evm test impairements

May 2, 2012 Dror Regev Presto-Engineering

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EVM Test Impairments Dror Regev PRESTO-ENGINEERING

May 2, 2012


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About Presto Engineering

• 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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Leader in Integrated Test & Product Engineering and Back-end Production services


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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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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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Thermal Noise and EVM

Thermal Noise reflects random fluctuations in sub-symbol’s amplitude.

These fluctuations are normally distributed.

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I

Q

Thermal Noise Fluctuations in

Symbol’s Amplitude

𝐴 𝑡 = 𝑄 𝑡 2 + 𝐼 𝑡 2 + TN(t)

For symbol’s duration:

Thermal Noise


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Phase Noise and EVM

Phase Noise reflects random fluctuations in the sub-symbol’s phase.

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Phase Noise over Frequency:

f

Loop BW

Carrier

Reference Noise

VCO Noise

I

Q

Phase Noise Symbol

Fluctuations

𝜑 𝑡 = tan−1 𝑄 𝑡

𝐼 𝑡 + PN(t)

𝜑(𝑡)

For symbol’s duration:

Phase Noise


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Total Noise and EVM

The total sub-symbol noise uncertainty will for a cloud in the IQ constellation Plan.

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I

Q

Thermal and Phase Noise Fluctuations in the

Sub-Symbol’s Constellation Plan

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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Spurious Signal and EVM

When a spur exists during symbol’s duration, the different sub-symbols will be distorted.

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t

A

The Spur will form a circle around constellation point

Sub-symbol and Spur presence in time domain:

Spur Effect on EVM:

Constellation Plan under Spur presence:

Amplitude Error

Phase Error


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Amplitude non-linearity and EVM Advanced QAM modulations include multiple sub-carriers (sub-symbols), hence it is fairly complicated to predict linearity EVM analytically.

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4 sub-carrier voltages in Frequency domain Example:

f

f1 f2 f3 f4

Δf ∆𝑓 =

1

𝑇 =

1

𝑺𝒚𝒎𝒃𝒐𝒍 𝑫𝒖𝒓𝒂𝒕𝒊𝒐𝒏

3

3

2

210

0

)(

vgvgvgg

vgvVi i

iDCout

Assuming Non-Linear output current of the form:

Non-Linear terms

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.

)cos()cos(

)cos()cos(

4433

2211

tvtv

tvtvv


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Amplitude Saturation and EVM

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4 sub-carrier voltages in Time domain Example:

t

v

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.

QAM modulation symbols usually have high Peak to Average Ratios during symbol duration.

Test equipment needs to have high enough saturation levels such that transmitted peaks will not be clipped.

Amplitude Peak


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Filtering Amplitude Effect on EVM

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

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.

Multi carrier base band signals, may encounter different filter amplitude transfer functions for the different carriers.

In-band Ripple

f f

1 1

Two common LPF topology examples:

Butterworth Chebyshev


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Filtering Phase Effect on EVM

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𝐻 𝑗𝜔 = 𝐻 𝑗𝜔 𝑒𝑗𝜃(𝜔)

Filters have a transfer function of the form:

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.

Group delay is defined as: 𝜏 𝜔 = −𝜕𝜃(𝜔)

𝜕𝜔

and will be constant for a linear phase filter.


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Filter Group Delay & EVM

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Amplitude |H(jω)| and phase θ(ω) transfer functions are related, hence Group Delay 𝝉 𝝎 is also amplitude dependent.

Amplitude & Group Delay both change at

filter’s BW edges.

Change will depend on Filter’s type and order

f

Amplitude

Group Delay

Qualitative LPF Amplitude and Group Delay example:

|H(jω)| 𝝉 𝝎

BW Edge

• 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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Vector Origin shift DC Offset & LO leakage effects

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LO Leakage signals will be direct down converted at the

receiver to I & Q DC offsets and have a similar effect on EVM.

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

I

Shifted Origin


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IQ Amplitude Mismatch EVM Impairment

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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:

I

Q AI=|HI(jω)|*I AQ=|HQ(jω)|*Q

AI

AQ

Amplitude IQ mismatch can generate both amplitude and

phase errors


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IQ Phase Mismatch EVM Impairment

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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:

θε(ω)=θI(ω)-θQ(ω)

Phase IQ mismatch can generate both amplitude and

phase errors

I

Q


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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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evm test impairements

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