maxim seminar

8/3/2019 Maxim Seminar

1/26

10/26/01 Walid Y. Ali-Ahmad 1

Architectures and RF System Design Issuesfor Integrated Receivers and Transmitters in

3rd Generation Wireless Handsets

Walid Y. Ali-Ahmad

Senior Member of Technical Staff

Wireless Communications GroupMaxim Integrated Products

Sunnyvale, CA


2/26


TALK OUTLINE

Introduction: Example of present level of integration in an RF chipsetfor CDMA cellular radio

Receiver Architectures

Heterodyne Receiver

Image-Reject Receiver

Direct-Conversion Receiver

Low-IF Receiver

Transmitter Architectures

IF-Modulation / Up-conversion Transmitter

Direct-Modulation Transmitter

Offset-PLL Transmitter

Summary


3/26


Example of Present Level of Integration in an RF chipsetfor CDMA Cellular Radio


4/26


Heterodyne Receiver

Advantages:

Down-conversion to baseband I&Q is done at an Intermediate-Frequency (IF) lower than RF.

This results in superior I & Q matching.

Its selectivity, which measures the receivers capability to process a desired small channel in

the presence of close-in strong interferes, is done partly at IF using a highly selective SAW filterand at baseband I&Q using low-pass baseband filters.

The use of the IF SAW filter relaxes the linearity requirements (IIP2, IIP3) of the succeeding IF

and baseband stages.

DC offsets at baseband I&Q do not limit its sensitivity because they are minimized by the factthat the first LO frequency is not equal to the input RF carrier frequency.

DUPLEXER

LNARF SAW

BPF Mixer

IF SAW

BPF

1st LO 2nd LO

090

IF

AGC

PLL1 PLL2

XREF XREF


5/26


Heterodyne Receiver (contd)Design Issues:

The need to use an an off-chip passive BPF for image rejection adds cost and board spacerequirements.

Currently, due to technology limitations, this image-reject BPF and the IF SAW filter can notbe integrated on-chip.

The trade-off between image rejection and channel selection is key in determining the IFfrequency.

Low frequency IF SAW filters (40-150MHz) have high Q and provide high adjacent channelselectivity. However, These filters tend to be large.

High frequency IF SAW filters (150-400MHz) have a relatively smaller physical size, butprovide a lower adjacent channel selectivity.

Good frequency planning is essential in order to minimize spurious responses generated in the

receivers front-end (Fs = mFRF nFLO1 pFLO2) .

The half-IF spurious response at (FRF+ FLO)/2 can be a serious problem in the case of low IFfrequency. The front-end mixer after LNA should have a low 2nd-order distortion and a high

suppression of the (2FLO2FRF) product.

Wanted

Signal

RF

IF

LO

fRFfLO fRF- fIF/2

Half-IF

2x2 product

Wanted

Signal

IF


6/26


Importance of IF Selectivity for Suppression of 3rd-order IM products

Typical Two-stages cascaded IIP3 equation, in linear format::

;g1 is gain of stage 1; il is insertion loss of IF filter

Equivalent IIP3of IF block when including selectivity S (dB) ahead of IF stage:

Generalized equation for overall IIP3 of a receiver chain with M cascaded stages:

IL

S

IF Filter#1

IIP31

G1

Block #1

RF Block IF Block

IIM31

IIP32

G2

IIM32

IIP3

IIM3

PI

P1

P2

Block #2

On-Channel

Passband

PCW_tone

PIIM3

Off-Channel

InterferersOn-Channel

Signal

IM3 product

(C/I)

2

1

1 331

3

1

iip

il)g(

iipiip

(dBm)2333)

233(2332)(33 2221212 S;IIPIIPSIIPPIIPSPIIM

eoo

23121

12123

213

2123

12

1

1 3333

1

3

1/

M-M

M// )ss(siip

ggg

)s(siip

gg

siip

g

iipiip


7/26


Importance of RF Selectivity for Suppression of 2rd-order IM products

Two-stages cascaded IIP2 equation, in linear format:

; g1 is gain of stage 1; il is insertion loss of image-reject filter

Equivalent IIP2of mixers block, including selectivity S (dB) ahead of mixer stage:

Generalized equation for overall IIP2 of a receiver chain with M cascaded stages:

On-Channel

Passband

IIM2

IM2 product

On-Channel

Signal

fRF

fLO

fLO

- fIF

/2

Half-IF

PI

(C/I)

IL

S

RF Filter#2

IIP21

G1

Block #1

RF Block

IIM21

IIP22

G2

IIM22

IIP2

IIM2

PI

P1

P2

Block #2

1st Mixer

2

1

1 22

1

2

1

iip

il)g(

iipiip

2121

1212

213

21212

1

1 2222

1

2

1

)ss(siip

ggg

)s(siip

gg

siip

g

iipiipM-M

M

(dBm)222)22(22)(22 2221212 S;IIPIIPSIIPPIIPSPIIMe

oo


8/2610/26/01 Walid Y. Ali-Ahmad 8

Image-Reject Receiver

Advantages:

It facilitates the integration of the heterodyne receivers front-end by eliminating the use of the

off-chip RF image-reject filter and accomplishing the image rejection on-chip through phasing. It works nicely in receiver systems which do not suffer from strong out-of-band blockers and do

not require interstage filter between front-end LNA and MIXER blocks.

It is suitable for receiver systems using a very low IF frequency (e.g. 10.7MHz, 45MHz), since it

eliminates the need for a very high Q bandpass RF filter in order to reject the image .

Hartley Image-Reject Receiver Weaver Image-Reject Receiver

Mixer I

Mixer Q

LO1

Desired

Ima

ge IF

IF

RF

Input

LO1

090

sin(wLO1

t)

cos(wLO1

t)

LO2

090

sin(wLO2

t)

cos(wLO2

t)BPF

LO

090

Mixer I

Mixer Q

sin(wLO

t)

cos(wLO

t)

LO

Desired

Ima

ge IF

IF

RF

Input

R

C

C

R

LPF



Image-Reject Receiver (Contd)

Design Issues:

1Hartley Architecture:

Image rejection is limited by amplitude and quadrature phase mismatches. Amplitude

mismatches are minimum when using wIF= 1/RC. Phase mismatches are due mainly to errors inthe LO quadrature generation circuit.

Image Rejection Ratio IRR (dB) = 10LOG([(A/A)2 2 ]/4); for (A/A)1, 1rad.

For typical matching in integrated circuits, image suppression falls in the range of 30 to 40dB

(0.2-0.6dB gain mismatch & 1-5 quadrature phase mismatch). In most RF systems, 60-70dB of

image rejection is required. The front-end filter (duplexer) normally makes up for the remainingrequired image rejection (~30dB).

For the RC-CR 90 phase-shift network, (A/A) = (R/R) (C/C), at wRC1.

2Weaver Architecture:

It is also sensitive to mismatches, but it is free from gain imbalances due to the RC-CR phaseshift network, thereby achieving greater image rejection despite process and T variations.

The Weaver architecture suffers from the secondary image problem because of the use of a

second mixing operation. The LPF (or HPF) in between 1st and 2nd mixing stages is used tosuppress the secondary image.

The problem of secondary image can be eliminated if we choose a Zero IF frequency at theoutput (FLO1 FLO2 = FRF). To its advantage also, 2

nd-order distortion in the signal path can be

removed by the bandpass filters following the first mixing operation.



Image-Reject Receiver (Contd)

A 1.9GHz Wide-Band IF Double Conversion Receiver (J.C. Rudell, et al., UC Berkeley, IEEEJSSC, December 1997):



Direct-Conversion Receiver

In current cellular systems where signals are either frequency- or phase-modulated, direct

downconversion must provide quadrature outputs so as to avoid loss of information, since thetwo sidebands of the RF spectrum contain different phase information.

Advantages:

The problems of image frequency and half-IF spurious response are eliminated since FIF = 0.

RF BPF after LNA is optional; it is only needed for additional rejection of out-of-band interferers

and TX power leakage.

The bulky off-chip IF SAW filter is eliminated. All channel selectivity is done at baseband with

low-pass filters and baseband amplifiers.

One VCO and one PLL are needed for the whole receiver.

BPF #1 LNA

LPFI

Q

Mixer I

LPF

AGC

AGC

090

BPF2

Mixer Q

Cext

PLL1 XREF



Direct-Conversion Receiver (Contd)

Design Issues:

1DC offsets:

Static offsets are caused by process mismatch and drift of analog circuitry that vary slowly vs. T,

aging, and current gain setting.

Time-variant offsets are caused mainly by parasitic LO coupling to mixer RF port, LNA input port,

and antenna port. LO self-mixing occurs in mixer and it produces a dc component at the

mixers I & Q baseband outputs. Time-variant offsets can also be caused by a large interferer which can leak from LNA or mixer

input to LO input port and self-mix with itself to produce a dc offset at mixers outputs.

The time-variance is due to reflection of LO leakage against moving objects back to receiver and

due to receiver movements.

Maximum frequency content of time-variant DC offset due to Doppler shift = 2max/; where

max: maximum moving object or car speed.

BPF #1 LNA

LPFI

Q

Mixer I

LPF

AGC

AGC

090

BPF2

1st LO

Mixer QLO

Leakage

Interferer

Leakage

Cext




2DC offsets Cancellation Techniques:

DC offsets can easily saturate the receivers final baseband output stages. Hence, DC offsetremoval or cancellation is required in direct-conversion receivers:

DC blocking or High pass filtering:

it is feasible in non-burst mode systems which are receiving continuously (FDD).

In order to minimize distortion of signal, the high-pass corner should be < 0.1% of the datarate for random binary Mary data.

The baseband signal in the transmitter can be encoded to result in dc-free modulationscheme, such as FSK with m 1 or wideband Direct-Sequence Spread-Spectrum signals.

DC calibration loop: In TDD systems, periodic offset cancellation can be performed during idletimes where the DC offset is stored on a capacitor and then subtracted from the received signal

during actual reception.

Adaptive DSP techniques have been used for DC offset-cancellation in TDD systems.




DC offset cancellation in Pager system

using HP filtering

Eye Diagram

Distortion with

HPF:a) No filtering;

b) Fc = 1% of

Rb;

c) Fc = 0.1% of

Rb




3LO leakage:

LO coupling to the antenna will be radiated out and will create interference in the receive band of

other users equipment using the same wireless standard.

In order to minimize this problem of LO leakage and re-radiation, it is important to use differential

LO and RF inputs to the receiver IC to cancel out common mode signals. In addition, LO leakage

is further reduced by fully integrating the RF VCO tank on chip.

4Flicker Noise:

The 1/f noise of devices in the baseband section of a Zero-IF receiver can substantially corruptthe down-converted signal after the mixers I/Q outputs, especially in MOS implementations (1/fcorner ~ 200kHz).

The effect of flicker noise can be reduced by the use of active mixers with bipolar transistors inswitching pairs and by the use of large MOS devices for baseband filters and amplifiers.

High pass filtering at baseband, when used as part of the DC offset cancellation, can reduce the

integrated 1/f noise at baseband.

Integrated total noise at baseband including 1/f noise can be expressed as following:

cornerfrequencyHigh:corner,frequencylow:corner,1/f:/1

density,spectralpowernoiseThermal:

;)()/1

ln(/1

2

Hf

cf

ff

thS

thS

cf

Hf

cf

ff

ff

thS

nV




Mixer Topology that minimizes 1/f

noise at its baseband output

Interference due to down-converted 1/f

noise and DC offset




5I/Q mismatch:

Assuming that the received signal can be written as vin(t)= I(t)sin(wCt)+Q(t)cos(wCt),and the

amplitude and quadrature phase imbalances in the I/Q down-converter are and ,respectively, we can write the baseband I/Q outputs, after demodulation, as:

vBB,I (t)= I(t).(12).cos(/2) Q(t).(12).sin(/2);

vBB,Q (t)= I(t).(12).sin(/2) Q(t).(12).cos(/2);

As we see from equations above, Gain error appears as a non-unity scale factor in the

amplitude, while phase imbalance

results in cross-talk between demodulated I and Qwaveforms degrading the SNR (I & Q data streams are usually uncorrelated).

In practice, 1dB and 5 for SNR degradation less than 1dB (for QPSK signals).

The full on-chip integration of the Zero-IF receiver and the minimization of devices mismatchreduce drastically the amplitude and quadrature phase imbalances in the I/Q down-converter.

6 Channel Filtering:

Baseband channel low pass filters in a Zero-IF receiver need to have a high dynamic range:

Receiver sensitivity cant be compromised.

Close-in interferes should be rejected without causing in-band distortion.

External capacitors at mixers I&Q outputs can be used to provide additional selectivity toblocking and out-of-band signals (pole @ 1/RCext)




7Even-Order Distortion:

Assume vRF(t) = A1cos(w1t)+A2cos(w2t), the LNA output will contain an IM term at frequency f1-

f2, resulting from 2nd order non-linearity in the LNA. This IM2 product at LNA output will leak to

mixers output because of finite feedthrough from RF input to IF output (3040dBc).

Special attention to the mixers design is required since IM2 product can also be generated in the

mixers RF port by two tones interferes after being amplified in LNA.

The 2nd

order non-linearity in the LNA and in the mixer will also demodulate any AM componenton the received signal due to fading during propagation or Nyquist filtering.

Based on input two-tone interferers level and the resultant low-frequency IM2 level at baseband

output, a receivers 2nd order intercept point (IP2) can be derived.

Using differential LNA output and differential mixers input will suppress the generated common-

mode 2nd-order IM products. As a result, receivers IIP2 can be improved.

RF

LO

IF

Wanted

Signal

fRFf2 f1

Wanted

Signal

IM2Interferer

LNA

0

Feedthrough

0

f1-f

2

Interferers



Low-IF Receiver

Advantages:

The received signal is down converted to a low-IF frequency, which is normally one to two times

the signal BW.

It has the same advantage of Zero-IF receiver in terms of the integration of channel filters. It is less susceptible to 1/f noise.

It is less susceptible to DC offsets since the bulk of signal energy is not centered around DC.

DC offsets cancellation scheme can be simplified.

Very low frequency IM2 products can be easily blocked.

BPF #1 LNA

I

Q

Mixer I

ComplexPolyphase

Filter

AGC

AGC

090

BPF2

Mixer Q

Cext

PLL1 XREF



Low-IF Receiver (contd)

Design Issues:

The A/D converters at baseband output require to have a higher sampling rate than in the case

of a Zero-IF receiver. Image suppression is an issue; Very good amplitude and phase matching between I & Q

baseband channels is required to obtain > 35dB image suppression.

Careful choice of the IF can place the image signal in the adjacent channel.

In order to discriminate between these two signals, it is essential to process I & Q outputs as

a complex pair.

Complex Polyphase filters is essential to obtain the necessary reject in the adjacent channel

2nd -Order Distortion can still result in in-band channel intereference.



Baseband I&Q signals undergo quadrature modulation at an intermediate IF frequency (wIF).

The following IF filter (BPF1) rejects the harmonics of the IF signal. The IF modulated signal isthen up-converted to (FIF FLO2).

The unwanted sideband imposes tough rejection requirements on BPF2, typically 50-60dB, in

order to meet transmitters spurious emissions levels imposed by standards.

This topology does not allow full transmitters integration because of use of off-chip passivedevices such BPF2 and BPF1.

On-chip I and Q matching is superior since modulation is done at IF and not at RF. This willlead to better EVMs and lower cross-talk between I & Q channels.

IF filtering reduces transmitted noise in RX band.

Wide power control dynamic range because control it is distributed between RF and IF sections.

IF-Modulation / Up-Conversion Transmitter

090

Mixer I

Mixer Q

I

Q

Tank

PA

LO1

cos(wIF

t)

sin(wIF

t)

Tank

LO2

IF RF

DUPLEXER



Direct-Modulation Transmitter

In direct-conversion transmitters, the baseband signal is directly modulated unto the RF carrier.

The output carrier frequency is equal to the LO frequency at mixers inputs.

This topology is attractive for full transmitters integration since it does not use an intermediate IF

stage with upconversion and interstage IF filter.

Its main disadvantage is the corruption through injection pulling of the VCO spectrum by the

high level PA output. Isolation required is normally > 60dB. The isolation can be highly improved by offsetting the LO frequency by using 2xLO off-chip and

dividing by 2 on-chip or by adding or subtracting another oscillator.

The power control dynamic range is limited by the carrier feedthrough. A fully integrated

differential transmitter architecture will minimize carrier feedthrough because of higher of

common mode rejection (differential LO inputs and modulator output).

090

Mixer I

Mixer Q

I

Q

Tank

PA

2xLO

/2

carrier

Feedthrough

090

Mixer I

Mixer Q

I

Q

Tank

PA

LO

carrier

Feedthrough

cos(wLO

t)

sin(wLO

t)

cos(wLO

t)

sin(wLO

t)



Direct-Modulation Transmitter (contd)

22

2

2

2

2

1

22

e

)cos(21)cos(21LOG10(dB)nSuppressioSideband

)cos(214

1sin2LOG10(dB)nSuppressioCarrier

)sincossignalmodulatingtonesingle(assuming

:followingascalculatedbecannsuppressiosidebandandcarrierThe-2

)sin())(()()1()cos()()1)cos(tanEVP(t)

))cos()()1)cos(())sin())(()()1((EVM(t)

:followingascalculatedbecancomponents(EVP)phaseand(EVM)magnitudeVectorErrorThe-1

signals.basebandinputareanderror;phasequadraturemodulatorisly.respectivepaths,Q&Ioferrorsamplitudeare&ly;respectiveinputs,Q&IbasebandatoffsetsDCare&

I

Qe

I

Q

I

Qe

I

Q

I

Qe

I

QQ

I

QeQI

I

QI

mmm

eQQIIIeQQeQ

eQQeQeQQIII

QIQI

A

A

A

A

A

A

A

A

A

A

A

AO

A

A)(OO

A

AO

t)(t); Q(t)(; I(t)

AOtQAOtIAAOtQA

AOtQAAOtQAOtIA

Q(t)I(t)AAOO

090

Mixer I

Mixer Q

I(t)

Q(t)

Tank

PA

LO

carrier

Feedthrough

cos(wLO

t)

sin(wLO

t + fe)

OI

AI

AQ

Desired

Vector

Measured

Vector

Error

Vector

OQ



Direct-Modulation Transmitter (contd)

Modulation error or EVM in Transmitters



Baseband I&Q signals undergo quadrature modulation at an intermediate IF frequency (wIF).

Instead of upconverting the IF signal, the phase modulation in the IF signal is transferred

faithfully to the TX VCO via the offset PLL topology, with condition that the loop BW of PLL ischosen properly.

This phase translation scheme to RF is only valid for constant envelop modulation signals suchin GSM (GMSK).

The PLL LPF helps tremendously at suppressing the out-of-band noise generated by the

modulator and, hence, meeting the stringent GSM requirements for the thermal noise in the

receive band. This eliminates the need for the off-chip bulky Duplexer.

This topology is quite attractive for low-cost high performance integrated transmitters using

constant-envelop modulation. However, enough isolation is required between the TX VCO and

the PA in order to suppress VCO pulling by PA output noise.

Offset-PLL Transmitter

090

Mixer I

Mixer Q

I

Q

Tank

PA

LO1

cos(wIF

t)

sin(wIF

t)

Tank

LO2

LPF

IF

LPF

PLLTank

TX

VCOLPF

MXR

PD

Offset

MXR


26/26

SUMMARY

Need for Fully Integrated VCOs.

Need for Mixers with low level of 2nd-order Distortion (High IIP2).

DC offset cancellation schemes have to be implemented without distorting signal.

High Dynamic Range Baseband filters are key for direct-conversion receivers.

Variable Gain PAs improve power control dynamic range in Direct-ModulationTransmitters.

Low Noise Direct-Modulation Transmitters and Direct-Conversion Receivers enablelow-cost radios for 3G applications.

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