ee08b097 akshay paper

8/3/2019 Ee08b097 Akshay Paper

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Introduction

A fully integrated 2X1 Dual Band DirectConversion Mobile WiMAX tranceiver.

• Features :

–Dual Mode Fractional Divider

– Noise Shaping Transimpedance Amplifier

– 65nm CMOS technology

Presented by

Akshay Bhardwaj

EE08B097

PAPER-PRESENTATION



Outline

Introduction

Transceiver Architecture

Frequency Planning

Circuit DesignMeasurement results

Conclusions



Introduction

WiMAX (Worldwide Interoperability for MicroWave Access) provides

wireless HIGH speed INTERNET service globally.

Mobile WiMAX uses:

• MIMO technique.

• OFDMA modulation scheme.

• High Data rates.

Challenges :• Single chip implementation.

• Low power Consumption

• Floor Planning.



Tranceiver Architecture

The transceiver is composed of a

PLL, LO distribution block, two

receivers and a transmitter.

Covers frequency ranges of

dual band Mobile WiMAX Low-Band (2.3-2.7GHz)

High-Band(3.3-3.8GHz)

Adopts Direct Conversion

Architecture• Minimise circuit Components

• Minimise power consumption



Direct Conversion Receiver Receiver has two RF paths for LOW and HIGH band respectively

RF input first amplified by cascoded common source LNA with LC

tank and down-converted to baseband I/Q signals by I/Q mixers.

The receiver adopts passive mixers to reduce the flicker noise from

switching transistors.

Then, flicker noise from the following TIA op-amp becomesdominant , hence a noise shaping TIA is proposed to improve the

noise figure(NF) of receiver.

In each I/Q path , the down-converted signals are filtered and

amplified by noise shaping TIA, an LPF andd two variable gain

amplifiers.

Finally, signals converted to digital signals by ADCs.



Receiver Block Diagram



Direct Conversion Transmitter The transmitter has two output paths for dual band

operations.Digital I/Q signals are converted to analog baseband

I/Q signals by current steering DACs in I/Q paths.

Next, baseband signals are filtered by reconstructionLPFs and up-converted to RF signal by I/Q mixers.

To reduce circuit components , I/Q mixers are shared

in the LB and HB paths by employing wide-band

passive mixers. The output power of the up-converted RF signal is

controlled by programmable gain stages.



Transmitter Block Diagram



Frequency Planning The single VCO in the PLL is covers frequency tuning range of 8.8-10.8

GHz. The operating frequency of the VCO is divided by 2 and LO signal of 4.4-

5.4Ghz is globally distributed.

LB operation:

For LB ,global LO signal is divided by 2 just before passive mixers in thereceiver and the transmitter.

HB operation:

For HB, global LO signal is divided by 4/3 and then the HB LO signal of

3.3-4.05 G Hz is obtained.

Process : Global LO signal of 4.4-5.4GHz is divided by 2 locally and mixed

with global LO signal to generate LO signal of 6.6-8.1GHz and finally

divided by 2 to generate 3.3-4.05GHz.



Frequency Plan



Circuit Design

Totally three key circuits.

Noise shaping TIA – for mitigating flicker noise

of scaled CCMOS devices

Dual Mode fractional divider – employed in

the inductor less LO distribution path.

Wide-band passive mixer –employed in the

transmitter



A. Noise shaping transimpedance amplifier

In the receiver design , passive mixer topology is adopted toreduce flicker noise , thus reducing the flicker noise from the

TIA becomes very important.

Large Input transistor size of Op-amp can be straightaway

used but it may make Op-amp design much more complex

due to other constraints

- gain, bandwidth, linearity and stability issues.



Basic TIA

Block Diagram :



Noise Shaping TIA

2.



NS-TIA with feedback cap

2.



NS-TIA contd…. I. For basic TIA ,

Signal transfer function(STF) and Noise Transfer Function(NTF) are derived.

• GI =conductance seen from mixer output

• GF = feedback conductance of TIA

• AV = Voltage gain of op-amp

• Vn2 = input referred noise of op-amp



NS-TIA contd…. II ) To suppress flicker noise , we place an additional gain component

before the

op-amp and as input referred noise is suppressed by that gain.

• Ya in the first block is replaced by Yb which is subdivided into Ya and

which is additional gain component.

III ). Since the actual implementation of TIA has 1st

order LPF characteristics ,weconsider

influence of feedback capacitance.

Cf reduces impedance at node X and noise is selectively suppressed only at low

frequencies and NTF is shape into a high pass characteristic.



NS-TIA contd….

• STF and NTF :



NS-TIA contd…. • With the aid of the additional gain boost by , the

STF of the noise-shaping TIA becomes closer tothe ideal low-pass characteristics than the onewithout the gain boost.

• Since the negative conductance cell is placedbetween the virtual ground nodes of the TIA,linearity of the negative conductance cell itself isnot critical.

• Linearity of the proposed TIA is better than aconventional one without the negativeconductance cell because the open loop gain of the TIA is larger



B. Dual-Mode Fractional Divider• It is composed of the divide-by-2 circuit, the gm-cell and the mixer.

• For suppressing the lower side-band spur generated by the mixer in the high-band

operation, we have adopted single side-band (SSB) mixer architecture.



C. Wide-Band Passive Mixer in Transmitter

• Actual implementation is the double-balanced topology for both I/Q

paths.

• The mixer is composed of resistors R1 , a capacitor C1 for AC coupling anda switching transistor M1 . The resistor R1 is the termination resistor as a

load of the mixer, which is also used for biasing the following

programmable gain stage.

• Wideband characteristics are achieved by making small enough to exclude

the effect of parasitic capacitance on impedance at the node X.



Measurement results

By enabling the noise-shaping function, 4 dB

improvement of the receiver output noise at

7 kHz is achieved

Difference of STF is negligibly

small with or without the noise-shaping

function



contd…. • In TIA , by making delta small, we can improve the noise

suppression ratio but we cannot make it too small as when GNbecomes larger than GI + GF , polarity of delta becomes

negative nad the feedback loop becomes unstable.



Layout of the compact dual-mode fractional divider

• The size is 56 um by 118 um

• It is laid out to minimize the lower side-band spur of 1/3*fc

caused by the device mismatches and layout asymmetry of

the single-gate mixers employed in the dual-mode fractional

divider.

• As a result of “fractional-then-distribute” plan all the spurs

become very small.

• All the spur levels are below 60 dBc.



Wide-band spectrum measured at transmitter output.



• Error Vector Magnitudes:

The constellation plots measured at +1 dBm

transmitter output are shown in Figure. Theerror vector magnitudes (EVM) in 2.5 GHz and

3.5 GHz bands are 34.0 dB and 31.0 dB,

respectively



TX constellation plots measured at +1 dBm output.

.



Summary of Transceiver Performance

.



Die micrograph of test chip



Conclusions• A fully integrated 2 RX X 1 TX dual-band direct-conversion mobile WiMAX

transceiver in 65 nm CMOS technology was presented.

• The “distribute-then-fractional” frequency plan is proposed, whichemploys the inductor-less LO distribution with the compact dual-mode

fractional dividers.

• The frequency plan provides higher flexibility for the floor planning of the

long LO distribution path.

• To mitigate the flicker noise of scaled CMOS devices, the noise-shaping TIA

with the negative conductance cell is proposed which keeps signal transfer

function the same and suppresses flicker noise.

• the power consumption of the receiver employing the noise-shaping TIA is

lower than that of the mobile WiMAX transceivers reported previously,

total NF of 3.8 dB and 4.5 dB is achieved in the low-band and the high-band operations , respectively.



THANK YOU

ee08b097 akshay paper

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