Simplified Transceiver Architecture

Role of a Transmitter

090

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Power Supply

Power Amplifier

Mixer

Oscillator

Baseband Processor

Modulator

bias

I Data

Q Data

1. create carrier

2. add data to carrier

4. amplify to broadcast

3. shift to high frequency

Information

Antenna

bias

uP/DSP

Transceiver

Role of a Receiver

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Power Supply

uP/DSP

Low Noise Amplifier

Mixer

Oscillator

Baseband Processor

De-Modulator

bias

I Data

Q Data

1. amplify received signal with min. added noise

2. shift to lower frequency (cost and/or performance)

3. LO for down conversion

4. discard carrier and recover data

Information

bias bias

Antenna

Mixer = Multiplying up/down conversion

• Frequency translation device

• Ideal mixer:

– Doesn’t “mix”; it multiplies

A

B

AB

Image problem converting to IF

A1cos(wRFt)

A has desired signal at IF

plus an interference at IM

A2cos(wIMt)

B is at LO

And:

RF - LO = LO - IM = IF

Both converted to IF,Can’t be cleaned once corrupted

Image Problem

Problem of Image Signal

IF

RFimage LO

Problem of Image Signal

• Solution: Image Rejection Filter

Problem of Half IF

• Second order harmonic

0

Superheterodyne Receiver

Multi IF Stage Receivers• Received RF signal is down converted stage by

stage until the desired final IF is obtained• Frequency conversion ratio of each stage is

usually kept lower than 10. – For example, RF 1800 MHz IF1 450 MHz, then

IF2 90 MHz, and finally IF3 18 MHz.– Corresponding ratios are: 4; 5; 5; total 100.

• Each stage has it’s own image problem• Each stage requires demanding filtering

– Typically done off chip or using SAW– Complicated, bulky, expensive

IF and LO frequency selection• Fixed RF filter before LNA for band selection

– One for each standard– Off-chip, high quality, no freedom

• IF frequency is selected at design– Fixed for each product

• LO frequency is tuned in real time– |RF–LO|=IF– Actual RF freq depends on which channel is

assigned to device– LO tuning range must cover RF bandwidth

Superheterodyne Receiver(cont.)

Selection of IF

• If IF is large, – better separation between RF and image– better image rejection– easier image rejection filter design– More stages of down conversion

• Other IF selection criteria– Select IF so that image freq is outside of RF

band IF >= (RF BW)/2

• Sometime may not be possible, if (RF BW)/2 is within RF Band

• For each channel assignment, there are two choices of LO freq that meets the requirement |RF–LO|=IF.

• Q: should LO > RF, or LO < RF??

Example: AM Radio• AM radio band: 530 to 1610 KHz• BW/2 = (1610-530)/2=1080/2=540, in band• IF has to be lower. Commonly: 455kHz• Image can be in AM band• If LO is on low side, LO tuning range is:

– (530 to 1610) – 455 = (75 to 1155)– LO lowest to highest is a factor of 15.4

• If LO is on high side, LO tuning range is:– (530 to 1610) + 455 = (985 to 2065)– LO lowest to highest is a factor of 2.01

Direct Conversion Receiver

No image problem

PGA

PGA

LNA

RFSAW ADC

ADC

DIG

ITA

LF

ILT

ER

I

Q

RFPLL

sin cos

270 kS/s

925-960MHz

925 - 960MHz

Direct Conversion Receiver

LO is at same frequency as RF1/f noise here can end up in channelSelf mixing cause DC problem

+ Eliminate IF SAW, IF PLL and image filtering+ Integration+ Avoids image problem

- Quadrature RF down conversion required- DC problem- Typically requires offset or 2x LO to avoid coupling

DC Offset (Self-mixing)

ADc

aLO(t)=ALOcos(c+)

0

ADc 0

c

c

)(, tx LOoffset

)(, tx RFoffset

capacitive couplingsubstrate couplingbondwire coupling

capacitive couplingsubstrate couplingbondwire coupling

Saturates the following stagesSaturates the following stages

DC Offset (Self-mixing)

θωω

θωθωω

ω

tAA

tAtAA

tataAtx

cLOcrosstalk

cLOcLOcrosstalk

LOLOcrosstalkLOoffset

2cos1)(

coscos)(

)()()()(

221

,

φωω

φωω

ω

)(2cos1)(

)(cos)(

)()()()(

2interferer2

1

22interferer

interfererinterfererinterferer,

tmtAA

tmtAA

tataAtx

ccrosstalk

ccrosstalk

crosstalkoffset

level

DC Offset

+

-

t

DC Offset Cancellation

• Capacitive Coupling– Requires a large capacitor

• Negative Feedback– Nonlinear

• TDMA Offset Cancellation– Requires a large capacitor

-A

1/f noise effect

• CMOS transistors has significant 1/f noise at low to DC frequency

• Significantly noise performance of direct conversion receivers

Receive signal1/f noise

f

Even-Order Distortion

0

0

Direct feed through

Direct feed through

Mirror Signal

• Upper sideband and lower sideband are identical

RF

LO

0

0

Mirror Signal • Upper sideband and lower sideband are not

identical

RF

LO

0

0

Mirror Signal Suppression

AD

090

AD

a(t)

ui(t)

uq(t)

vi(t)

vq(t)

• Quadrature Down Conversion

I

Q

Quadrature Conversion

)(

)(tan)(

))(tan()(

)(

))(sin()(

))(cos()(

)sin()()(

)cos()()(

))(cos()(

1

21

21

tv

tvtm

tmtv

tv

tmtv

tmtv

ttatu

ttatu

tmtta

i

q

i

q

q

i

LOq

LOi

RF

Quadrature Down Conversion

0

0

I/Q Mismatch

090

I

Q

Phase & Gain Error

Phase & Gain Error

Phase & Gain Error

a(t)

I/Q Mismatch due to LO errors

2)(sin

21

2)(

2)(cos

21

2)(

2sin

21)(

2cos

21)(

)(cos)(

,

,

,

,

tmA

tx

tmA

tx

ttx

ttx

tmtAta

QBB

IBB

cQLO

cILO

c

2

2ˆ

ˆ

2222

1

)(

)()(ˆ Find :Exercise

unbiased. is )(ˆ Hence,

)())(ˆ( Clearly,

0)(,))((,))(( and

0)( ,0)( Suppose

)2/)(cos()2/1(

)2/)(sin()2/1(tan)(ˆ

tm

tmtmEσ

tm

tmtmE

EσEσE

EE

tm

tmtm

def

m

m

ε

Phase errorPhase errorGain errorGain error

Effect of gain mismatch Effect of phase mismatch

Use of I/Q down conversion recovers the nonsymmetrical receive signal spectrumBut port isolation becomes more challengingSelfmixing and even order distortion may affect both channels and affect each other, causing additional I/Q mismatch

090

a(t)

A/D

A/D

Base Band

DSP

Phase and gain mismatchcompensation

DC and 1/fcancellation

Summary of Direct Conversion Receiver

• No need for imager reject filter• Suitable for monolithic integration with baseband • DC offsets due to crosstalk of input ports of

mixer• Even order IM direct feed through to baseband• Quadrature down conversion suppresses mirror• I/Q mismatch due to mismatches in parasitics• Low power consumption attributes to less

hardware

ADC

ADC

PGA

PGA

LNA

RFPLL

DIG

ITA

LF

ILT

ER

100 kHz

I

Q

sin cos

RFSAW

sin cos

270 kS/s

925-960MHz

925.1 - 960.1MHz

Low IF receiver

- Quadrature RF down conversion required- Require higher performance ADC-Additional mixer-Slower RF PLL settling-Even order distortion still problem-Low freq IF filters require large chip area

+ Eliminate IF SAW, IF PLL and image filtering

+ Integration

+ Relaxes image rejection requirements

+ Avoids DC problems, relaxes 1/f noise problem

Low-IF Down Conversion

Complex BPF

Mirror signal, needs removal

Mirror Signal Suppression (1)

ComplexBandpass

Filter

I Q I Q

LO1 LO2

Mirror Signal Suppression (2)

I Q I Q

LO1 LO2

Both schemes used in heterodyne receivers for image rejectionMathematical analysis very similar

Image rejection architectures

• Use additional hardware (LO’s, mixers, and filters)

• Use I/Q channels which process + or – frequencies differently

• Two steps of I/Q to solve both image and mirror problems

• Effects limited by I/Q channel/filter matching accuracies

Image Reject Receiver

• Hartley Architecture

-90°

090

RFinput

IFoutput

A

B

C

LO

tLOωsin

tLOωcos

Hartley Architecture

LOLO LO0

)(BX 0

0

2/j

2/j

0)(AX

90 0

0

0

xcos

xsin

IQ error effect

• Ideal IQ: image completely rejected

• If signal and image not single tone, 90o shift is not exact

• Local oscillator’s sine and cosine not matched in magnitude and phase

• 90o phase shifter may have both gain and phase error

• All lead to incomplete image rejection

IPR Evaluation and IRR – LO error

tAA

tAA

tx imLOimLO

RFLORFLO

A )sin(2

)sin(2

)(

)(cos2

)()(cos2

)()( tA

AtA

Atx imLOim

LORFLORF

LOB

)cos(2

)cos(2

)(

t

At

AAtx imLO

imRFLO

RFLOC

cos2

cos2

)()( t

AAt

AAtx RFLO

RFLORFLO

RFLOsig

cos2

cos2

)()( t

AAt

AAtx imLO

imLOimLO

imLOim

cos)(2)(

cos)(2)(.

22

22

2

2

LOLOLOLO

LOLOLOLO

RF

im

outsig

im

AAAA

AAAA

A

A

P

P

Input image power ratio

44

4

4

)cos1)((2

cos)(2)(

cos)(2)(

22

22

2

222

2

2

22

22

AA

AIRR

A

A

A

AA

AAAA

AAAAIRR

LO

LO

LO

LOLO

LOLOLOLO

LOLOLOLO

Image Reject ReceiverHartley Architecture with simple 90 deg phase shiftor

 

1

1

1

1222222

CRCCRR

CCRR

A

A

2

1

2

C

C

R

RC

CR

RC

CR

R

A

A

Gain Mismatch due to R, C errors

At w = 1/RC:

Weaver Architecture

1LOω 1LOω0

0

0

0

)(CX

)(DX

0

2/j

2/j

0)(AX

2/j

2/j

02LOω2LOω

0

)(BX2/1 2/1

00

2LOω 2LOω

Weaver Architecture

1LOω

1LOω0

0

0

0

)(CX

)(DX

0

2/j

2/j

0)(AX

2/j

2/j

02LOω2LOω

0

)(BX2/1 2/1

00

2LOω 2LOω

RFLOLO ω2ω2ω 21

RFLOLO ω2ωω 21

1ωω LORF 21 ωωω LOLORF

Digital IF?• To avoid 0-IF or low-IF issues, IF frequencies

can’t be too low• Recall: RF-IF ratio within 10• Typical RF freq is in 1 to 5 GHz, IF needs

to be more than 100 to 500 MHz• But dynamic range requirements requires >=

14 bit resolution• No such ADC’s are available

• But signal bandwidth much smaller,• Subsampling Receivers

• Example: 1.8 GHz GSM Specifications: IF carrier frequency = 246 MHz, Channel BW = 200 KHz, Input Dynamic Range = 90 dB.

2 digital low frequency mixers, no noise and distortion. Easier I&Q matching. No DC offset and 1/f noise. More digital means easier integration on a CMOS process.x SNR degradation due to noise foldingx ADC & SH have to run at high clock to minimize noise folding.

Noise folding problem

IF

White noise

0

… …

fs 2fs

IF0

… …

Baseband noise increased by IF/fs factor

• The aliased noise, once happened, cannot be removed in the digital domain

• Band pass filtering of IF before sampling can reduce the noise in lower frequency– Requires expensive IF filters– Against the spirit of moving more things to

digital

• Reduce IF frequency and increase fs frequency so that IF/fs ratio is not large– More stringent requirement on RF filtering and

image rejection– Requires faster ADC

ExampleUMTS/DCS1800 Specifications

Frequency BandChannel BWSystem SensitivityBER

BlockingCharacteristics

Adjacent ChannelInterference

DCS18001805 - 1880 MHz200 kHz-102 dBm1e-3600 - 800 kHz: -43 dBm800 - 1600 kHz: -43 dBm1600 - 3000 kHz: -33 dBm> 3000 kHz: -26 dBmCochannel: -9 dBc200 kHz: 9 dBc400 kHz: 41 dBc600 kHz: 49 dBc

UMTS2110 - 2170 MHz5 MHz-117 dBm(@32ksps)1e-310 - 15 MHz: -56 dBm15 - 60 MHz: -44 dBm60 - 85 MHz: -30 dBm> 85 MHz: -15 dBm

5 MHz: -52 dBm

Sensitivity

• Adjacent Channel Interference

• Co-Channel Interference

Desired Channel

Adjacent Channel

Adjacent Channel

MHz890.4 890.4 890.6

890.4

890.4 890.4

890.4890.4

890.4 890.4

Multi-Channel, Multi-Mode DynamicRange, DCS1800

Multi-Channel, Multi-Mode DynamicRange, DCS1800

PB = 13 dBm, Px = -60 dBm

PB:Px = 73 dB

If want FS:1LSB > 73 dB >12 bit resolution

If want digital channel selection + filtering, fs >= 2BW fs >= 150MHz

If want noise floor 20 dB below wanted signal SFDR >= 13 – (-60) + 20 = 93 dB

Type of ADC needed: 150 MSPS, 13-14 bit, 95-100 dB SFDR

Sensitivity

Receiver Thermal Noise

Receiver Added Noise

Desired Signal

(dB)(dBm)

(dB)(dB)(dBm)(dBm)

min

minmin,

SNRP

SNRFkTBPSensitvity

nf

receiverin

(dB)(dBm)(dBm) receivernf FkTBPFloorNoise

Sensitivity

ERP = +50 dBm

Power to Antenna: +40 dBm

TX. Antenna Gain: +10 dB

Frequency: 10 GHz

Bandwidth: 100MHz

Rcvr. Antenna Gain: +60 dB

Transmitter:

ERP

Path LossesRcvr. Ant. Gain

Power to Receiver

Receiver:

Noise Floor @ 290KNoise in 100 MHz BWReceiver N.F.

Receiver Sensitivity

Margin: 4 dB

+ 50 dBm-200 dB

60 dB

-80 dBm

- 174 dBm/Hz+ 80 dB+10 dB

-84 dBm

How to increase Margin by 3dB ?

Path Losses: 200 dB

Selectivity

Ch1

Ch2

Chn

Ch3

RF Filter

freqfRF

Ch1

Ch2

Chn

Ch3

freqfIF

IF Filter

freqfLO

Selectivity

RF Filter

IF Filter

• IF filter rejection at the adjacent channel

• LO spurious in IF bandwidth

• Phase noise of LO

Receiver Added Noise

Receiver Thermal Noise

Noise Figure Calculation

receiver

BasebandRF input

nfP

minSNR

NFinSNR

Standard Bandwidth10log(BW) Sensitivity(dBm) Noise Floor (dBm) SNRin(dB) NF(dB) SNRmin(dB)

DECT 1.70E+06 62.30 -83.00 -111.50 28.50 18.20 10.3

GSM 2.00E+05 53.01 -102.00 -120.79 18.79 9.79 9

WLAN 2.00E+06 63.01 -80.00 -110.79 30.79 15.69 15.1

BW

RdB

NE

dBSNRBW

R

NE

SNR symbolssymbols log10)()(0

min0

min

17!17!

Es/No or Eb/No=?

IP3 Calculation

10

23

3)(2

31

min3

min3

IIPdB

nfsfIIP

nfIIPsf

PP

PSNRDRP

SNRPP

DR

Standard DR SFDR IIP3 Pmax Bandwidth 10log(BW) Sensitivity(dBm)Bluetooth 50.00 36.57 -10.00 -20.00 1.00E+06 60.00 -70.00GSM 87.00 61.67 -5.00 -15.00 2.00E+05 53.01 -102.00WLAN 76.00 52.30 6.00 -4.00 2.00E+06 63.01 -80.00

Image Rejection Calculation

SNRmin

fIF

IRrequired

fRFfLO

Pdesired

PImage

minSNRPPIR desiredimagerequired

(all in dB’s)

Transmitter Architecture

• Direct Conversion Transmitter

• Two-step Conversion Transmitter

• Offset PLL Transmitter

Transmit Specifications• Transmit spectrum mask

Receiver Specifications

20 20

4040

Adjacent channel

alternate adjacent channel

Direct-conversion transmitter

090

I

QLO

Pros: less spurious synthesizedCons: more LO pulling

• Direct-conversion transmitter with offset LO

090

I

Q

LO1

2

Pros: less LO pullingCons: more spurious synthesized

• Two-step transmitter

090

I

Q

cos1tcos2t

12

Pros: less LO pulling superior IQ matchingCons: required high-Q bandpass filter

• Offset PLL transmitter

090

I

Q

cos1tPD/LPF VCO

1/N

Weaver Architecture

Wideband IF Architecture