Chapter VI A Detailed Look at Wireless Signal Chain ... · 3 Blockers – a closer look Blockers...

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ADI 2006 RF Seminar

Chapter VIA Detailed Look at Wireless Signal Chain Architectures

Chapter VIA Detailed Look at Wireless Signal Chain Architectures

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Receiver Architectures

Receivers are designed to detect and demodulate the desired signal and remove unwanted blockersReceiver must also get rid of unwanted signals that it generates (e.g. mixer spurs)Receiver uses variable gain and power detection Most Receivers will have some form of Automatic Gain ControlDiversity: Some Receiver Systems have two separate Receive Paths (Antennas separated by a quarter wavelength).A Diversity Receiver will either pick the strongest signal or “intelligently” combine both signals to increase signal power

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Blockers – a closer look

Blockers can be orders of magnitude larger than the desired signalLarge Blockers can jam a receiverBlockers can inter-modulate with each other and produce IMD products right at the frequency of the desired signalSome Blockers can be filtered (e.g. out-of-band) but others must be tolerated.

A

Rx BandDCDC

Freq

Tx Band

Power (dBm)Out-of-Band

Blocker

In-BandBlockers

DesiredSignal

TransmitSignal

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A Superheterodyne (Single Conversion) IF Sampling Receiver

Mixes the received signal from RF down to a single IF Uses SAW filters to remove blockers and unwanted mixing componentsDetects signal power and implements AGC at the IFReduces number of down-conversions by sampling the spectrum at an Intermediate Frequency but requires a high performance ADCIs the most popular architecture in non-cellular applications

IF SAMPLINGADC

MIXER

DUPLEXER RSSI/AGC

VGA

Band/ImageFilter

A

B C D E F G HI

TRANSMITTER

Channel SelectFilter

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IF Sampling Signal Flow

A

Rx BandDCDC

Freq

Tx Band


Blocker

In-BandBlockers

DesiredSignal

TransmitSignal

B

Rx BandDCDC

Freq

Tx Band


Blocker

In-BandBlockers

DesiredSignal

TransmitSignal

C/D

Rx BandDCD

Freq

Tx Band

Power (dBm)

In-BandBlockers

DesiredSignal

EDesiredSignal

DCDFreq

Power (dBm)

LOLeakage

FLO-FRF FLO+FRF

F Channel Select Filter

DCDFreq

Power (dBm)

H2nd

Harmonic

DCD

Freq

Power (dBm)

I

DCDFreq

Power (dBm)

SAMPLINGCLOCK

FIF FS

FS-FIF

FFT

FS/2

FLO

AGC&

NqyuistFilter

BlockerIMD

Product

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How IF sampling works

The receiver uses RF and IF filters to eliminate the transmit signal and blockers so that only the desired signal is sampledThe ADC must sample at twice the signal bandwidth to meet Nyquist criteriaOversampling can be used to improve the signal to noise ratio by 3 dB for each doubling of the sample frequencyHarmonics of ADC driver amp that are not filtered will degrade performanceThere is usually a clock recovery loop in an FPGA or DSP or both that locks the sampling rate to a multiple of the symbol rate

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Direct Conversion Receiver

Saves money by mixing RF spectrum to baseband in a single stepReduces component count and eliminates IF SAW filtersThere is a reason why RF engineers have not tried this sooner – removing DC offsets at baseband

ADC

ADC

900

IQ DEMOD

DUPLEXER

RSSI/AGC

VGA

BandFilter

A

B C D E F G

TRANSMITTER

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A

Rx BandDCDC

Freq

Tx Band

Power (dBm)Transmit

Signal

C

DCDCFreq

Tx Band

Power (dBm)

TransmitSignal

F

DC

FreqFSFS/2

Rx Band

E

DCDC

TransmitSignal

In-BandBlockers

DesiredSignal

Freq

Tx Band

Power (dBm)

Rx Band

In-BandBlockersDesired

Signal

DC Offset fromLO Self Mixing

&IP2 Intermodulation

Blocker IMDProduct

G

DC

FreqFSFS/2

NqyuistFilter

DesiredCarrier

In-BandBlockers

DesiredSignal

In-BandBlockers

DesiredSignal

Blocker IMDProduct

BB AmpDistortion

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In-Band Blockers can only be eliminated at the end of the signal chain or in the digital domain.In-Band Blockers can mix in the Front End (before mixer) to produce an unwanted product at basebandLO leakage to the RF input causes self-mixing and produces an unwanted dc offset at dc (right in the middle of the desired signal)Non-Ideal 90 degree balance in the Demodulator produces unwanted images of blockers which can be close to the carrierDirect Conversion Receivers are cheaper and smaller (no IF SAW filters, cheaper ADCs, only one mixer)

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Transmitter Architectures

Super Heterodyne with IQ ModulatorSuper Heterodyne with Real IF DAC SynthesisDirect ConversionLow IF to RF Conversion

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Superheterodyne Transmitter using IQ Modulator

Superheterodyne Transmitter uses one or more Intermediate Frequencies.DAC constructs the baseband signal, centered either at dc or at a low Intermediate Frequency (IF)Gain control and filtering may be implemented at RF, IF, and baseband.Lots of power back-off to avoid distortion in non-constant envelope systems

ACTIVEMIXER

IFAMP

Diffto SE

DAC

DAC

-15 dBm380 MHz

SAW

PADRIVER

-25 dBm

-10 dB

+10dB

0 to -20dB

+15dB

-15 dBm -18 dBm

BANDFILTER

-3 dB

-3 dBm

-5 dBm

380 MHz1760 +/-30 MHz1580 +/-30 MHz1462.5 +/-37.5 MHz

AD8345

PA

48 dB

AD836260 dB RMS Detector

AD836260 dB RMS DetectorADF4212L (Int-N)

ADF4252 (Frac-N)

TxDAC

+45 dBm

Gain=10dBNF=12 dBOIP3=20 dBmP1 dB=10 dBm

A B C D E F G

12


ATx Band

FTx Band

GTx BandDCDC

F

F

DCDC

BTx BandDCDC

C

F

F

F

IF

IF

IF Tx Band

DF

IF Tx Band

ETx Band

LOIMAGE

F

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Noise and Spurs generated in the IF stage can be filteredAfter mix to RF, band filtering removes out of band noise along with the imageIn-Band noise generated in mix to RF cannot be removed

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Example: Superhet with IF Synthesis of signal in IQ format

Driving IQ mod with a low IF creates a single-sideband-like spectrum at the modulator output.Once IF has been filtered (removing unwanted sideband and LO), modulation quality (EVM) is excellent.

AD9777

AD8345PowerAmplifier

StepAttenuatorAD8343

40dB

16-bit

16-bit

402 MHz

380 MHz

1.52 GHz

22 Mhz

BandFilter

A B

C D E F G

15

A

FTx Band

GTx BandDCDC

F

FLOW IF

DF

IF

ETx Band

IMAGES

FB

FLOW IF

CFIF

LOLEAKAGE

LOLEAKAGE

UNDESIREDUPPERSIDEBAND

F

Example: Superheterodyne Receiver with IF Synthesis of signal in IQ format

Unwanted LO leakage and Upper Sideband are filtered at IF, resulting in excellent EVMIf low IF is high enough, do a single up-conversion to RF

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Direct Conversion Zero IF Architecture

DAC

DAC

-15 dBm

PADRIVER

0 to -20dB

+15dB

-18 dBm

BANDFILTER

-3 dB

-3 dBm

-5 dBm

1760 +/-30 MHz1580 +/-30 MHz1462.5 +/-37.5 MHz

AD8349

PA

48 dB



AD9767 TxDAC

+45 dBm

Direct Conversion mixes a base-band signal from a dual DAC up to the transmission frequency in a single step. With no IF, gain control, filtering, and equalization must be performed either in the digital backend, at the reconstructed analog base-band output or at RF.Effects of LO leakage and Upper Sideband Leakage occur in-band potentially interfering with the signal’s EVM.Dual channels are required to generate the complex signal, any channel mismatch causes In-band distortion which cannot be filtered. High quality components are required to generate an accurate signal In-Band Modulator Noise cannot be filteredCalibration of LO leakage and Quadrature balance is generally necessaryPA to LO leakage can modulate or “pull” the PLL

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Example: Direct Conversion Transmitter

DAC

DAC

-15 dBm

PADRIVER

0 to -20dB

+15dB

-18 dBm

BANDFILTER

-3 dB

-3 dBm

-5 dBm

1760 +/-30 MHz1580 +/-30 MHz1462.5 +/-37.5 MHz

AD8349

PA

48 dB



AD9767 TxDAC

+45 dBm

A BC D E

ATx Band

DTx BandDCDC

ETx BandDCDC

Freq

Freq

DCDC

BTx BandDCDC

CTx Band

Freq

Freq

Freq

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Poor OIP3 causes Adjacent Channel Leakage

Think of a broadband spectrum multiple tones inter-modulating with each otherIM3 products produce Adjacent Channel Power/Leakage/DistortionUse 3-to-1 decay of IMD products to reduce dBc IMD but this degrades SNR

SNR

AdjacentChannelLeakage

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ACPR and Noise vs. Output Power

-65.0

-64.0

-63.0

-62.0

-61.0

-60.0

-59.0

-58.0

-57.0

-56.0

-55.0

-30 -28 -26 -24 -22 -20 -18

Per-Carrier Output Power - dBm

ACP

- dB

c

-160

-159

-158

-157

-156

-155

-154

-153

-152

-151

-150

Nois

e Fl

oor

- dBm

/Hz

(25

Mhz

Car

rier O

ffset

)

ACP 2140 MHz

Noise Floor 2140 MHz

ACP degrades with increased output power due to IMDNoise is independent of input and output powerAt low power levels ACP degrades because of falling SNR

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Example: Low IF to RF Transmitter using IF Synthesizing DAC and Passive Mixer

AD9786DAC

Pout -15 dBm190 MHz

ANTIALIAS

-16 dBm

-1 dB0 to -20dB

-11 dBm

-5 dBm

PASSIVEMIXER

IFAMP

PADRIVER

+15dB

-14 dBm

BANDFILTER

-3 dB

+1 dBm

PA

44 dB



+45 dBm

2.33 GHz2.15 GHz2.03 GHz

+5 dB

10 dB

-6 dBmGain= -5 dBNF= 5 dBOIP3= +35 dBmP1 dB= 25 dBm

Baseband DAC, IQ Modulator and PLL are replaced by an IF Synthesizing DAC or DDS modulatorTrade Off: High Performance DDS/DAC + SAW + Mixer + PLL vs. IQ DAC + Modulator + PLLNone of the problems typically associated with Direct ConversionProbably more expensive than Direct Conversion

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Low IF to RF Architectureω

Tx Band

ADC

LO

BIF

DC

LOLO- IF LO+ IFω ω ω

ω

ω ω

ω

ω

IFω

Tx Band

High Performance DAC generates “real” IF at a low IF (100-200 MHz)Mixer performs Double Sideband ModulationAdvantage: Unwanted LO and Sideband are removed -> excellent EVMChallenge: To move unwanted LO and upper sideband out of band means that the IF must be quite high

Chapter VI A Detailed Look at Wireless Signal Chain ... · 3 Blockers – a closer look Blockers...

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Transcript of Chapter VI A Detailed Look at Wireless Signal Chain ... · 3 Blockers – a closer look Blockers...