A Fully Differential Low-Power High-Linearity 77-GHz SiGe Receiver Frontend for Automotive Radar Systems Dietmar Kissinger*, Benjamin Sewiolo*, Hans-Peter Forstner", Linus Maurer", and Robert Weigel* *Institute for Electronics Engineering, University of Erlangen-Nuremberg, Cauerstr. 9, 91058 Erlangen, Germany Email:
[email protected] tInfineon Technologies, Am Campeon 1-12, 85579 Neubiberg, Germany +Danube Integrated Circuit Engineering (DICE), Freistadter Str. 400, 4040 Linz, Austria
Abstract-This paper presents a single-chip receiver frontend consisting of a low-noise amplifier and an active downconversion mixer, intended for application in automotive radar systems at 77 GHz. The circuit has been implemented in a SiGe:C HBT technology with ft/fmax = 200/250 GHz and can operate either fully differential or in single-ended mode. The receiver frontend shows a conversion gain of 24 dB and a single sideband noise figure of 14 dB when driven single-ended. Linearity measurements show a 1 dB input referred compression point of -10 dBm. The circuit draws 40 rnA from a 3.3 V supply and occupies a chip area of 728 x 1028 fJm2 including bond pads. I. INTRODUCTION
In the past years, automotive radar is finding an increased interest for comfort and safety applications. It enables the integration of a wide variety of active and passive systems for improved vehicular safety. Long-Range Radar (LRR) applications use the frequency band from 76 to 77 GHz. Additionally the band ranging from 77 to 81 GHz has been allocated for Short-Range Automotive Radar (SRR). Silicon-based technologies featuring SiGe Heterojunction Bipolar Transistors offer the possibility to manufacture costefficient radar frontends with a high level of integration. These technologies show maximum oscillation frequencies and cutoff frequencies above 200 GHz [1]-[6] and their suitability for millimeter-wave applications has been shown by a number of publications. Over the recent years several architectures for SiGe-based downconversion mixers have been published, resembling standard double-balanced Gilbert cell approaches [7], [8] as well as micromixer topologies [9], [10]. Published receiver frontends feature an additional low-noise amplifying stage prior to the mixer to reduce the overall noise figure of the receiver chain [11]-[16]. Hard specifications for the automotive environment define a large dynamic range of the received signal. Besides a low noise figure this necessitates a high linearity of the RF frontend. This paper presents the design and measurement results of a highlinearity receiver frontend for application in 77 GHz FMCW radar systems.
II. CIRCUIT DESIGN
The proposed receiver consists of a cascade of a low-noise amplifier (LNA) followed by a mixer stage for direct downconversion of the received signal. Both the LNA and the mixing stage are designed differentially. The inputs for the RF and La signals feature a A/ 2 transmission line connected between the differential pads. This line acts as a balun and enables the circuit to be additionally driven in single-ended mode through the conversion of the 100 n differential impedance to 25 n for single-ended measurement equipment. Fig. 1 shows the schematic of the low-noise amplifier. It consists of a cascode stage which is inductively degenerated for simultaneous noise and power matching. The input and output matching networks with integrated DC decoupling are realized through transmission lines and capacitors. DC bias for the common-emitter and cascode stage is fed through the virtual ground nodes along the symmetry axis. The schematic of the mixer is shown in Fig. 2. It consists of a double balanced switching quad with L-matching networks for the La and RF input ports. Instead of featuring the additional transconductance stage of a Gilbert mixer the RF
RFout
RF;bt1
Fig. 1.
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Schematic of the proposed cascode low-noise amplifier
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LO power (dBm)
Fig. 4. Fig. 2.
Receiver gain and noise figure versus LO power
Schematic of the proposed high-linearity mixer 26 ,-------~---~---~---~--_____,
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Fig. 5.
Fig. 3.
Die photograph of the fabricated receiver frontend
signal path is decoupled from the current source through AI4 lines. The IF output of the mixer is connected to differential emitter followers (not shown) which transform the output impedance to 100 n (differential).
III.
EXPERIMENTAL RESULTS
Fig. 3 shows a die photograph of the receiver frontend with different indicated stages of the circuit. The overall pad limited chip area is 728 x 1028 urn2 . The characterization of the receiver frontend has been done by single-ended on-wafer measurements with a measurement setup described in [17]. Noise parameters have been measured at an intermediate frequency of 4.8 MHz.
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Receiver gain and noise figure versus LO frequency
Fig.4 shows the vanation of noise figure and conversion gain over the local oscillators power level for a fixed RF input level of -16 dBm . The mixing stage works down to a level of odBm without significant performance degradation. At 0 dBm La power noise figure and conversion gain are 14 dB and 24 dB respectively. It can be expected that the noise figure is further improved by approximately 2 dB when the circuit is driven differentially. Referring to Fig.5 the frontend shows nearly constant behavior for the gain and noise figure across the intended frequency range around 77 GHz at an La power of 0 dBm. Fig. 6 shows the measured conversion gain versus the RF input power for a La power of 0 dBm . The input referred 1 dB compression point is -10 dBm. S-Parameter measurement have been carried out using an Agilent PNA8361A in combination with waveguide modules . Fig.7 shows the S-Parameter Measurements for the La and RF port. The input return loss is better than -10 dB for both ports at the operation frequency of 77 GHz.
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TABLE I COMPARISO N OF SIG E-BAS ED STANDALO NE MIX ERS AND RECEIV ERS IN TH E
76 -81 GH z BAN D
Gain (dB)
NF (dB) b
P-1dB(in) (dBm)
Vee (V)
Pile (mW) C
PI.O (dBm)
FOMI
24
14
-30
5.0
300
2
154
127
11
16.5
-0.3
5.5
412
-3
168
145
[9]
Architecture" Mixer Mixer Mixer
13.4
18.4
-12
4.5
176
4
157
131
[10]
Mixer
15.5
16
-3
5.5
187
-2
171
150
[11]
LNA + Mixer
28
II
-16
5.5
1072
I
175
144
Ref. [7] [8]
[12]
LNA + Mixer + YCO
37
8
-28.5
2.5,3 .5
161
[13]
LNA + Mixer
30
11.5
-26
5.5
440
[14]
LNA + Mixer + YCO
21.7
10.2 (sim.)
-35
5.5
(595)
[15]
LNA + Mixer + YCO
40
6.9
-35
2.5
[16]
LNA + Mixer (+ YCO)
40
7-9
-38
This work
LNA + Mixer
24
14
-10
3.3
FOM2
175 0
167
140
151 172
115 122 (195)
-2
168
149
132
0
174
153
abraekets denote different published realizations with external and on-ch ip YCO respectively bsingle-sideband (SSB) noise figure, reported double-sideband (DSB) noise figures have been increased by 3 dB Ctotal power consumpt ion without YCO, power consumpt ion including YCO in brackets
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IV. CONCLUSION 24
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RF power (dBm)
Fig. 6.
Receiver gain versus RF input power
A high-linearity integrated receiver frontend in a highperformance SiGe:C technology for application in 77 GHz automotive radar is presented. The fabricated chip can be operated in differential or single-ended mode and on-wafer measurements show a gain of 24 dB and a noise figure of 14 dB when driven single-ended with an LO power of 0 dBm. An input related I dB compression point of -10 dBm is achieved with a total power consumption of 132mW from a 3.3 V supply. The overall occupied chip area is 728 x 1028 f-lm 2 including bond pads. Table I shows a summary of published downconversion mixers and receiver frontends in the frequency range of 76 to 81 GHz in SiGe technology. In (I) the calculation of the figure of merit for the performance of FOM, is shown. An additional figure of merit FOM 2 which also takes the power consumption and the necessary LO drive into account is presented in (2). FOM, FOM 2
co
174 + Gain - NF + P.'dB(in) FOM, - Pile (dBm) - Pw
(I) (2)
-2
In comparison to other published receiver frontends the performance related figure of merit FOM, of this work is among the highest published so far. In addition it simultaneously achieves the best performance to power consumption ratio which is expressed through FOM 2 •
-2
ACKNOWLEDGMENTS
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Frequency (GHz)
Fig. 7.
S-Parameter Measurement of La and RF input return loss
The authors would like to thank the team from Infineon Technologies for the fabrication of the presented chip, as well as the radar design group from DICE for their support and helpful discussions . This work has been supported by the German Bundesministerium fiir Bildung und Forschung (BMBF) through the research project RoCC - Radar on Chip for Cars under contract number 13N9821.
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