Outline

• Direct conversion architecture• Time-varying DC offsets• Solutions on offset• Harmonic mixing principle• FLEX pager receiver• Individual receiver blocks• Conclusion

CMOS Direct Conversion

90º

I

Q

Simple architecture No image problem No 50ohm interfaces High integration level Low cost, low power

DC offsets Flicker noise LO leakage Even-order distortion

ProsCons

Time-Varying DC Offsets

Zero IF

LO Leakage

+ Offset

• The offset originates from self-mixing.• It can be as large as mV range at the mixer output.• It varies with the environment and moving speed of the mobile.• The offset signal bandwidth also changes with time. • The maximum bandwidth can be as large as kHz range.

Pow

er

Narrow Band Broad Band

Frequency

Pow

er

Frequency

Pow

er

Frequency

Pow

er

Frequency

Signal

DC

Off

sets

Off

set-

Free

DC offset

Time-Varying DC Offsets

Flicker noise

High-pass corner

Solutions on Offset• Autozeroing or correlated double

sampling• AC coupling or high pass filtering• Digital cancellation• Double LO frequency method [ISSCC99]

• Adaptive dual-loop algorithm combined with the mixer [RAWCON00]

• Pulse-width-modulation based bipolar harmonic mixer [CICC97]

• Square–law based CMOS harmonic mixer [Our work: RAWCON00]

Harmonic Mixing Principle

2flo

Con

ven

tion

al

Th

is w

ork

Square-law Based Mixer

• Ideally self-mixing free.• Traditional voltage controlled switches are replaced by current controlled time-varying transconductances.• Current injection is used to reduce flicker noise.• No noise contribution from LO stage and current source.

M1 M2

M3 M4

LO

2

RF IF

Current

Voltage

Voltage

CouplingNo

DC offset effect

10 -1

10 -2

10 0

BE

R

Eb/N0 (dB)4 8 12 16

A: 0.2 B: 0.4 C: 0.6 D: 0.8 E: 1.0

ED

C

B

A

Offset / Signal

DC

Off

set

Eff

ect

Difficulties in FLEX Pager

-60

-50

-40

-30

-20

-10

0

-15 -10 -5 0 5 10 15

[dB]

FLEX 6400, 4FSK0

-20

-40

-60

dB

1050-5-10kHz

• Narrow band modulation• Significant energy near DC• High pass filtering is not viable• DC offset problem• Flicker noise is significant

Hig

h p

ass

eff

ect

A: Zero OffsetB: 1e-7 OffsetC: 2e-7 Offset

CBA

BE

R @

12

dB E

b/N

0

High pass corner (Hz)

100020050-2

-1

10

Higher corner, Larger BER.

4-FSK Pager Receiver

• Harmonic mixers are used to solve time-varying DC offset.• Peak detectors are used to cancel static DC offset.• High front-end gain and current injection to reduce flicker noise.

LNA• Non-quasi-static

phenomenon makes it unnecessary to do on-chip matching.

• Off-chip matching by a single inductor and a balun.

• |S11|<-20dB @ 930MHz

• Both on-chip and off-chip inductive loads were tried.

Double Balanced Mixer

Improve the linearity; Provide constant impedance to LNA;Current injection provides more than 20dB flicker noise reduction.

Ring Oscillator

It provides 45º phase shift.

AGC

• Gain: -14.5dB~18.6dB. • The linear resistor R0 is used to improve the linearity.• The signal level is sensed by the peak detector.

Static DC Offset Cancellation

Peak Detector

Fmin>200Hz

Zero-IF 4-FSKSignal

LPF

• 5th order elliptic gyrator-C filter• Pass-band gain –6.2dB, ripple ≤ 0.5dB (≤ 9kHz)• Stop-band attenuation ≥ 63dB (≥17.8kHz)

Gai

n [d

B]

0 10 20 30Frequency [kHz]

0

-20

-80

-60

-40

-100

4-FSK Demodulator

Modified zero-intermediate frequency zero-crossing demodulator (ZIFZCD)

Clock Recovery Circuit

VCO is a source-coupled multi-vibrator.

Measured Demodulated Signal

The measured asynchronous/synchronized speed signal.

The measured asynchronous/synchronized direction signal.

The function of demodulator was verified.

Die Photo

DE

MO

D

LP

F

AG

C

Mixer

OS

C

LN

A

LN

A

OS

C

Mixer

Base Band Circuitry

RF Front-End

RF Front-End

Conclusion• Feasibility of direct conversion has been

demonstrated. • Proposed harmonic mixing technique

solves self-mixing induced DC offset problem successfully.

• With the help of static DC offset cancellation, the total DC offset is less than 1mV at the receiver output.

• The modified ZIFZCD 4-FSK demodulator functions correctly.

• A 4-FSK FLEX pager receiver in a single chip has been implemented.