Testing RF front ends for mobile device designs€¦ · Envelope Tracking (ET) Power Added...

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Testing RF front ends for mobile device designs Chris Ziomek VP of zSeries


What is an RFFE? Skyworks’ SkyOne™ Example •  Multi-Mode Multi-Band SOI FEM •  Integrates all RF and analog devices between transceiver and antennas

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RFFE Complexity & Importance is Growing Requirements for Mobile Devices •  Multiple band coverage •  Higher wireless data rates •  Longer battery life •  Better reception Challenges of Multi-Mode Multi-Band World Phones •  40+ LTE bands (9 to 15+ for world phone) •  Inter/intra band carrier aggregation •  Unlicensed LTE bands @ 5 GHz •  Lower cost & smaller/thinner phones •  More ports, more RFFE components,

shared antennas, more complexity

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RFFE System Example: Qualcomm’s RF360™ •  Multi-Mode Multi-Band CMOS FEM •  Antenna Matching •  Envelope Tracking •  Digital Pre-Distortion •  Integrated 3D Packaging including Filters & Duplexers (RF POP)

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RF FEM Components • Components within an RFFE - Switches, filters, duplexers, matching - Low Noise Amplifier (LNA) in RX path - Power Amplifier (PA) in TX path

• PA determines mobile device design & performance - Dynamic EVM & long packet effects - Spectral Mask and ACLR - Digital Pre-Distortion (DPD) - Power Added Efficiency (PAE) & Envelope Tracking (ET)

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Dynamic EVM & Amplitude Droop •  PA is pulsed for power efficiency •  Dynamic EVM (pulsed) degraded by power supply & thermal transients •  Static EVM (always ON) often better than dynamic EVM •  Output power can drop over long packets due to self heating •  Small amplitude droop can significantly impair EVM •  Difficult to identify on power vs time plot •  Can identify amplitude droop using pilot amplitude tracking

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Adjacent Channel Leakage Ratio (ACLR)

For low-order modulation, spectral emission mask and ACLR limit Pout •  PA is driven into non-linear operating region to increase power efficiency

•  Non-linear PA determines TX mask and ACLR performance •  Higher Peak-to-Average Power Ratio (PAPR) causes more non-linear distortion

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Carrier Aggregation (CA) and PAPR

•  PAPR on 10 MHz uplink signal ~6-8 dB (thanks to SC-FDMA)

•  CA 2x10 MHz ! PAPR increased by ~2 dB

•  CA negates some of the benefits of SC-FDMA

1CC 10 MHz Full filled QPSK RCM=3.27

2CC 10 MHz Full filled QPSK RCM=7.24

CCDF

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Power Added Efficiency (PAE)

+5.0Vcc

+2.3Vcc

+5.0Vcc

+2.3Vcc

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•  Power Added Efficiency (PAE) •  DC ! RF efficiency •  PAE = 𝑃𝑜𝑢𝑡 − 𝑃𝑖𝑛/𝑃𝐷𝐶  x 100%

•  PA VCC voltage affects PAE, EVM, ACLR, gain, linearity


Digital Pre-Distortion (DPD)

Pout

Pin

Linearized Gain Predistorter P(v)

Predistorter P(v)

PA

•  Pre-distortion causes gain expansion

•  Increases PAPR •  DPD often used in conjunction

with crest factor reduction (CFR)

Amplifier Gain, G(v)

Pmax

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DPD Bandwidth Expansion

+

+

+

IM products 3-5x BW

IM products (anti-phase)

DPD causes bandwidth expansion of input signal of 3-5x depending upon required degree of correction

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DPD Bandwidth Expansion Example

•  DPD improves in-band EVM and adjacent channel power

•  VHT160: 3x160 = 480 MHz to correct adjacent channels

•  Need sufficient source bandwidth to correct adjacent channel power

-250 -200 -150 -100 -50 0 50 100 150 200 250-100

-90

-80

-70

-60

-50

-40

Offset Frequency (MHz)

Pow

er (d

Bm)

DPD with Limited Tx BW = 200 MHz

Unable to correct foradjacent channel leakage

11ac VHT160: 480 MHz signal bandwidth

Correction limited to VSG bandwidth

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Crest Factor Reduction (CFR)

•  Pre-distortion yields gain expansion & increases PAPR

•  CFR used to reduce PAPR •  Several CFR techniques: - Clip & filter: EVM + ACLR

degradation - Peak windowing: ACLR degradation - Tone reservation: unused sub-

carriers carry sequences to reduce PAPR

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DPD Characterization

•  Integrated zSeries PA/FEM test solution - VSA, VSG, AWG, Digitizer, SMU, MIPI, …. - Measure Dynamic EVM, ACLR, PAE, Gain, AM/AM, AM/PM, DPD, …

•  Key performance specifications - Full coverage to 6 GHz for connectivity and cellular PA testing - ≥ 500 MHz VSG/VSA modulation bandwidth - Extremely low residual EVM: VHT80 <-47dB - Fast test time: Dynamic EVM and DPD sweeps in seconds

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Envelope Tracking (ET)

Power Added Efficiency (PAE) •  Example: PA class AB •  Fixed Vcc ! 20% PAE •  ET Vcc ! 57% PAE

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ET – The Software Defined Amplifier

Reproduced with permission from Nujira

Shaping Table

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RF-to-Vcc Time Alignment

Sweep AWG-to-VSG ∆t •  Different propagation delays between RF and ET supply paths •  EVM “V” defines optimum time skew •  Typically need <100 ps time alignment between RF and ET supply

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ET Characterization

ET Test Board •  ET supply is part of PA reference design •  High-efficiency ET supply close to PA minimizes parasitics Characterization of ET reference design •  TX quality measurements •  Power added efficiency •  Linearity measurements (AM-AM/AM-PM) •  Combined ET + DPD performance •  ET can improve the linearity of the PA

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Conclusions

• RFFE is critical for next generation wireless devices • Much more signal processing is required to test RFFEs - DPD to extend linearity for higher Pout - ET to improve power efficiency

• High performance test instrumentation is needed to speed time to market for RFFE screening and design verification - Very low residual EVM floor (e.g. -47dB for 256-QAM) - Wide bandwidth (e.g. 480 MHz for VHT-160) - Integrated test solution including multiple instruments and solution

software

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Further information: litepoint.com

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Date post:	05-Mar-2021
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Testing RF front ends for mobile device designs€¦ · Envelope Tracking (ET) Power Added...

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