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EVM Test Impairments
      Dror Regev
      PRESTO-ENGINEERING




         May 2, 2012 2012 Regev
             May 2, Dror
                                  1
            Presto-Engineering
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
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
EVM Introduction
Error vector
measures the
distance on the IQ
plan between the
ideal constellation
point of the
symbol and the
actual point




                      May 2, 2012 Dror Regev   4
                                                   4
                         Presto-Engineering
Thermal Noise and EVM
                         For symbol’s duration:

                             𝐴 𝑡 =       𝑄 𝑡      2   + 𝐼 𝑡   2   + TN(t)
Thermal Noise        Q
                                                                   Thermal
reflects random                                                     Noise

fluctuations in                      Thermal Noise
                                     Fluctuations in
sub-symbol’s                       Symbol’s Amplitude
amplitude.
These fluctuations
are normally
distributed.
                                                              I

                     May 2, 2012 Dror Regev                         5
                                                                             5
                        Presto-Engineering
Phase Noise and EVM
                                                   For symbol’s duration:
Phase Noise
                                                                      𝑄 𝑡
reflects random                                   𝜑 𝑡 =     tan−1            + PN(t)
                                                                      𝐼 𝑡
fluctuations in the
sub-symbol’s                                    Q                                Phase
                                                                                 Noise
phase.
                                                                  Phase Noise
Phase 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
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.



                                                                          I

Since 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
Spurious Signal and EVM
                                   Sub-symbol and Spur presence in time domain:
                             A
When a spur exists during
symbol’s duration, the
different sub-symbols will
be distorted.                                                                     t
                                                          Phase
                                                           Error
                                         Amplitude
Spur 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
Amplitude non-linearity and EVM
Advanced 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
       1
𝑺𝒚𝒎𝒃𝒐𝒍 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 contribute
intermodulation 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
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
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 base
1                    In-band       1
                      Ripple                                        band signals, may
                                                                    encounter different
                                                                    filter amplitude
                                                                    transfer functions
                                                                    for the different
                                                                    carriers.
                               f                                f


Since filter in-band ripple or BW “roll-off” can be measured, their effects may be mostly
compensated at system level.

                                       May 2, 2012 Dror Regev                   11
                                                                                            11
                                          Presto-Engineering
Filtering Phase Effect on EVM
Filters 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
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
Vector Origin shift
    DC Offset & LO leakage effects
I 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
IQ Amplitude Mismatch EVM
             Impairment
I 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
IQ Phase Mismatch EVM
                 Impairment
I and Q phase transfer functions may differ at all or some of the frequencies
effectively 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
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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Evm Test Impairements

  • 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 Introduction Error vector measures the distance on the IQ plan between the ideal constellation point of the symbol and the actual 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 Thermal reflects random Noise fluctuations in Thermal Noise Fluctuations in sub-symbol’s Symbol’s Amplitude amplitude. These fluctuations are normally distributed. I May 2, 2012 Dror Regev 5 5 Presto-Engineering
  • 6. Phase Noise and EVM For symbol’s duration: Phase Noise 𝑄 𝑡 reflects random 𝜑 𝑡 = tan−1 + PN(t) 𝐼 𝑡 fluctuations in the sub-symbol’s Q Phase Noise phase. Phase Noise Phase 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. I Since 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: A When a spur exists during symbol’s duration, the different sub-symbols will be distorted. t Phase Error Amplitude Spur 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 EVM Advanced 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 1 𝑺𝒚𝒎𝒃𝒐𝒍 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 contribute intermodulation 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 base 1 In-band 1 Ripple band signals, may encounter different filter amplitude transfer functions for the different carriers. f f Since filter in-band ripple or BW “roll-off” can be measured, their effects may be mostly compensated at system level. May 2, 2012 Dror Regev 11 11 Presto-Engineering
  • 12. Filtering Phase Effect on EVM Filters 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 effects I 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 Impairment I 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 Impairment I and Q phase transfer functions may differ at all or some of the frequencies effectively 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