ADI ChapterVI a DetailedLookAtWirelessSignalChainArchitectures

8/11/2019 ADI ChapterVI a DetailedLookAtWirelessSignalChainArchitectures

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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 ArchitecturesReceivers are designed to detect and demodulate the

desired signal and remove unwanted blockers

Receiver must also get r id of unwanted signals that i tgenerates (e.g. mixer spurs)

Receiver uses variable gain and power detection

Most Receivers will have some form of Automatic Gain

Control

Diversity: 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 desiredsignal

Large Blockers can jam a receiver

Blockers can inter-modulate with each other and produce IMDproducts r ight at the frequency of the desired signal

Some Blockers can be filtered (e.g. out-of-band) but others mustbe tolerated.

A

Rx BandDCDC

Freq

Tx Band

Power (dBm)

Out-of-BandBlocker

In-BandBlockers

DesiredSignal

TransmitSignal


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A Superheterodyne (Single Conversion) IFSampling Receiver

Mixes the received signal from RF down to a single IF

Uses SAW filters to remove blockers and unwanted mixing components

Detects signal power and implements AGC at the IF

Reduces number of down-conversions by sampling the spectrum at an

Intermediate Frequency but requires a high performance ADC

Is the most popular architecture in non-cellular applications

IF SAMPLINGADC

MIXER

DUPLEXER RSSI/AGC

VGA

Band/ImageFilter

A

B C D E F G H

I

TRANSMITTER

Channel SelectFilter


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IF Sampling Signal Flow

A

Rx BandDCDC

Freq

Tx Band

Power (dBm)

Out-of-BandBlocker

In-BandBlockers

DesiredSignal

TransmitSignal

B

Rx BandDCDC

Freq

Tx Band

Power (dBm)Out-of-Band

Blocker

In-BandBlockers

DesiredSignal

TransmitSignal

C/D

Rx BandDCD

Freq

Tx Band

Power (dBm)

In-BandBlockers

DesiredSignal

E

DesiredSignal

DCD

Freq

Power (dBm)

LOLeakage

FLO-FRF FLO+FRF

F Channel Select Filter

DCDFreq

Power (dBm)

H 2ndHarmonic

DCD

Freq

Power (dBm)

I

DCD

Freq

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 issampled

The ADC must sample at twice the signal bandwidth to meet

Nyquist criteria

Oversampling can be used to improve the signal to noiseratio by 3 dB for each doubling of the sample frequency

Harmonics of ADC driver amp that are not filtered will

degrade performance

There is usually a clock recovery loop in an FPGA or DSP or

both that locks the sampling rate to a mult iple of the symbol

rate


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Direct Conversion Receiver

Saves money by mixing RF spectrum to baseband in a single

step

Reduces component count and eliminates IF SAW filters

There is a reason why RF engineers have not tried this

sooner removing DC offsets at baseband

ADC

ADC

90

0

IQ DEMOD

DUPLEXER

RSSI/AGC

VGA

BandFilter

A

B C D EF

G

TRANSMITTER


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Direct Conversion Receiver

A

Rx BandDCDC

Freq

Tx Band

Power (dBm)Transmit

Signal

C

DCDC

Freq

Tx Band

Power (dBm)

TransmitSignal

F

DC

Freq

FSFS/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

Freq

FSFS/2

NqyuistFilter

DesiredCarrier

In-BandBlockers

DesiredSignal

In-BandBlockers

DesiredSignal

Blocker IMDProduct

BB AmpDistortion


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Direct Conversion ReceiverIn-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) toproduce an unwanted product at baseband

LO 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

carrier

Direct Conversion Receivers are cheaper and smaller (no IFSAW filters, cheaper ADCs, only one mixer)


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Transmitter Architectures

Super Heterodyne with IQ Modulator

Super Heterodyne with Real IF DAC SynthesisDirect Conversion

Low IF to RF Conversion


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Superheterodyne Transmitter using IQModulator

Superheterodyne Transmit ter 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 fil tering 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 MHz 1760 +/-30 MHz

1580 +/-30 MHz1462.5 +/-37.5 MHz

AD8345

PA

48 dB

AD836260 dB RMS Detector


ADF4212L (Int-N)ADF4252 (Frac-N)

TxDAC

+45 dBm

Gain=10dBNF=12 dBOIP3=20 dBm

P1 dB=10 dBm

A B C D E F G


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A

TxBand

F

TxBand

G

TxBandDCDC

F

F

DCDC

B

TxBandDCDC

C

F

F

F

IF

IF

IF Tx Band

D

FIF Tx Band

E

TxBand

LOIMAGE

F


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Noise and Spurs generated in the IF stage can be filtered

After mix to RF, band filtering removes out of band noisealong with the image

In-Band noise generated in mix to RF cannot be removed


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A

F

TxBand

G

TxBandDCDC

F

FLOW IF

DF

IF

E

TxBand

IMAGES

F

BF

LOW IF

CF

IF

LOLEAKAGE

LOLEAKAGE

UNDESIREDUPPERSIDEBAND

F

Example: Superheterodyne Receiver withIF Synthesis of signal in IQ format

Unwanted LO leakage and Upper Sideband are fil tered at IF,resulting in excellent EVM

If low IF is high enough, do a single up-conversion to RF


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Direct Conversion Zero IF ArchitectureDAC

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 asingle step.

With no IF, gain control, filtering, and equalization must be performed either in the digital backend, atthe reconstructed analog base-band output or at RF.

Effects of LO leakage and Upper Sideband Leakage occur in-band potentially interfering wi th thesignals EVM.

Dual channels are required to generate the complex signal, any channel mismatch causes In-banddistortion which cannot be filtered.

High quality components are required to generate an accurate signal

In-Band Modulator Noise cannot be filtered

Calibration of LO leakage and Quadrature balance is generally necessary

PA 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


AD8362

60 dB RMS Detector

AD9767 TxDAC

+45 dBm

A B

C D E

A

Tx Band

D

Tx BandDCDC

E

Tx BandDCDC

Freq

Freq

DCDC

BTx BandDCDC

C

Tx Band

Freq

Freq

Freq


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Poor OIP3 causes Adjacent ChannelLeakage

Think of a broadband spectrum multiple tones inter-modulating with eachother

IM3 products produce Adjacent Channel Power/Leakage/Distortion

Use 3-to-1 decay of IMD products to reduce dBc IMD but this degradesSNR

SNR

Adjacent

Channel

Leakage


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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-dBc

-160

-159

-158

-157

-156

-155

-154

-153

-152

-151

-150

NoiseFloor

-dBm

/Hz

(25MhzCarrierO

ffset)

ACP 2140 MHz

Noise Floor 2140 MHz

ACP degrades with increased output power due to IMD

Noise is independent of input and output power

At low power levels ACP degrades because of falling SNR


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

AD9786DAC

Pout -15 dBm190 MHz

ANTIALIAS

-16 dBm

-1 dB 0 to -20dB

-11 dBm

-5 dBm

PASSIVEMIXER

IFAMP

PADRIVER

+15dB

-14 dBm

BANDFILTER

-3 dB

+1 dBm

PA

44 dB



+45 dBm

2.33 GHz

2.15 GHz2.03 GHz

+5 dB

10 dB

-6 dBm

Gain= -5 dBNF= 5 dBOIP3= +35 dBmP1 dB= 25 dBm

Baseband DAC, IQ Modulator and PLL are replaced by an IF

Synthesizing DAC or DDS modulator

Trade Off: High Performance DDS/DAC + SAW + Mixer + PLL vs.IQ DAC + Modulator + PLL

None of the problems typically associated with Direct Conversion

Probably more expensive than Direct Conversion


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Documents

ADI ChapterVI a DetailedLookAtWirelessSignalChainArchitectures