Testing RF front ends for mobile device designs€¦ · Envelope Tracking (ET) Power Added...
Transcript of Testing RF front ends for mobile device designs€¦ · Envelope Tracking (ET) Power Added...
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© 2013 L i tePo int , A Teradyne Company . A l l r i ghts reserved .
Testing RF front ends for mobile device designs Chris Ziomek VP of zSeries
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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
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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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