GAAS MMICS FOR AUTOMOTIVE RADAR APPLICATIONS
M. Camiade, D. Domnesque, P.F. Alleaume, A. Mallet*, H. Daembkes
United Monolithic Semiconductors, BP46, Route Départementale 128, 91401 Orsay, France
e-mail: camiade@ums.thomson.fr
* IRCOM, Université de Limoges, 87060 Limoges Cedex, France
ABSTRACT
This paper describes design, fabrication, and performance of a GaAs based MMIC chip set for a
production oriented Automotive Cruise Control (ACC) radar system at 76GHz. All functions of the
high frequency front-end are fabricated in three different MMICs: Oscillator, transmitter, and
receiver. Simulated and measured performances of these elements are presented. The results show
that today MMICs provide a competitive solution for production type ACC application. Future
trends and developments are also briefly discussed.
INTRODUCTION
After more than 20 years of R&D work, 1998 will be the first year to see Automotive Cruise
Control radar systems as standard parts in production. German car manufacturers are taking the lead
in this domain. The RF-part of the first production-generation systems will be based either on
completely hybrid assembly using GUNN and Schottky diodes or on a mixture of GUNN and
Schottky diodes together with one MMIC for switching. First experiences with these solutions show
the limitations especially in the overall cost-, size-, and performance objectives. Clear
improvements are expected by using a solution completely based on MMICs. The advantages are :
- significantly reduced tuning of the oscillator
and no tuning for the transmitter and receiver elements.
- reduction of size
- improved thermal stability
- improved reliability
- and finally reduced cost.
The cost reduction will be achieved not only directly at the component level but at the module level
which today is dominated by assembly, tuning, and packaging efforts.
In this paper we report about a complete MMIC solution for all RF functions of a typical ACC
radar. This chip set was tested in several generations of pre-production systems and is in production
now.
RADAR SYSTEM CONSIDERATIONS
For ACC applications, the radar system has to measure distance and relative speed of a car driving
in front of the own car. Typical specs for the radar sensor are: a range of 150m to 200m, angular
resolution of about 0.5° to 3°, speed of up to ± 180km/h with an accuracy of 0.2 to 0.5km/h /1,2,3/.
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Presently detection of fixed targets (obstacle warning, stop and go operation) is not yet required,
even if some of the systems could already support these functions.
Among the different radar principles /4/ FM-CW is the most popular one. The chip set described
later on supports narrow band FM-CW in analog or stepped frequency mode. A modified version
allowing wide band FM-CW and random frequency shape systems will be described in the last part
of this paper.
The partitioning of the system reflects the results of extensive system simulation concerning overall
system performance and handling requirements. The key importance of assembly yield, directly
determining the production cost, led us to the decision to cut the frequency chain at 38GHz and
avoid one interface at 76GHz. The partitioning of TX and RX chips was determined following
different customer requirements concerning the receiver concept.
OUT
38.25 GHz
x2
19.125 GHz
x2
76.5 GHz
OSC40
MFC 38/76
MOD
RF1
RF2
DMIX 77
IF2
IF1
Fig. 1: Block diagram of the ACC radar RF-front end for a quasi bistatic system with two receiver
channels. Single channel receivers are available also as image rejection mixers. /5/
COMPONENTS
Oscillator
The oscillator is the most important part of the today’s radar systems. Stability, output power level,
and phase noise are the critical parameters. Especially the phase noise near carrier is a parameter of
very strong influence to the resolution and so to the sensitivity of a radar system.
Phase noise (dBc/Hz)
-20
-40
-60
-80
-100
-120
0.1
1
10
100
Offset frequency from carrier (kHz)
Fig. 2: Measured phase noise characteristics
of the 38.25GHz narrow band VCO
1000
The oscillator was designed to operate at fo/4
equal to 19.125GHz. This signal is multiplied by
2 inside the oscillator chip and also buffered
against external influences. We use 0.25µm
PHEMT technology in connection with a mixture
of lumped and distributed elements. The design
was done in microstrip technology. The
frequency is determined by an off-chip resonator,
which allows either wide band or narrow band.
characteristics. Tuning is done by application of
appropriate gate voltage. Tuning bandwidth at
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38.25GHz between 700MHz (low Q) and 20MHz (high Q) is achieved for about 2.5V voltage
swing. The output buffer stage delivers a power level of typically 9±1dBm. Frequency stability is
better than 5ppm/°C and can be influenced via the characteristics of the resonator material.
Measurement of the phase noise was done in test fixtures and delivered values of -100dBc/Hz at
100KkHz offset from carrier.
Transmitter
The transmitter was redesigned starting from a former version /6/. It is based on 0.15µm PHEMT
and microstrip technology. The improvements concerned the further reduction of noise in the TX
chain and reduction of cross talk.
OUT
s
um
Fig. 3 :
Chip photograph of the
transmitter chip in 0.15µm
PHEMT and microstrip
technology.
The input of the circuit is formed by a buffer amplifier, driving a PHEMT frequency doubler, and a
three stage output amplifier / power splitter. It delivers 12dBm at the main OUT port. A second
output port is buffered by a single stage amplifier, providing the drive signal to the receiver with
good isolation. Evaluation of the phase noise of this chip delivers an excellent value of better than
-154dBc/Hz at 200kHz off carrier. The circuit is very insensitive against input signal level variation.
Receiver
Depending on the application, one, two or three receiver channels are required today. For the single
channel receivers image rejection mixers are realized. The earlier generations of receivers were
fabricated using also 0.15µm PHEMT technology as for the TX function. The PHEMT receiver
however delivered significantly higher 1/f noise performance and conversion loss. The conversion
loss may be compensated by adding a LNA in front /7,8/, but the low frequency noise remains a
problem for base band receivers. Therefore a redesign was done usíng a well proven diode process.
This is a GaAs Schottky diode process with a minimum feature size of 1µm. Due to the very low
series resistance and small capacitance, extremely high extrinsic cutoff frequencies in excess of
3THz are achieved. Further on the low frequency noise is typically around 20dB to 30dB better than
that of equivalent PHEMTs. These performance advantages could also be realized in the diode
based mixer MMIC as displayed in table 1. Therefore, with the new diode based mixers, excellent
sensitivity equal to that of optimized hybrid diode based is achieved avoiding external tuning.
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Criteria
Conversion loss
AM noise @ 10kHz
AM noise @ 100kHz
Isolation between RF ports
PHEMT based design
10 dB
- 135dBc/Hz
18dB
Diode based design
6dB
-160dBc/Hz
-167dBc/Hz
35dB
Table 1: Comparison of measured performance of single ended mixers for 76GHz RF signal.
TECHNOLOGY
A standard PHEMT technology developed for communication application is used for the ACC
MMICs. All levels are generated by stepper lithography except for the gate which is presently done
by e-beam lithography. The gate recess process uses an etch stop layer which delivers excellent
homogeneity across the wafer and from wafer to wafer. A temperature stable Aluminum based
Schottky contact is used for the gate, eliminating the gate sinking and hydrogen related degradation
observed for some Gold based gates.
The diode process for the mixers starts with a MBE grown layer sequence. All litho steps are done
by stepper. The minimum feature size is 1µm, enabling electrical yields in excess of 90%. For the
interconnects and the passive components the same process and the identical design rules as for the
PHEMT circuits are applied.
For reasons of compatibility to the user interfaces, the designs were done using the well establish
microstrip technology. Substrate thickness is 100µm. Via holes are used for contacting the backside.
FUTURE DEVELOPMENTS
For applications requiring a wider frequency sweep bandwidth, a different oscillator principle will
be used. All the other circuits have no bandwidth limitation and remain unchanged.
With simple VCOs the noise figure is insufficient for bandwidth specs of a few hundred MHz.
Further on, for standard FM-CW type radars, a linearisation loop has to be included.
Fout/3
Chip 1 : VCO
Fout =38.25GHz
MEDIUM Q
RESONATOR
(Printed)
x3
Tuning
(Fout-Fif)/3
HIGH Q
RESONATOR
(Dielectric)
Fig. 4:
Generic block diagram for general
purpose source at 38GHz /9/
Chip 2 : Reference
x3
IF
LOOP
MOD
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A multi purpose, programmable oscillator for nearly all presently employed radar principles, a
source architecture as given in Fig. 4 was developed and is presently under evaluation.
The new source consists of a millimeter wave VCO combined with a stable reference oscillator in
order to extract the intermediate frequency and build a loop for VCO linearisation and VCO phase
noise compression. Details are reported in /9/.
SUMMARY
Based on more than 10 years of experience in the development of automotive radar MMICs, a
comprehensive chip set for the RF-front end of a narrow band FM-CW / frequency stepping ACC
system is now available on an industrial basis. Excellent performance could be demonstrated in
commercial systems. A new source under development will enable us to provide a complete chip set
for a wide range of different radar principles, allowing system designers to implement their new
system in a flexible mode using standard off the shelf components.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support received from French Ministry of Industry
in the frame of the Eureka project PROTECH and from the German Ministry for Education and
Research. The support by numerous colleagues in Ulm and Orsay is gratefully appreciated.
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