Stroet INTRODUCTION This Application Note describes method accurately
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Application Note October 2005 Measuring Phase Delay Errors Accurately Modulators
Stroet INTRODUCTION This Application Note describes method accurately measure internal external phase timing errors high performance direct modulator. direct modulator, such LT5528, translates baseband signals combines them produce modulated single sideband signal with (ideally) minimal residual carrier feedthrough) image signals (undesired sideband). ideal modulator, with perfect phase shift between mixer mixer local oscillators (LOI LOQ), with other undesired phase gain impairments, modulator output will contain only desired sideband. practice, this very difficult accomplish. example, with requirement -60dBc image suppression, residual phase error required below 0.16°. practice, there other sources phase error, particularly baseband signal processing. Examples include baseband skew other frequency dependent phase shifts modulator baseband circuitry; skew errors phase delay mismatched baseband connection paths (e.g., cabling); phase mismatch between paths baseband signal source (e.g., baseband DACs signal generators.). These phase errors cause output spectra shaped like those shown Figures each plot, (single) channel chosen -7.5MHz, -2.5MHz, 2.5MHz 7.5MHz offset from carrier choosing frequency offset function baseband generator. seen, residual sideband spectra flat frequency. Usually, image rejection calibration done using (baseband) frequency, preferably center desired channel. However, uncalibrated residual sideband flat
Figure Measurement Compilation Four One-Channel W-CDMA Modulator Output Spectra Selected -7.5MHz, -2.5MHz, 2.5MHz 7.5MHz Offset Frequency from 2.14GHz Carrier Using Baseband W-CDMA Channel Selection with Uncalibrated Image
Figure Measurement Compilation Four One-Channel W-CDMA Modulator Output Spectra Selected -7.5MHz, -2.5MHz, 2.5MHz 7.5MHz Offset Frequency from 2.14GHz Carrier Using Baseband W-CDMA Channel Selection with Uncalibrated Image
registered trademarks Linear Technology Corporation. other trademarks property their respective owners.
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Application Note
Figure Measurement Compilation Four One-Channel W-CDMA Modulator Output Spectra Selected -7.5MHz, -2.5MHz, 2.5MHz 7.5MHz Offset Frequency from 2.14GHz Carrier Using Baseband W-CDMA Channel Selection with Uncalibrated Image
Figure Measurement One-Channel W-CDMA Spectrum Modulator Output After Image Nulling 7.5MHz Baseband Calibration Frequency Using Baseband Delay Correction. W-CDMA Channel Located Offset 7.5MHz Image Located Offset -7.5MHz with Respect Carrier
versus frequency, causes image rejection after calibration degrade edges channel. This seen Figure where image rejection less than 60dBc image channel shape form letter "M". delay difference between baseband paths cause image power falling frequency Figure rising Figure have shape Figure sign magnitude quadrature phase error modulator, sign magnitude baseband delay difference determine whether situation Figure residual sideband spectrum Figure improved adding compensating delay baseband paths. This shown Figure
order achieve best image rejection broadband communications channel (such W-CDMA), important understand what error source(s) causes image response non-flat over frequency. This Application Note provides measurement method determine sources both baseband phase error, whether comes from baseband generator and/or modulator. method consists three different measurements, each with slightly different measurement setup. From these measurements, determine quadrature error modulator baseband phase error modulator MOD, baseband phase error baseband signal generator DGEN. very likely that DGEN result from internal skew time delay errors (DGEN MOD, respectively). Therefore, write more general case different baseband frequencies (BB): DGEN DGEN0 DGEN MOD0 analysis that follows, disregard amplitude mismatches, because measurements indicate that phase errors dominant, greatly simplifies math. order resolve uncontrolled, systematic phase errors, DGEN MOD, technique requires there controllable, adjustable baseband phase offset, GEN. This adjustable phase used null image signal under various measurement conditions. nulling phases used calculate individual system phase errors.
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Figure Measurement One-Channel W-CDMA Spectrum Modulator Output After Image Nulling 7.5MHz Baseband Calibration Frequency. W-CDMA Channel Located Offset 7.5MHz Image Located Offset -7.5MHz with Respect Carrier
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Application Note
MEASUREMENTS IIA. First Measurement-Null Modulator Image Signal with Normal Signal Connections (Figure phase error exists between quadrature signals modulator. cancel this with extra phase shift GEN1 between baseband signals However, shown defined Figure there delay differences between path both baseband generator (DGEN) within modulator itself (MOD). particular baseband frequency 2fBB, baseband signals modulator's mixers given cos(BB sin(BB GEN1 DGEN MOD) Note that placement error terms DGEN, paths arbitrary does affect final conclusions this analysis. Here GEN1 controllable phase offset that adjusted needed compensate other phase errors system. cos(LO sin(LO cos(BB cos(LO sin(BB GEN1 DGEN MOD) sin(LO cos() cos() cos( cos( sin() sin() cos( cos( cos[(LO BB)t] cos[(LO cos[(LO BB)t GEN1 DGEN MOD] cos[(LO BB)t GEN1 DGEN MOD] cos( cos()cos() sin()sin() cos[(LO BB)t] cos[(LO cos(LO GEN1 DGEN MOD) cos[(LO BB)t] sin(LO GEN1 DGEN MOD) sin[(LO BB)t] cos(LO GEN1 DGEN MOD) cos[(LO BB)t] sin(LO GEN1 DGEN MOD) sin[(LO BB)t] (Small angle approximation) cos() sin() 5040
(Small angle approximation) cos[(LO 1/2(LO GEN1 DGEN MOD) sin[(LO 1/2(LO GEN1 DGEN MOD) sin[(LO BB)t] addition desired lower sideband signal also some upper sideband signal BB).
BBPI BBMI BASEBAND GENERATOR BBPQ BBMQ
MODPI MODMI BALUN MODULATOR MODPQ MODMQ
GEN1
DGEN
ADJUSTABLE UNDESIRED PHASE GENERATOR IMPAIRMENT PHASE ERROR SETTING (DEG) SKEW (DEG)
UNDESIRED MODULATOR PHASE UNDESIRED ERROR MODULATOR PHASE PHASE SHIFT ERROR MISMATCH (DEG) BASEBAND SKEW (DEG)
AN102
Figure Measurement Setup Configuration
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Application Note
small phase errors, upper sideband amplitude approximately given AUSB 1/2(LO GEN1 DGEN MOD) upper sideband suppression given (dB) log[1/2(LO GEN1 DGEN1 MOD)] log(LO GEN1 DGEN MOD) 6.02 (dB) Note that phases radians. image term minimized adjusting generator (impairment) phase setting GEN1 DGEN Second Measurement-Null Modulator Image Signal with Reversed Differential Baseband Signals Modulator's Differential I-Channel Inputs (Figure This configuration differs from that Figure that differential baseband signals modulator's inputs reversed. this configuration image component output signal measured nulled adjustment controllable signal generator phase, GEN2. Note that length signal path assumed change flipping BBPI BBMI; connectors baseband generator just flipped. -cos(BB sin(BB GEN2 DGEN MOD) cos(LO sin(LO -cos(BB cos(LO sin(BB GEN2 DGEN MOD) sin(LO
BBPI BBMI BASEBAND GENERATOR BBPQ BBMQ MODPQ MODMQ MODULATOR MODPI MODMI BALUN
Using trigonometric identities, this expanded -1/2 cos[(LO BB)t] cos[(LO cos(LO GEN2 DGEN MOD) cos[(LO BB)t] sin(LO GEN2 DGEN MOD) sin[(LO BB)t] cos(LO GEN2 DGEN MOD) cos[(LO BB)t] sin(LO GEN2 DGEN MOD) sin[(LO BB)t] Again using small angle approximations, this becomes: -cos[(LO 1/2(LO GEN2 DGEN MOD) sin[(LO 1/2(LO GEN2 DGEN MOD) sin[(LO BB)t] Now, desired signal upper sideband signal BB), image signal BB). small phase errors, lower side band amplitude given ALSB 1/2(LO GEN2 DGEN MOD) lower sideband suppression given (dB) log[1/2(LO GEN2 DGEN MOD)] log(LO GEN2 DGEN MOD) 6.02 (dB) this configuration, image minimized adjusting: GEN2 DGEN
GEN2
DGEN
ADJUSTABLE UNDESIRED PHASE GENERATOR IMPAIRMENT PHASE ERROR SETTING (DEG) SKEW (DEG)
UNDESIRED MODULATOR PHASE UNDESIRED ERROR MODULATOR PHASE PHASE SHIFT ERROR MISMATCH (DEG) BASEBAND SKEW (DEG)
AN102
Figure Measurement Setup Configuration
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Application Note
Third Measurement-Null Modulator Image Signal After Reversing Inputs Modulator (Figure This configuration differs from that Figure that differential inputs exchanged. Note that connection lengths path change reversing BBQ, connectors baseband generator just flipped. this configuration, image component output signal measured nulled adjustment controllable signal generator phase, GEN3. cos(BB MOD) sin(BB GEN3 DGEN) cos(LO sin(LO sin(BB GEN3 DGEN) cos(LO cos(BB MOD) sin(LO Using trigonometric identities small angle approximations, this expanded 1/2(GEN3 DGEN MOD) cos[(LO BB)t] 1/2(GEN3 DGEN MOD) cos[(LO BB)t] sin[(LO Now, desired signal upper sideband frequency component image lower sideband signal BB). small phase errors, lower sideband amplitude given ALSB 1/2(GEN3 DGEN MOD) lower sideband suppression given (dB) log[1/2(GEN3 DGEN MOD)] log(GEN3 DGEN MOD) 6.02 (dB) this configuration, image minimized adjusting: GEN3 DGEN
BBPI BBMI BASEBAND GENERATOR BBPQ BBMQ
MODPI MODMI BALUN MODULATOR MODPQ MODMQ
GEN3
DGEN
ADJUSTABLE UNDESIRED PHASE GENERATOR IMPAIRMENT PHASE ERROR SETTING (DEG) SKEW (DEG)
UNDESIRED MODULATOR PHASE UNDESIRED ERROR MODULATOR PHASE PHASE SHIFT ERROR MISMATCH (DEG) BASEBAND SKEW (DEG)
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Figure Measurement Setup Configuration
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Application Note
Calculation Phase Impairments From GEN1= DGEN GEN2 DGEN GEN3 DGEN solve these three equations, with three unknowns, give: (GEN2 GEN1)/2 DGEN -(GEN2 GEN3)/2 (GEN3 GEN1)/2 Using modulator with latter relationship will affect derivations somewhat. this case, configuration desired signal will then upper sideband image nulling will achieved for: GEN1 DGEN configuration desired signal will lower sideband image nulling will achieved for: GEN2 DGEN configuration desired signal will again lower sideband image nulling will achieved for: GEN3 DGEN again solve these three equations, with three unknowns, give: (GEN1 GEN2)/2 DGEN -(GEN2 GEN3)/2 (GEN3 GEN1)/2
Note that express phases these equations radians degrees. equations above hold modulator with output relationship given cos(LO sin(LO (2D_A) modulators provided Linear Technology will satisfy above equation. However, other modulators have following output characteristic: cos(LO sin(LO (2D_B) There international convention which modulator equation "right" one.
Note that sign different calculation, equations DGEN stay same.
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Application Note
III. APPLYING METHOD five different 5528 boards image rejection nullvectors configurations described sections measured logged Table baseband frequencies 5MHz 10MHz. QPSK signal programmed into Rohde Schwartz AMIQ baseband generator with sequence 00011011. symbol rate 40MHz with oversampling 10MHz baseband frequency, symbol rate 20MHz with oversampling 5MHz baseband frequency,
both resulting sample rate 80MHz. cases better than 75dBc image rejection achieved after nulling. quadrature phase error LT5528 baseband phase error LT5528 baseband phase error generator, DGEN determined using Equations amplitude mismatch results discarded. results phase errors degrees given Table Also equivalent delays derived from phase errors, assuming phase error caused delay.
Table Image Rejection Null Vectors Configurations 5MHz 10MHz Baseband Frequency. Amplitude Adjustment Required Nulling (Not Shown) 0.35% (Worst Case)
BASEBAND FREQUENCY 5MHz config1 DGEN1 UNIT DEGREE -0.90 1.13 0.32 0.36 0.51 config2 DGEN2 DEGREE 1.93 -0.07 0.81 0.74 0.60 config3 DGEN3 DEGREE -0.83 1.24 0.37 0.44 0.62 config1 DGEN1 DEGREE -0.48 1.60 0.73 0.80 -0.03 BASEBAND FREQUENCY 10MHz config2 DGEN2 DEGREE 2.41 0.30 1.30 1.20 0.10 config3 DGEN3 DEGREE -0.41 1.74 0.84 0.92 0.03
Table Phase Error Measurement Results LT5528
BOARD UNIT DEGREE 1.415 -0.60 0.245 0.19 0.045 BASEBAND FREQUENCY 5MHz DEGREE 0.035 0.055 0.025 0.04 0.055 19.4 30.6 13.9 22.2 30.6 DGEN DEGREE -0.55 -0.585 -0.59 -0.59 -0.61 DGEN DEGREE 1.445 -0.65 0.285 0.20 0.08 BASEBAND FREQUENCY 10MHz DEGREE 0.035 0.07 0.055 0.06 19.4 15.3 16.7 27.8 DGEN DEGREE -1.0 -1.02 -0.785 -0.86 -1.08 DGEN
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Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
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Application Note
CONCLUSION method described here capable accurately measuring various sources phase error. measured quadrature error using 5MHz 10MHz baseband frequencies equal within 0.05 degrees, suggesting quadrature error measured quite accurately relatively high baseband frequencies. seen that baseband signal generator phase error DGEN dominant this setup. DGEN about 300ps, compared LT5528's baseband delay error MOD, which only about 25ps 30ps. somewhat surprising result magnitude phase error baseband signal source DGEN. This baseband signal source phase error dominant error source direct modulation scheme. should carefully characterized compensated. Otherwise, limit extent image suppression. This especially important broadband application, such W-CDMA, baseband source phase error skew (time delay) error, which results frequency dependent phase error.
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Linear Technology Corporation
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(408) 432-1900
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LINEAR TECHNOLOGY CORPORATION 2005
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