3/18/2004 1 A Direct Conversion CMOS Transceiver for IEEE802.11a WLANs Pengfei Zhang RF Micro Devices, San Jose, California
Mar 27, 2015
3/18/2004 1
A Direct Conversion CMOS Transceiver for IEEE802.11a
WLANs
Pengfei Zhang RF Micro Devices, San Jose, California
3/18/2004 2
Outline
• Introduction
• Transceiver Architecture
• Frequency Planning
• Circuit Design
• Measured Results
• Conclusion
3/18/2004 3
Wireless Connectivity
WLAN
Bluetooth™
Cellular
GPS
3/18/2004 4
WLAN - Mobility vs. Bit Rate
Mb/s1 10 1000.1
Ou
tdo
or
Stationary
Walk
Vehicle
Ind
oo
r
Stationary/Desktop
WalkMo
bili
ty WLAN
(IEEE 802.11a/b/g)
Bit Rate
LAN
W-CDMA/ EDGE
BluetoothZigbee (IEEE 802.15)
3/18/2004 5
WLAN - Cost vs. Bit Rate
Mb/s1 10 1000,1
Use
r C
ost /
bit
HIPERLAN/2
Bit Rate
LAN
Low
Medium
High
Very Low
$
WLAN(IEEE
802.11a/b/g) LAN
W-CDMA/ EDGE
W-CDMA/EDGE
BluetoothZigbee (IEEE 802.15)
3/18/2004 6
Two-Chip WLAN System
Ref. J. Bohac, Flexible chipset arms 802.11a/b/g WLANs, Microwaves and RF, May 2003
3/18/2004 7
Design GoalsPerformance Condition 11a Requirement
Frequency Range In-door 4.90*-5.35 GHz (*incl. Jpn
bands) 54 Mb/s -65 dBm
Minimum Receive Sensitivity 6 Mb/s -82 dBm
Maximum Transmit Constellation Error
54 Mb/s 16 dBm
-25 dB
• Design Goals: To exceed standard performance requirement, achieve low power consumption and low cost
3/18/2004 8
Direct Conversion vs. Low-IF Conversion (I)
Direct Conversion
Low-IF Conversion
Flicker Noise X DC Offset X Even Order Distortion
X X
LO Pulling X X LO Leakage X X
3/18/2004 9
Direct Conversion vs. Low-IF Conversion (II)
• Image Response– Direct conversion: No Image– Low-IF conversion:
• Analog image rejection – I/Q matching, Circuit complexity• DSP image rejection – large ADC dynamic range required
• ADC Sampling Rate f flow-IF conversion= 2*fdirect conversion
• Baseband Filters– Direct conversion: BW ~ 10 MHz– Low-IF conversion: BW ~ 20 MHz More Power!
3/18/2004 10
Transceiver Architecture
LNA
5.25GHz
90o
0o
LO-0o
LO-90o
RF IN
RF OUT
AGC
LPF
LPF
AGC
To ADC
To ADC
Receiver
LPF
LPF
LO-0o
LO-90o
FromDAC
FromDAC
PAD
Transmitter
Synthesizer
3/18/2004 11
Example of Frequency Planning (Ref.[2])
• Avoid pulling, reduce LO-RF interaction• However, unwanted sideband at /3:
– RX: image in highly populated band (1.8GHz) – TX: degrades efficiency; FCC requirement
Unwanted SidebandMixerDividerVCO
2
cos(2t/3) cos(t/3)
sin(t/3)
cos(t)+cos(t/3)
sin(t)+sin(t/3)
3/18/2004 12
Proposed Frequency Planning
• Quadrature VCO: cross coupled LC resonators• SSB Mixer: suppress unwanted sideband
LO DriverSSB MixerDividerBufferVCO
2
3.5
GH
z
1.75GHz 5.25GHz
cos(2t/3)
sin(2t/3)
cos(t/3)
sin(t/3)
cos(t)
sin(t)
3/18/2004 13
IM2 of Multi-carrier
Ref. K. Cai, P. Zhang, Microwave Journal, Feb. 2004
3/18/2004 14
ADS Simulation of IM2 Impairment
Ref. K. Cai, P. Zhang, Microwave Journal, Feb. 2004
3/18/2004 15
Receiver Architecture
• 7th order Chebyshev “leapfrog” LPF• Improved IP2: AC coupled RF, fully diff baseband• DC offset compensation: 7b DAC; Look-up-table
NotchFilter
AGC
LPF
LPF
AGC
To ADC
To ADC
Notch Filter
DAC
DACLNA V/I Converter
LO-0o
LO-90o
RF IN
From baseband
LUT I
Q
3/18/2004 16
Mixer Design
• Conventional single balanced mixer: tradeoff between 1/f noise and linearity
• Two-stage mixer: Independent biasing
• I2: linearity, I1: 1/f noise
I1
M1
M2 M3
R1 R2
Out+ Out-
LO+ LO-
RFC1
L1
I1
M1M2 M3
R1 R2
Out+ Out-
LO+ LO-
C1
I2RF
C2C2
3/18/2004 17
0 10 20 30 40 50 60-60
-50
-40
-30
-20
-10
0
Gai
n (
dB
)Digital Control Word
Digital Gain Control: R-2R LadderR/6 R/6 R/6 R/6R/6 R/6
Sj1 Sj3 Sj4 Sj5Sj2Sj0 S(j-1)0I1
VOUTR 2R 2R 2R
R R RS00Sn0 Sj0 S10
Mixer Current I1
VDD
When Sjk=1, Gjk=-6*j+20LOG(1+k/6) (dB)
• Mixer resistive load: 10-section R-2R ladder
• Monotonic, linear, fast settling and Constant Rout
3/18/2004 18
Notch Filter: Active-LC Trap
10M 100M-16
-12
-8
-4
0
Mag
nit
ud
e (d
B)
Frequency (Hz)
• Partial channel selection filter to relax linearity requirement of baseband stages• Synthesized L takes minimal silicon area• Large low frequency impedance of C1 minimize flicker noise contribution from M5
R1
I1
C2
C1
M5
VDD
VoutMixer
3/18/2004 19
AGC
60 48 36 24 12 0
-60
-40
-20
0
20
40
60
80
Output SNR
output noise power
output signal power
No
ise
Fig
ure
(d
B)
Po
wer
(d
Bm
)
Receiver Gain (dB)
For an input signal ramping in power, the RX gain is adjusted to maintain a constant output signal power level. Although the RX noise figure increases in low gain settings, the output SNR remains higher than 34 dB in the entire dynamic range.
3/18/2004 20
RX LPF Frequency Response
3/18/2004 21
Transmitter Architecture
• LPF: 4th order Butterworth• Off-chip PA used for system power saving
OffsetControl
LPF
OffsetControl
LPF
LO-0o
LO-90o
FromDAC
PA Driver
FromDAC Power Amp
I
Q
3/18/2004 22
TX LPF Frequency Response
3/18/2004 23
Transmitter RF Front-end
• LO leakage: Mixer DC offset tuning• D/S converter: No need of off-chip balun• Programmable Pout: Binary weighted PA driver
offsettuning
Vbbi
Vbbq
LOq
LOi
L1 L2
VddRFout
R0
M0C0
M4
M3
R1
M1C1
R2
M2C2
D0
D1
D2
C3
D/S Converter
3/18/2004 24
Frequency Synthesizer• Automatic
VCO band selection
• Accumulation mode NMOS varactors
• Off-chip 40 MHz XTAL and loop filter (~200 KHz)
PFD N
decision
timer
M3.5GHz
XT
AL
VH
VL Band Selection
3/18/2004 25
VCO Frequency Bands
3/18/2004 26
Die Photo
0.18 m 1P-6M CMOS with MIM-C
3/18/2004 27
Receiver Gain & Noise Figure
0 2M 4M 6M 8M 10M4
6
8
10
12
14
16RBW=50KHz
-3dB @ 8.7MHz
Baseband Output Frequency (Hz)
No
ise
Fig
ure
(d
B)
40
44
48
52
56
60
64G
ain (d
B)
3/18/2004 28
Receiver Sensitivity
-90 -80 -70
0.0
0.2
0.4
0.6
PER:10%
36 Mb/s 50 frm
54 Mb/s100 frm6 Mb/s
30 frm
Pac
ket
Err
or
Rat
e
Input Power (dBm)
3/18/2004 29
TX Transmit Spectrum
3/18/2004 30
TX Transmit Spectrum
3/18/2004 31
PAPR in OFDM System
• 52 Subcarriers:17dB (=10*log52) PAPR
0 50 100 150 200 250 3000
10
20
30
40
50
60
70
3/18/2004 32
Peak Power Probability
• Large peaks do not occur very often (Gaussian distribution)
4 5 6 7 8 9 10 11 12 13 140
2000
4000
6000
8000
10000
12000
PAPR(dB)
3/18/2004 33
TX Transmit Spectrum
5.28G 5.32G 5.36G-60
-50
-40
-30
-20
-10
0TransmitSpectrum
Mask
RBW=100KHzVBW=30KHz
Output Frequency (Hz)
Tra
nsm
it P
ow
er (
dB
m)
3/18/2004 34
Transmit Constellation
• Output power: 16.2dBm
• 64QAM, 54Mb/s
• Average EVM: 3.41%, -29.3dB
3/18/2004 35
LO Spectrum
1.72 1.74 1.76-80
-60
-40
-20
0
Divided-by-2Output
-66dBc13.3MHz
RBW: 10KHzVBW: 500Hz
Po
wer
(dB
m)
Frequency (GHz)
5.238 5.240
TXOutput
3/18/2004 36
LO Phase Noise
1M 10M 100M-160
-140
-120
-100
-80 Divided-by-2Output 1.75GHzOpen Loop
Ph
ase N
ois
e (
dB
c/H
z)
Offset Frequency (Hz)
10k 100k 1M 10M-130
-120
-110
-100
-90
-80
Phase error=1.5 degree
TX Output5.24GHzClose Loop
3/18/2004 37
RX Switch On Settling Time
-2µ -1µ 0 1µ 2µ0.0
0.4
0.8
1.2
1.6
1.6s
Signal
Clock
Time (second)
Clo
ck V
olt
age
(v)
-0.3
0.0
0.3S
ign
al Vo
ltage (v)
3/18/2004 38
TX Switch On Settling Time
-2µ -1µ 0 1µ 2µ0.0
0.4
0.8
1.2
1.6
Time (second)
Clo
ck V
olt
age
(v)
-0.15
0.00
0.15
1.6s
SignalClock
Sig
nal V
oltag
e (v)
3/18/2004 39
Performance Summary
Condition IEEE802.11a Requirement
This Work
6 Mb/s -82 dBm -91 dBm Sensitivity 54 Mb/s -65 dBm -74 dBm RX
Power Consumption
VDD=1.8v NA 171 mW
EVM 16 dBm Pout
-25 dB -29.3 dB*
LO Rejection 15 dBc 38 dBc TX Power Consumption
VDD=1.8v NA 135 mW
Chip Area 13 mm2
*External PA used
3/18/2004 40
Conclusion
• CMOS Direct-Conversion Transceiver with integrated VCO and frequency synthesizer fully compliant with IEEE 802.11a standard
• Exceeding standard performance requirement with low power consumption and small die area