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Dror Regev, Presto

Engineering

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- 1. EVM Test Impairments Dror Regev PRESTO-ENGINEERING May 2, 2012 2012 Regev May 2, Dror 1 Presto-Engineering
- 2. About Presto Engineering Leader in Integrated Test & Product Engineering and Back- end Production services• Service hubs in USA, Europe and Israel• Jan/12: Acquisition of ITH operations• ~100 WW team expert in: – Test Engineering (Test HW and SW) – Qualification & Reliability – Failure Analysis• Special focus in RF testing May 2, 2012 Dror Regev 2 2 Presto-Engineering
- 3. Agenda• Error Vector Magnitude (EVM) Introduction• Thermal Noise & EVM• Phase Noise impairment & EVM• EVM Total Noise Effects• Spurious Impairment EVM• Amplitude linearity EVM impairment• Phase linearity EVM impairment• DC Offset & LO Leakage EVM Effects• IQ Amplitude and Phase EVM impairments May 2, 2012 Dror Regev 3 Presto-Engineering
- 4. EVM IntroductionError vectormeasures thedistance on the IQplan between theideal constellationpoint of thesymbol and theactual point May 2, 2012 Dror Regev 4 4 Presto-Engineering
- 5. Thermal Noise and EVM For symbol’s duration: 𝐴 𝑡 = 𝑄 𝑡 2 + 𝐼 𝑡 2 + TN(t)Thermal Noise Q Thermalreflects random Noisefluctuations in Thermal Noise Fluctuations insub-symbol’s Symbol’s Amplitudeamplitude.These fluctuationsare normallydistributed. I May 2, 2012 Dror Regev 5 5 Presto-Engineering
- 6. Phase Noise and EVM For symbol’s duration:Phase Noise 𝑄 𝑡reflects random 𝜑 𝑡 = tan−1 + PN(t) 𝐼 𝑡fluctuations in thesub-symbol’s Q Phase Noisephase. Phase NoisePhase Noise over Frequency: Symbol Carrier Fluctuations Loop BW Reference Noise VCO Noise f 𝜑(𝑡) I May 2, 2012 Dror Regev 6 6 Presto-Engineering
- 7. Total Noise and EVM Q Thermal and Phase Noise The total sub-symbol Fluctuations in the noise uncertainty will for Sub-Symbol’s Constellation Plan a cloud in the IQ constellation Plan. ISince noise is stochastic these EVM errors can not be calibrated.Different averaging techniques may be implemented but will lengthen EVM test time. May 2, 2012 Dror Regev 7 7 Presto-Engineering
- 8. Spurious Signal and EVM Sub-symbol and Spur presence in time domain: AWhen a spur exists duringsymbol’s duration, thedifferent sub-symbols willbe distorted. t Phase Error AmplitudeSpur Effect on EVM: Error Constellation Plan The Spur will under Spur presence: form a circle around constellation point May 2, 2012 Dror Regev 8 8 Presto-Engineering
- 9. Amplitude non-linearity and EVMAdvanced QAM modulations include multiple sub-carriers (sub-symbols),hence it is fairly complicated to predict linearity EVM analytically. 4 sub-carrier voltages in Frequency domain Assuming Non-Linear output current Example: of the form: iout (VDC v ) g i v i f1 f2 f3 f4 1 ∆𝑓 = = 0 𝑇 Δf g 0 g1v g 2 v 2 g 3v 3 1𝑺𝒚𝒎𝒃𝒐𝒍 𝑫𝒖𝒓𝒂𝒕𝒊𝒐𝒏 Non-Linear v v1 cos(1t ) v2 cos(2t ) terms f v3 cos(3t ) v4 cos(4t )At Base Band frequencies, both squared (like IP2) and cubic (like IP3) terms contributeintermodulation products at the original sub-carrier frequencies and distort sub-symbols.At RF frequencies, it is the cubic term that generates intermodulation products. May 2, 2012 Dror Regev 9 9 Presto-Engineering
- 10. Amplitude Saturation and EVM QAM modulation symbols usually have high Peak to Average Ratios during symbol duration. 4 sub-carrier voltages in Time domain Example:v Amplitude Test equipment needs Peak to have high enough saturation levels such t that transmitted peaks will not be clipped. Another known saturation effect is dependency of transmission phase in input/output power level. This power to phase dependency will also distort the symbol at high power. Pre-distortion techniques may be available to negate some of these effects. May 2, 2012 Dror Regev 10 10 Presto-Engineering
- 11. Filtering Amplitude Effect on EVM Filters are common in test instruments and especially important are those employed at IQ base bands. These Low Pass Filters (LPFs) are necessary for rejecting I and Q signal’s alias but have the potential of degrading EVM. Two common LPF topology examples: Chebyshev Butterworth Multi carrier base1 In-band 1 Ripple band signals, may encounter different filter amplitude transfer functions for the different carriers. f fSince filter in-band ripple or BW “roll-off” can be measured, their effects may be mostlycompensated at system level. May 2, 2012 Dror Regev 11 11 Presto-Engineering
- 12. Filtering Phase Effect on EVMFilters have a transfer function of the form: 𝐻 𝑗𝜔 = 𝐻 𝑗𝜔 𝑒 𝑗𝜃(𝜔)Where the frequency dependent amplitude is given by: |H(jω)| θ(ω)- Phase transfer function should be linear over frequency to support phase accuracy of different sub-symbols. 𝜕𝜃(𝜔) Group delay is defined as: 𝜏 𝜔 =− 𝜕𝜔 and will be constant for a linear phase filter. May 2, 2012 Dror Regev 12 12 Presto-Engineering
- 13. Filter Group Delay & EVM Amplitude |H(jω)| and phase θ(ω) transfer functions are related, hence Group Delay 𝝉 𝝎 is also amplitude dependent. Qualitative LPF Amplitude and Group Delay example: |H(jω)| 𝝉 𝝎 Amplitude & Group Delay Amplitude both change at filter’s BW edges. Group Delay Change will depend on BW Edge Filter’s type and order f• Hence at filter’s BW “roll-off” frequencies Phase transfer function is not linear.• Choosing LPF with BW wider than signal’s BW is usually not practical as it degrades filtering.• These phase nonlinearities are measurable and their effects may be compensated. May 2, 2012 Dror Regev 13 13 Presto-Engineering
- 14. Vector Origin shift DC Offset & LO leakage effectsI and/or Q offsets in the DC level will skew the origin of the IQ constellation plan.The effect is a constant error vector added to all constellation points as seen below: Q LO Leakage signals will be direct down converted at the I receiver to I & Q DC offsets and have a similar effect on EVM. Shifted Origin May 2, 2012 Dror Regev 14 14 Presto-Engineering
- 15. IQ Amplitude Mismatch EVM ImpairmentI and Q gain offsets or different amplitude ripple performance, will degrade EVM.The different amplitude transfer functions will shift all constellation points as shown: AI Q AI=|HI(jω)|*I AQ=|HQ(jω)|*Q AQ I Amplitude IQ mismatch can generate both amplitude and phase errors May 2, 2012 Dror Regev 15 15 Presto-Engineering
- 16. IQ Phase Mismatch EVM ImpairmentI and Q phase transfer functions may differ at all or some of the frequencieseffectively skewing the ideal 900 phase between I and Q degrading EVM.The different phase transfer functions will shift all constellation points as shown: Q θε(ω)=θI(ω)-θQ(ω) I Phase IQ mismatch can generate both amplitude and phase errors May 2, 2012 Dror Regev 16 16 Presto-Engineering
- 17. Summary• Common EVM test impairments reviewed.• Designing an accurate EVM test bench, requires a low internal EVM and mastering minimization of the different impairments.• Calibrations of many residual impairments are possible at test level to enable higher EVM dynamic range measurements.• Presto Engineering is a WW leading test house for mm Wave EVM testing May 2, 2012 Dror Regev 17 17 Presto-Engineering

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