A fully integrated low-IF CMOS GPS radio with on

IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 37, NO. 12, DECEMBER 2002
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A Fully Integrated Low-IF CMOS GPS Radio With
On-Chip Analog Image Rejection
Farbod Behbahani, Member, IEEE, Hamid Firouzkouhi, Member, IEEE, Ramesh Chokkalingam, Siamak Delshadpour,
Alireza Kheirkhahi, Mohammad Nariman, Student Member, IEEE, Matteo Conta, and Saket Bhatia
Abstract—A fully integrated Global Positioning System (GPS)
radio is presented. Low-IF architecture was used for a high level of
integration and low power consumption. An on-chip analog imagereject filter provides 18 dB of image-noise rejection to prevent noise
figure (NF) degradation. With image rejection performed in the
analog radio, a single-path (nonquadrature) output was used. The
integrated synthesizer only requires an off-chip phase-locked loopfilter to function. Implemented in a 0.35- m 2P4M CMOS process,
the integrated radio has a chip area of 9.5 mm2 . The radio operates
over a wide range of voltage and temperature, from 2.2 to 3.6 V and
from 40 C to 85 C and consumes 27 mW from a 2.2-V supply.
The receiver has 4 dB NF.
+
Index Terms—CMOS radio, Global Positioning System (GPS),
image rejection, receiver, VCO.
Fig. 1.
I. INTRODUCTION
T
HE MARKET for Global Positioning System (GPS) radios is rapidly growing. Soon, GPS radios will be standard
equipment for cars. The FCC has mandated location finding for
cell phones with GPS as the prevailing technology. Manufacturers of other portable devices, e.g., portable digital assistants,
have plans to integrate GPS receivers in their systems. The main
requirements for all these systems are low cost, low power consumption, and minimum footprint. Only a single-chip solution
can meet those requirements.
Typically, GPS radios are implemented in bipolar or
BiCMOS processes and cannot be integrated with the digital
signal processor chip due to higher cost. Furthermore, they
use multi-downconversion receiver architectures that require
off-chip filters, adding to the footprint and cost. This paper
describes a highly integrated CMOS GPS radio with minimum
off-chip components, which is the gating factor for the ultimate
single-chip solution.
Section II describes the nature of GPS systems and signals.
Sections III and IV explain the details of receiver and synthesizer design, respectively. Measurement results are presented in
Section V.
GPS signals.
spread spectrum system functioning at two bands: L1 (1575.42
MHz) and L2 (1227.6 MHz) [1]. All satellites transmit at both
frequencies and their signals are distinguished by different
Gold codes used to spread the signal, where most commercial
GPS receivers use the L1 signal only. The system is based on
time-of-arrival (TOA) of signals from the visible satellites.
Using the location and time information sent by satellites and
TOA from at least four visible satellites, four equations can be
solved for altitude, latitude, longitude, and time.
The GPS has two sets of codes in the L1 band, Coarse-Acquisition (C/A) and Precision (P). The original 50-b/s data is spread
over a 2-MHz bandwidth (BW) for the C/A code (Fig. 1). With a
130-dBm received signal, the power spectral density (PSD) is
about 193 dBm/Hz and is lower than the thermal noise level.
On the other hand, the P code has a 20-MHz BW with a PSD
even lower than the C/A code and is mainly used for military applications. Across the P code 20-MHz band, the signal is dominated by the thermal noise and no other signal is present. This
characteristic was used to choose the optimum architecture for
this GPS receiver.
III. RECEIVER PATH
II. GPS SYSTEM
The GPS is a satellite-based location/time finding system
with 24 satellites orbiting the earth. It is a direct sequence
Manuscript received May 7, 2002; revised June 29, 2002.
F. Behbahani and H. Firouzkouhi were with Valence Semiconductor, Inc.,
Irvine, CA 92618 USA. They are now with SiART, Inc., Irvine, CA 92606 USA
(e-mail: fbehbahani@siart.net).
R. Chokkalingam, S. Delshadpour, A. Kheirkhahi, M. Nariman, M. Conta,
and S. Bhatia are with Valence Semiconductor, Inc., Irvine, CA 92618 USA.
Digital Object Identifier 10.1109/JSSC.2002.804355
A. Architecture
This radio was designed for the L1 band and C/A code.
A low-IF architecture with 1.023 MHz IF was chosen for
minimum off-chip components and low power consumption
(Fig. 2). The choice of 1 MHz low-IF results in an image
frequency within the P-code 20 MHz BW. Thermal noise
dominates the 20 MHz P-code band (Fig. 1). With IF of 1 MHz,
since the image frequency of the C/A code lies in the P-code
band, no other strong signals are present in this band and only
0018-9200/02$17.00 © 2002 IEEE
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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 37, NO. 12, DECEMBER 2002
Fig. 4. Third-order Chebychev g =C implementation.
Fig. 2. Radio block diagram.
circuit de-emphasizes the input referred noise and improves the
NF. A specially designed ESD structure provides more than 2
kVof ESD protection, without noticeable NF degradation. The
series resistance usually found in an ESD structure is not used
here. The output current of M1 is divided into I and Q branches
(by M2 and M3, respectively) and goes to the switching devices
of these two single-balanced mixers (M4–M5 and M6–M7).
M2 and M3 isolate the two mixers as well as improving the
LO-to-RF isolation. M2 and M3 each provides a relatively low
impedance path, looking from the source of the other cascode.
This increases the noise contribution of the cascode MOSFETs
compared to a simple LNA.
The image reject filter rejects correlated image noise at the
output of the mixers, while the uncorrelated image noise increases the NF. Here, the image noise of M1 is correlated at the
output and cancelled. M2 and M3 have partly correlated noise
contributions at the output.
C. Channel Select Filter
Fig. 3.
Low-noise mixer.
15 dB image rejection is needed to limit the noise figure (NF)
degradation due to image noise to less than 0.15 dB [2], [3].
An active antenna comprising an LNA and a filter receives
the GPS signal from the satellites. After a matching circuit, the
signal is input to the on-chip low-noise mixers and is downconverted to a quadrature IF of 1.023 MHz. The third-order Chebychev filters following the mixers are used for channel selection.
Then, the signal passes through a fifth-order complex elliptic
filter that rejects the image noise by an average of 18 dB and
further attenuates out-of-band interferers. A chain of variable
gain amplifiers (VGAs) then provides up to 80 dB of gain for
the filtered noise and signal and sets the noise level for quantizer input. The quantizer is programmable to provide 1-, 1.5-, or
2-b outputs. The following sections describe the main building
blocks of the receiver path in detail.
B. Low Noise Mixer
Fig. 3 shows the low-noise mixer [4]. M1 is an inductively
degenerated CS stage at the input, designed in the same way
as for an LNA [5]. The voltage amplification in the matching
The low-noise mixer is followed by a third-order Chebychev
channel select filter with a 3-MHz BW. This filter provides the
first step of out-of-band interference rejection and relaxes the
linearity requirement on the image-reject filter. The filter uses a
implementation of a ladder structure. As shown in Fig. 4,
the floating inductor in the LPF is replaced by a Gyrator block
stages in feedback.
and the resistors are implemented using
The filter is fully scaled for amplitude and noise [6]. A loop is
and, therefore, the bandwidth of the
used to control the
filter.
D. Image Reject Filter
The quadrature low-noise mixer provides quadrature signals
at its output. The IF image-reject filter provides an average of
18 dB in band image rejection, without any trimming. It limits
the NF degradation to below 0.07 dB.
The frequency response of a real system is symmetric around
dc, as shown in Fig. 5. It passes both the positive frequencies
(desired signal) and negative frequencies (image signal). An
ideal image-reject filter is single-sided and only passes positive or negative frequencies. A complex frequency-shift operation converts a real low-pass function into the required complex
function, as Fig. 5(a) shows [7]. The shift frequency is
(1)
BEHBAHANI et al.: FULLY INTEGRATED LOW-IF CMOS GPS RADIO WITH ON-CHIP ANALOG IMAGE REJECTION
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(a)
(b)
Fig. 5. (a) Conversion of a real LPF frequency response to a complex BPF
by frequency shifting. (b) Capacitor transformation to implement frequency
shifting in an g =C filter.
Fig. 7. VGA blocks and AGC loop.
to provide a sharp transition close to dc and limit the in-band
image noise.
At the output of the image reject filter, image noise is sufficiently attenuated. Since the main signal at this point is real, the
quadrature path is no longer required.
E. VGA and AGC
(a)
(b)
Fig. 6. (a) Implementation of the image-reject filter using two coupled
fifth-order elliptic low-pass filters. (b) Complex frequency response.
Applying this operation to the current equation of a grounded
capacitor yields
(2)
are available. This can
In a quadrature system, both and
be easily implemented by adding a coupling transconductor of
which the input is connected to the quadrature node, as shown
in Fig. 5(b). This technique was used to implement the analog
polyphase image-reject filter.
Fig. 6 shows the image-reject filter. It has two fifth-order elliptic LPFs with a 1.2-MHz BW for the I and Q paths. The
coupling transconductors shift the low-pass frequency response
by 1.2 MHz and create the complex image-reject frequency response required for image rejection. Elliptic filters were used
VGA stages amplify the filtered noise and GPS signal to optimally fit the input dynamic range of the ADC. Their gain is set
by the automatic-gain-control (AGC) loop (Fig. 7). The VGAs
provide up to 80 dB of gain with 70 dB of gain variation. These
stages have process-dependant biasing to minimize their gain
variation. This gain range was implemented to accommodate
different gain levels in the off-chip active antenna. Each of the
VGA stages has a frequency-dependent load that reduces the
gain at dc and keeps the stage gain at dc less than 1. They are
implemented using a transconductor with a first-order LPF in its
feedback path. This prevents the dc offset from being amplified
and saturating the amplifier stages.
The output of VGA stages is full-wave rectified and compared
to the desired level. The output is filtered by an off-chip capacitor and controls the gain of the VGA stages.
The VGA stages are implemented using two cross-coupled
) is high,
parallel amplifiers. When the control voltage (
all of the current passes through the main amplifier (M1 and
M2) and provides the maximum gain. By lowering the control
voltage, some bias current passes through the cross-coupled amplifier (M3 and M4) and the generated signal will be subtracted
from the output, reducing the total gain.
F. Quantizer
Most available GPS baseband chips available have a 1-,
1.5-, or 2-b quantized output signal. A programmable quantizer
provides these three options and is implemented using three
comparators. For 1-b operation, a single comparator is used.
It simply compares the VGA output to “0” and generates the
1-b output. For 1.5-b operation, two comparators are used to
compare the VGA output with optimum threshold levels for
the GPS signal [8]. For 2-b operation, all three comparators
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IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 37, NO. 12, DECEMBER 2002
Fig. 8. The quadrature VCO.
are used to generate four levels. A simple digital coder is used
to convert the comparator outputs to the quantizer output bits.
To accommodate different types of baseband signal processing
chips, the output is provided in both CMOS full swing level
and differential limited swing (with signal level of VDD and
VDD-0.8 V).
The 1-b quantizer acts as a limiter. As the in-band noise dominates the signal, such a limiter is captured by the noise. This
results in a signal-to-noise ratio (SNR) loss of about 2.2 dB [1].
Fig. 9. The charge pump.
IV. SYNTHESIZER
A. Structure
The synthesizer is comprised of a crystal oscillator that generates the 18.414-MHz reference frequency using an off-chip
crystal. Its output is divided by four and used as the comparison
frequency. The VCO generates the quadrature local oscillator
(LO) signals for the receiver mixers. The LO signal is divided
by 342 and input to the PFD along with the comparison frequency. The PFD has a delay element in its feedback loop to
prevent a dead- zone region in the charge pump. The resulting
filtered charge-pump output controls the VCO. Here, an off-chip
second-order filter was used (Fig. 2). GPS is a single-frequency
system and a start-up time of less than 5 mS is acceptable.
Therefore, synthesizer-settling time was not an important design factor.
B. VCO
Two coupled VCOs generate the quadrature LO (Fig. 8).
Coupling devices are sized at 25% of the main MOSFETs to
limit their power consumption and noise contribution, while
providing the required coupling factor. Then, 17-nH inductors
are implemented by four layers of metal in series and have a Q
of 3.5. NMOS transistors are used as varactors.
Most of the published coupled quadrature VCOs use separate current sources for I and Q VCOs [9], [10]. This design
shares one current source for both I and Q VCOs, resulting
in higher output swing with the same total bias current. This
effect can be explained in the following way. For simplicity,
lets assume that coupling MOSFETs (M5–M8) drain negligible
current. When one quadrature node is at its peak (e.g., IP), its
voltage is higher than the other three output nodes (IN, QP, QN).
One core MOSFET (M2) carrying all the tail current at its peak
Fig. 10.
Chip micrograph.
current would be
. However, when using two separate cur. Thererent sources, the maximum current would be
fore, the peak current is higher compared to a circuit using separate current sources with half of the current for each differential
VCO. This provides a higher swing for the same total bias current. Current in the coupling MOSFETs exists in both cases of
shared current source and separate current sources and does not
affect the estimated conclusion. This VCO has 15% frequency
range to cover 4% frequency variation due to temperature and
11% process variation.
C. Charge Pump
Source switching current sources are used for the charge
pump (Fig. 9). In the steady state, the output current sources
are off most of the time and have no power consumption. When
current sources turn off, charge trapped at nodes A and B leaks
to the output and increases the PLL spurs. The auxiliary circuit
discharges these nodes to prevent leakage to the output. When
both current sources go off, SW1 and SW2 connect nodes
A and B to a buffered version of the output voltage. With a
BEHBAHANI et al.: FULLY INTEGRATED LOW-IF CMOS GPS RADIO WITH ON-CHIP ANALOG IMAGE REJECTION
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TABLE I
RADIO PERFORMANCE SUMMARY
Fig. 11. NF of the RX excluding the quantizer.
zero voltage drop across the output cascode MOSFETs, their
leakage current is eliminated. In practice, very small leakage
current still exists due to the opamp offset voltage.
V. MEASUREMENT RESULTS
Fig. 12. SNR at the output of the 1-b quantizer.
This radio was implemented in 2P4M 0.35- m CMOS with
a total chip area of 9.5 mm . A 48-pin TQFP package was used.
Fig. 10 shows the chip micrograph. Radio performance is summarized in Table I. This radio operates over a wide range of
supply voltages, from 2.2 to 3.6 V and over a wide range of
temperature from 40 C to 85 C. It drains 12.3 mA from a
2.2-V supply. It needs very few off-chip components to function, namely the active antenna, the matching circuit, the AGC
cap, crystal, phase-locked loop (PLL) filter, and bias reference
resistor. The single-ended input was matched to 50 using a
simple T network. It provided better than 10 dB S11 over the
supply, temperature, and component variation range.
As was explained before, the natural condition for a GPS receiver is for its output to be dominated by the in-band noise. The
same condition was reproduced during all measurements. With
about 3-MHz approximate noise BW and assuming the NF to be
around 4 dB, the equivalent input referent integrated noise level
is about 105 dBm. Therefore, a 120-dBm tone was chosen
as the desired in-band signal for measurements, which is at least
15 dB below the in-band noise level.
To measure the NF, a 120-dBm tone was applied at 1575
MHz to the input of the receiver and the SNR was measured. The
output was monitored on a spectrum analyzer with 100-Hz resolution BW (RBW), which limited the integrated noise referred
to the input of the receiver to about 150 dBm. The resulting
30 dB SNR can then be easily measured on the spectrum analyzer. The NF was calculated from the SNR using the following
equation:
Fig. 13. The 1-dB blocker desensitization level.
NF
dBm
dBm/Hz
dBHz
SNR
HzBW
(3)
A 1-b limiter captured by noise degrades the SNR. In order to
separate this effect from the radio performance, the output was
measured at the VGA output/quantizer input, which includes the
whole RX chain except the quantizer. Fig. 11 shows that the NF
is about 4 dB and is fairly constant over the temperature and
supply voltage range.
In practice, 1-b digital signal processors experience SNR
degradation from a 1-b quantizer. Therefore, SNR at the output
of the 1-b quantizer was measured here with the same setup
of a 120-dBm tone at 1575 MHz, the input, and 100-Hz
resolution BW for the spectrum analyzer. Fig. 12 shows that
the SNR is about 27.7 dB. This corresponds to about 2 dB SNR
degradation by the quantizer.
Another important characteristic of a GPS radio is its sensitivity to the blocker signals. The 1-dB desensitization of this
radio was measured in the following way: the same desired
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Fig. 14.
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 37, NO. 12, DECEMBER 2002
Image-rejection response.
image from dc to 2 MHz, there is no room for a transition
band in the image rejection response at dc. Therefore, we
have a transition band from dc to about 400 kHz for the image
rejection to get to 18 dB. The average image noise rejection
is 18 dB. The frequency response droop close to dc is caused
by the dc rejection. The IF filter section provides more than 60
dB of attenuation for out-of-band signals, as shown in Fig. 15.
With a 70-kHz PLL BW, the spurs level was measured to be
63 dBc and the in-band PLL phase noise floor was 72 dBc.
The main potential contributor to the output spurs level is
the mismatch between the output PMOS and NMOS current
sources. The VCO phase noise was 107 dBc/Hz measured at
a 1-MHz offset (Fig. 16).
VI. CONCLUSION
Fig. 15.
IF chain frequency response.
A low-IF GPS radio with very few off-chip components was
implemented in a pure 0.35- m CMOS process consuming
27 mW from a 2.2-V supply. It comprises a complete receiver
chain and a fully integrated synthesizer. Image noise is rejected
using an on-chip analog image-reject filter. This filter was
implemented by coupling two low-pass filters in the quadrature
paths and converting them to a complex filter.
ACKNOWLEDGMENT
The authors would like to thank B. Javid for the digital design,
the layout team of L. Doan, H. Nguyen, E. Rajabalian, M. Ho,
and D. Nguyen, and the test team of D. Patel and G. Soper.
REFERENCES
Fig. 16.
Phase noise and spurs at the output of the synthesizer.
signal was applied at the input and the SNR was measured at
the output of the quantizer. An interferer tone was applied at
the input over a wide range of frequencies and its power was
ramped up to the point that the SNR for the desired signal is
decreased by 1 dB. Fig. 13 shows the results. Thanks to the
filtering very early in the RX chain, this radio can tolerate a
high level of 20-dBm out-of-band interference, without any
off-chip filtering. The lower interference tolerance at 535 and
788 MHz is due to the in-band harmonics. The IIP3 of the radio
was measured using two tones at 8 and 14 MHz to be 15 dBm.
Fig. 14 shows the on-chip image rejection frequency response. Since the signal extends from dc to 2 MHz and the
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BEHBAHANI et al.: FULLY INTEGRATED LOW-IF CMOS GPS RADIO WITH ON-CHIP ANALOG IMAGE REJECTION
Farbod Behbahani (S’92–M’99) was born in
Tehran, Iran, in 1967. He received the B.S. and M.S.
degrees (with honors) in electrical engineering from
Sharif University of Technology, Tehran, Iran, in
1990 and 1993, respectively, and the Ph.D. degree
from the University of California, Los Angeles
(UCLA), in 1999.
From 1992 to 1995, he worked for Philips Semiconductors as an RF and Analog Integrated Circuit
Designer. From 1995 to 1999, he was a Research Assistant at UCLA. From 1999 to 2002, he was the Director of Engineering, RF and wireless at Valence Semiconductor Inc., Irvine,
CA. In 2002, he founded SiART, Inc., Irvine, CA, which is focused on devloping high-performance CMOS radio products. His research interests include
RF CMOS and adaptive architecture for wireless radio. He has received four
U.S. patents and is an author or coauthor of more than 25 technical papers in
the area of integrated circuit design and communication systems.
Dr. Behbahani was a co-recipient of the second prize in the annual Iranian
national research contest in 1996 and the IEEE Solid-State Circuits Society Predoctoral Fellowship in 1997. He received the Young Scientist Award from the
Association of Professors and Scholars of Iranian Heritage in California in 1999.
He was also recognized as the outstanding Ph.D. in electrical engineering from
UCLA in 2000.
Hamid Firouzkouhi (M’01) received the B.S.
degree from Khajeh Nasiredin Toosi University,
Tehran, Iran, in 1992 and the M.S. degree in
telecommunications engineering from California
State University, Long Beach, in 1998.
He worked in Iran Telecommunications Research
Center and Broadcasting Research Center on communication circuit designs from 1989 to 1992. From
1998 to 2002, he was with Valence Semiconductor,
Inc., Irvine, CA. Since 2002, he has been with SiART,
Inc., Irvine, CA, where he is developing CMOS radio
1727
Siamak Delshadpour received the B.S. degree from
the Sharif University of Technology, Tehran, Iran, in
1993 and the M.S. degree from Tehran University,
Tehran, Iran, in 1996, both in electronic engineering.
He worked as a Member of Technical Staff in
Sharif Electronic Research Center, Sharif University
of Technology, from 1993 to 1999, on system
and board level circuit design. He joined Valence
Semiconductor, Inc., Irvine, CA, in 1999, where
he is working on analog and mixed-signal circuit
design in deep submicrometer CMOS technology
for wireless products.
Alireza Kheirkhahi was born in 1975 in Tehran,
Iran. He received the B.S. degree from Sharif
Institute of Technology, Tehran, Iran, in 1996 and
the M.S. degree from the University of Tehran,
Tehran, Iran, in 1999.
He has been with Valence Semiconductor, Inc.,
Irvine, CA, since 1999 as a Senior Design Engineer.
His main research interests are wireless transceivers
and CMOS mixed-signal circuits.
Mohammad Nariman (S’97) received the B.S.
degree in electronics from Sharif University of
Technology, Tehran, Iran, in 1997 and the M.S.
degree in electrical engineering from the University
of Southern California, Los Angeles, in 1999. He is
currently pursuing working toward the Ph.D. degree
at the same university.
He joined Valence Semiconductor, Inc., Irvine,
CA, in May 2001 where he has been involved in
design of RF integrated tranceivers in deep submicrometer CMOS technology. He has also worked
on noise analysis, substrate modeling, design and modeling of integrated
inductors and varactors, and design of RF blocks including oscillators, mixers,
and amplifiers.
products.
Ramesh Chokkalingam received the B.S. degree
in electrical engineering from the Birla Institute of
Technology and Science, Pilani, India, in 1998 and
the M.S. degree from the University of Southern
California, Los Angeles, in 2000.
He is currently working on High Performance
Synthesizers for wireless chips at Valence Semiconductor, Inc., Irvine, CA.
Matteo Conta was born in Pavia, Italy, in 1972. He
received the M.E. degree in electrical engineering
from the University of Pavia, Pavia, Italy, in 1996
and the M.B.A. degree from the University of
California, Irvine, in 2002.
From August 1996 to October 2000, he worked as
an RF IC Design Engineer with Conexant Systems,
Newport Beach, CA, where he designed CMOS frequency synthesizers and phase-locked loops. Since
October 2000, he has been with Valence Semiconductor, Inc., Irvine, CA, engaging in the design and
development of integrated CMOS transceivers for wireless LAN and GPS. His
research interests include the design of RF, analog, and mixed-signal circuits.
Saket Bhatia, photograph and biography not available at the time of publication.