1 J.Dąbrowski, Radioelectronics 1 Radioelectronics ‘RF CMOS Transceiver Design’ Jerzy Dąbrowski Division of Electronic Devices Department of Electrical Engineering (ISY) Linköping University e-mail: [email protected]http://www.ek.isy.liu.se/ → courses/tsek26/ J.Dąbrowski, Radioelectronics 2 Objectives of the course • Learn principles of wireless digital communication transceivers • Gain knowledge of RF front-end circuits • Learn design methods and techniques for RF circuit design in CMOS technology • Understand the related possibilities and limitations
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J.Dąbrowski, Radioelectronics1
Radioelectronics‘RF CMOS Transceiver Design’
Jerzy DąbrowskiDivision of Electronic Devices
Department of Electrical Engineering (ISY)Linköping University
• Learn principles of wireless digital communication transceivers
• Gain knowledge of RF front-end circuits• Learn design methods and techniques for RF
circuit design in CMOS technology• Understand the related possibilities and
limitations
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J.Dąbrowski, Radioelectronics3
Organization of the course• Lectures 12 x 2h• Tutorials 6 x 2h (by Rashad Ramzan [email protected] )• Homework: optional• Laboratory work 3 x 4h (by Rashad Ramzan and
Henrik Fredriksson [email protected])• Course book: T.H.Lee, The Design of CMOS Radio-
Frequency Integrated Circuits, Cambridge Univ.Press, 2004 supporting book: Bosco Leung, VLSI for Wireless Communication, Prentice-Hall, Pearson Education, 2002/2004
• Examination: written exam
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Outline of the lecture
• Wireless communication systems today
• RF transceiver at glance
• CMOS RF design issues- architectures- circuits and devices
This IEEE 802.11a (in 5 GHz band) transceiver employs a direct-conversion architecture and includes an internal synthesizer. This is Intel's first RFIC used in a WLAN product.
fmax>100GHz, perform well up to 10 GHz or more• Good linearity for higher signal swing• With multiple metal layers good capacitors and inductors
(QL up to 20) can be integrated on a chip• Upper metal layers far from Si substrate – reduce substrate losses• Lower substrate doping helps to isolate RF blocks and reduce losses• Large digital bocks (DSP & control) can be integrated on one chip
(but introduce substrate noise), Bipolar process not suitable• CMOS on Si cheaper from other technologies (BiCMOS, or CMOS
on GaAs, SiGe)• Many successful RF CMOS designs performed recently
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Old & New receiver architectures
• Superheterodyne receiver /double conversion(good sensitivity and selectivity, good image rejection but tradeoffs)
IR Filter
RFFilter
LO1
IF Filter
LNA IFA
LO2
I Q
LP Filter ADC
• Discrete IR and IF filters not amenable for integration
• Low impedance of those filters raise power dissipation in LNA and first mixer (matching for off-chip needed)
BB signal
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Superheterodyne receiver (cont’d)
fk f
RF filter selects band, rejects off-band signals,
IR filter rejects off-band products, it has same band as RF filterfLO,k
Receiver band Bw
fk-1fLO,k-1
Constant intermediate frequency fIF
fIF
LO1 frequency is adjusted to select the channel for down-conversion
fIF f
IF filter selects channel,adjacent channels are partly suppress
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Old & New receiver architectures (cont’d)
• Direct receiver (homodyne) –(fewer components, image filtering avoided – no IR and IF filters)
RFFilter
LNA
LO
I Q
DC removal+ LPF ADC
• Large DC offset can corrupt weak signal or saturate LNA (LO mixes itself), notch filters or adaptive DC offset cancellation – eg. by DSP baseband control
• Flicker noise (1/f) be difficult to distinguish from signal
• Channel selection with LPF, easy to integrate, (noise-linearity-power tradeoff are critical, even-order distortions low-freq. beat)
LO Leakage fLO = fRF fIF = 0
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Direct receiver (cont’d)
fk f
RF filter selects band, rejects off-band signals,
Receiver band Bw
fk-1
fLO,k = fk
Wanted channel is corrupted by its mirror,IQ downconversion is needed to separate them with Hilbert transform
fIF = 0 f
fIF = 0 f
LP filter selects channel, It is also anti-alias filter for ADC
Useful for wideband systems, DC and 1/f noise can be removed by HPF
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Old & New receiver architectures (cont’d)
• Low-IF receiver
RFFilter
LNA
LO
I Q
DC notch + LPF ADC
LO Leakage
• DC offset and 1/f do not corrupt the signal, like in the superheterodyne, still DC offset must be removed /saturation threat
• But image problem reintroduced / close image !
• Still even-order distortions can result in low-freq. beat – differential circuits useful
Polyphase filtersupports IQ rejection
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Low-IF receiver (cont’d)
fk ffLO,k
fIF fIF
Desired channel
In-band image channelRF band
filter
Close-image problemImage and desired channel signal overlapat fIF frequency, but due to I and Q paths and Hilbert transform the image can be suppressed
More severe problem than in zero-IF since theimage can be much stronger than the signal.
Tough requirements for IQ match if image is large, otherwise signal strongly corrupted
Good for GSM std. since the adjacent channel only 9dB larger, so rejection of 20..30 dB enough
fIF = ½ BWch typical
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Need for IQ down conversion
-ω0 ω00
Inherent mirror spectrum
-ω0 ω00Aliasing by mirror
IQ mirror cancellation after using Hilbert transform
0
Down conversion to zero with one mixer
-ω0 ω00
Down conversion to zero with quadrature IQ mixer
and
0
)ω()ω()sgn(
2ILP
j
QLP YeY ±⋅− ωπ
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Need for IQ down conversion (cont’d)
sFSK (t) = cos(ωIF t + ϕ(t))
dϕ(t)/dt = k xBB (t)
s1 (t) = cos(ωIF ( t-t1) + ϕ(t-t1))= cos(ωIF t – 900+ ϕ(t-t1))= sin(ωIF t + ϕ(t-t1))
so the detection cannot be achieved since the other component is folded
y
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Direct conversion transmitter
MatchingNetworkPA
Duplexer
or Switch
Receiver
Asinωct Acosωct
LO
I
Q
Bas
e ba
nd
BPF
Leakage of PA
• Up-conversion is performed in one step, fLO= fc
• Simple modulation, e.g. QPSK can be done in the same process
• BPF suppresses harmonics
• LO must be shielded to reduce corruption
• I and Q paths must be symmetrical, otherwise crosstalk
High-power signal
FDD or TDD,respectively
Leakage of LO
Also effect on Rx can be critical
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Problem of carrier leakage
PABPF
Leakage of LO
Asinωct
Acosωct
I
Q
Calibrationfeedbackto BB
fRF
Wantedtransmitted signal
Carrier fallsin band
Constellation can be destroyed and EVM rises
Tx measures output when BB signal is absent and introduces offset in BB stage to compensate for the carrier leakage
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Circuits and devices
A suspended spiral inductor of about 100 nHon a heavily doped CMOS substrate. This element enabled the first RF CMOS circuits at 900 MHz.Without inductors you can hardly neutralize parasitic capacitances in CMOS
Today lightly doped substrates and upper-metal layers enable good inductors on chip
Low-noise amplifier styles commonly used in CMOS. (a) Common gate circuit, robust against parasitics, moderate noise figure. (b) Common source circuit, lowest noise figure.
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Circuits and devices (cont’d)
Differential style must be preferred to suppress common mode substrate noise and even order distortions
CMOS oscillators require good Q factors to suppress own noise (white and 1/f)
Shield helps to reduce substrate currents
But still eddy-currents
L
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Circuits and devices (cont’d)
Circuit techniques of minimizing Phase Noise, 2nd harmonic can be suppressed and less 1/f noise is upconverted in this way
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Circuits and devices (cont’d)
MOSFETs as switches enable discrete tuning to enhance freq. range and compensate for production variations (tolerances).
The same technique is popular to calibrate for IQ mismatch
CMOS process tolerances can be largebut a circuit correction is possible → DfC
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Summary• Wireless communication systems (mobile, cordless,
WLAN, GPS, … ) are in continuous progress• Wireless communication systems are very complex
multidisciplinary field • Design of RF IC’s is a multi-objective task• CMOS technology proves to be increasingly
competitive for RF IC’s design (even higher frequencies)