Dror Regev, Presto Engineering

1. EVM Test Impairments
Dror Regev
PRESTO-ENGINEERING
May 2, 2012 2012 Regev
May 2, Dror
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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
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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
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4. EVM Introduction
Error vector
measures the
distance on the IQ
plan between the
ideal constellation
point of the
symbol and the
actual point
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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
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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
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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
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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
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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
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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
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