013

  24. Implementation: DUT measurement Insert the DUT to make the NF measurement. B NOTE: It is worthwhile to check the absolute power being delivered to the VNA receiver to ensure that the receiver is not being overdriven. 24 EuMW Seminars 2013

  25. Implementation: Comparisons Preamp linearity issues 25 EuMW Seminars 2013

  26. Uncertainties NF measurement uncertainty can be affected by many factors: B Residual image responses, discrete leakage and RMS computation scatter DUT S-parameter uncertainty Mismatch Mismatch, composite receiver linearity (net gain), composite receiver NF, receiver cal accuracy 26 EuMW Seminars 2013

  27. Uncertainties: What matters? • Receiver gain when the DUT gain drops Fixed 40 dB rcvr gain With fixed composite receiver gain, as DUT gain drops, uncertainty increases More composite receiver gain is needed to keep the same uncertainty as the DUT gain drops 27 EuMW Seminars 2013

  28. Uncertainties: What matters? • Match, particularly between DUT and receiver 28 EuMW Seminars 2013

  29. Uncertainties: What matters? • Power cal/receiver cal accuracy At low power uncertainties, other terms dominate 29 EuMW Seminars 2013

  30. Uncertainties: Calculator • 3 basic modes of operation: • (enter fixed or actual DUT .s2p and NF data as well as various measurement parameters) • Uncertainty vs. frequency • Uncertainty vs. DUT gain (at one frequency) • Required receiver gain (vs. frequency) for a given uncertainty 30 EuMW Seminars 2013

  31. Summary • Background • Definition and Importance • Methods • Y-factor (or hot/cold) • Cold source • Implementations • Composite receiver • Importance of linearity and gain adequacy • Measurement steps • Uncertainties • The basic model, terms of interest • What really matters? • The Uncertainty Calculator MS4640B-041 Noise Figure Measurement 31 EuMW Seminars 2013

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