A means of mechanically altering the electrical length of a planar transmission line would greatly enhance the use of integrated circuit technology at millimeter and submillimeter wavelengths. Such a mechanically adjustable planar RF tuning element, successfully demonstrated at 100 GHz, is described here. It consists of a thin metallic sheet, with appropriately sized and spaced holes, which slides along on top of a dielectric-coated coplanar-strip transmission line. Multiple RF reflections caused by this structure add constructively, resulting in a movable RF short circuit, with |s11|&Gt;APX=/-0.3 dB, which can be used to vary the electrical length of a planar tuning stub. The sliding short is used here to produce a 2-dB improvement in the response of a diode detector. This tuning element can be integrated with planar circuits to compensate for the effect of parasitic reactance inherent in various devices including semiconductor diodes and superconductor-insulator-superconductor (SIS) junctions

Lubecke, Victor M.

McGrath, William R.

Rutledge, David B.

English

Caltech Authors - Main

IEEE MICROWAVE AND GUIDED WAVE LE'l-rERS, VOL. 3, NO. 12, DECEMBER 1993 441 
A 100 GHz Coplanar Strip Circuit 
Tuned with a Sliding Planar Backshort 
Victor M. Lubecke, William R. McGrath, and David B. Rutledge 
Abskuct- A means of mechanically altering the electrical 
length of a planar transmission line would greatly enhance the use 
of integrated circuit technology at millimeter and submillimeter 
wavelengths. Such a mechanically adjustable planar RF tuning 
element, successfully demonstrated at 100 GHz, is described here. 
It consists of a thin metallic sheet, with appropriately sized and 
spaced holes, which slides along on top of a dielectric-coated 
coplanar-strip transmission line. Multiple RF reflections caused 
by this structure add constructively, resulting in a movable RF 
short circuit, with lslll~ -0.3 dB, which can be used to vary 
the electrical length of a planar tuning stub. The sliding short 
is used here to produce a 2-dB improvement in the response 
of a diode detector. This tuning element can be integrated with 
planar circuits to compensate for the effect of parasitic reactance 
inherent in various devices including semiconductor diodes and 
superconductor-insulator-superconductor (SIS) junctions. 
I. INTRODUCTION 
HE allure of highly integrated components and systems T makes planar transmission line circuits attractive for use 
at frequencies in the millimeter wave spectrum and beyond. At 
these frequencies, planar transmission line circuits are straight- 
forward to fabricate and can provide seamless integration with 
various planar devices. However, the devices used in these 
circuits often have large parasitic reactances which require 
compensation to be included in the circuit. Consequently, 
at the highest frequencies in state-of-the-art systems, planar 
devices must often be implemented in tunable waveguide 
embedding circuits in order to extract optimum performance 
[l], [2]. The objective of this work is to demonstrate a 
mechanical means of tuning planar circuits, analogous to non- 
contacting waveguide backshorts. A sliding metal pattern, used 
to form a movable RF short circuit on a planar transmission 
line by spatially modulating the impedance, is introduced. This 
sliding, planar backshort can be readily fabricated by a variety 
of techniques, including micromachining techniques [3], which 
are particularly useful for short millimeter wavelength and 
submillimeter wavelength applications where characteristic 
dimensions are very small. 
Manuscript received July 28, 1993; revised September 23, 1993. The 
research described in this paper was jointly sponsored by the Innovative 
Science and Technology Office of the Ballistic Missile Defense Organization, 
and the National Aeronautics and Space Administration (NASA), Office of 
Advanced Concepts and Technology. V. M. Lubecke holds a NASA GSRP 
Fellowship. 
V. M. Lubecke and D. B. Rutledge are with the Division of Engineering 
and Applied Science, Califomia Institute of Technology, Pasadena, CA 91 125. 
W. R. McGrath is with the Center for Space Microelectronics Technology, 
Jet Propulsion Laboratory, Califomia Institute of Technology, Pasadena, CA 
91 109. 
IEEE Log Number 9214369. 
11. DESIGN AND FABRICATION 
The sliding short used in this experiment is based on a 
recently patented design [4] and consists of a movable metal 
sheet, with appropriately sized and spaced holes, placed on 
top of a dielectric-coated coplanar-strip transmission line, 
as shown in Fig. I .  In this position, the metal pattern is 
designed to create a sequence of alternating low- and high- 
impedance quarter-wavelength sections of transmission line, 
resulting in a broadband RF short circuit. Critical dimen- 
sions for the transmission line and sliding short are scaled, 
by frequency, from an empirically designed 2-GHz model 
[51. 
The performance of the sliding short was tested in a 
100-GHz detector circuit based on an existing 230-GHz planar 
heterodyne receiver [6]. This quasi-optical design utilizes a 
dielectric-filled parabola to focus an incident RF signal onto 
a planar dipole antenna. A GaAs Schottky beam-lead diode 
mounted across the antenna is used as the detector. DC bias 
and detector output are carried by the coplanar strips. 
The antenna circuit and the sliding short were fabricated 
by thermal evaporation of thin conducting films onto separate 
quartz wafers, each 250pm thick. A 0.28pm Cr/Au film was 
used for the antenna circuit and a 0.27pm Cr/Ag/Cr film was 
used for the sliding short. A photoresist liftoff stencil was 
used to define the geometry for each of these circuits. The 
quartz wafer served as a dielectric substrate for the antenna 
circuit. For the sliding short, however, the quartz wafer merely 
served as a support structure, providing a convenient means 
for achieving the high degree of flatness required at this 
frequency. 
The antenna circuit consisted of a 936pm-long, 87pm- 
wide metal-film dipole, center fed by two 50pm-wide 
metal-film strips as shown in Fig. l(a). A lOOpm gap 
separated the two strips and split the dipole. A thin layer 
of polyimide was spun onto the circuit and cured to a 
final thickness of 0.5pm. Small windows were reactive 
ion etched from the polyimide, in an oxygen plasma, to 
allow for attachment of the diode to the antenna and dc 
wires to the other end of the transmission line. The sliding 
short consisted of a rectangular metal-film pattern with two 
centered rectangular holes along its length. When positioned 
as shown in Fig. 1, three 486pm sections of transmission 
line are covered by the metal film, forming the lower 
impedance sections, with two sections, 388pm and 460pm 
long, left uncovered by the holes. A dicing saw was used 
to cut the quartz-backed sliding shorts to an appropriate 
size. 
1051-8207/93$03.00 0 1993 IEEE 
442 IEEE MICROWAVE AND GUIDED WAVE LEmERS, VOL. 3, NO. 12, DECEMBER 1993 
Transmission line 
r Hole in polyimide 
Narrow 
sliding Quartz support 
Metal pattern 
I ' short Polylmlda coated substrate 
(a) 
/Quartz support 
Metal pattern 
Polylmide 
Antenna 
Ouartz substrate \ Transmission line 
(b) 
Fig. 1. Top view (a) and cross section (b) of the mechanically tunable planar 
detector circuit. The metal pattem is moved along the transmission line to 
provide an adjustable impedance in parallel with the diode. 
111. EXPERIMENTAL RESULTS 
Measurements were made with the detector circuit mounted 
on the dielectric-filled parabola with the sliding short, posi- 
tioned metallization side down, resting freely on top of the 
polyimide-coated transmission line. The circuit was positioned 
horizontally beneath a flat copper reflector aligned to direct a 
100-GHz signal onto the circuit. An HP 83620A synthesized 
oscillator with an HP 8355A millimeter wave source module 
was used as the signal source. The source power was set to 
0 dBm with an internal 1-KHz square wave modulation, and an 
EG&G 5210 lock-in amplifier was used to monitor the detected 
power. Positioning marks were included in the antenna circuit 
pattern so that the sliding short could be manually aligned at 
various distances, dimension d in Fig. l(a), from the antenna 
and diode with the aid of a microscope. 
The circuit was tested by manually aligning the sliding 
short, with the aid of a sharp wooden probe, at increments of 
A,/16 along the transmission line and measuring the detector 
response at each position. The wavelength (A,) used here was 
that for the coplanar strip transmission line, and the sliding 
short was positioned up to three wavelengths away from 
the antenna and diode. This procedure was repeated using 
two sliding short designs, both having identical dimensions 
along the transmission line, but one with a wider metallization 
pattem across the line. The diode was dc-biased at 200pA, 
and results for both the narrow (0.34-A,) and wide (0.77- 
A,) sliding shorts are shown in Fig. 2, normalized to the 
response measured with no sliding short present (typically 
160pV). 
I I I I I 
I I I I I I 
0 1 2 3 
- 9' 
Sliding short position, d l l g  
(b) 
Fig. 2 Data for two typical trials (*)(x) is shown for both the wide (a) and 
narrow (b) sliding shorts. Theory is shown both without (-), and with (- -) 
the coupling effect included. 
A theoretical model for the circuit was developed. Circuit 
parameters for the beam-lead diode (M/A-COM MA40417) 
were taken from the manufacturer's specifications. A resis- 
tance of 500 was used for the antenna impedance [6].  The 
transmission line impedance (1900) and effective dielectric 
constant (2.4) were calculated using quasi-static closed-form 
expressions [7]. Loss for the line (0.83 dB/A,) was calculated 
as a sum of radiation loss into the substrate (0.15 dB/X,) 
[8], and conductor loss (0.68 dB/X,) 171 calculated using 
the measured dc conductivity (1.5 x lo7 S/m). The sliding 
short was modeled with l s l l l  = -0.3 dB, as measured in 
the 2-GHz experiment [ 5 ] ;  changes due to material variations 
and increased skin effect at this frequency were not included 
because the experiment was not expected to be accurate 
enough to show these effects. The calculated circuit response 
is included in Fig. 2. 
A parasitic coupling effect between the sliding short and 
the antenna was also observed. This was characterized by 
repeating the measurements with the sliding short moved 
incrementally away from the antenna as before, but now 
positioned on the side of the antenna exactly opposite to the 
transmission line. In this way the parasitic coupling between 
the antenna and sliding short could be recreated, isolated 
from the transmission line effect. The measured coupling 
effect, a perturbation of as much as -2 dB, was then added 
to the theoretical response, and is also shown in Fig. 2. 
This compensation improves agreement between theory and 
measurement for positions less than 2 A,. 
IV. DISCUSSION 
In this experiment, a new approach for tuning planar circuits 
was successfully demonstrated at millimeter wavelengths. The 
LUBECKE et al.: SLIDING PLANAR BACKSHORT 443 
detector response was increased by more than 2 dB over the REFERENCES 
untuned response at the peak position and nulled by nearly 
11 dB at the 442 position. Also, the theory was shown to 
adequately predict the circuit performance. 
The circuit used here demonstrates the concept and other 
circuits could be designed to take even better advantage of 
the sliding short. A different device, such as a monolithically 
fabricated diode or SIS tunnel junction, could be used as the 
detector and a second tuning stub could be incorporated to 
provide further improvement in the response. Future versions 
of the sliding short could be fabricated on the transmission 
line itself, captivated by a micromechanical guiding structure 
with an integral means of electromechanical drive[3]. 
ACKNOWLEDGMENT 
[l] P. H. Siegel, R. J. Dengler, I. Mehdi, W. L. Bishop, and T. W. Crowe, 
“A 200 GHz planar diode subharmonically pumped waveguide mixer 
with state-of-the-art performance,” IEEE MlT-S Int. Microwave Symp. 
Dig., June 1992, pp. 595-598. 
[2] W. R. McGrath, P. Febvre, P. Batelaan, H. G. LeDuc, B. Bumble, 
M. A. Frerking, and J. Hemichel, “A submillimeter wave SIS receiver 
for 547 GHz,” Fourth Int. Symp. Space Terahertz Technology Dig. (Los 
Angeles, CA, Mar. 3CApr. I),  1993.. pp. 50-58. 
[3] L-S. Fan, Y-C. Tai, and R. S. Muller, “Integrated movable micromechan- 
ical structures for sensors and actuators,” IEEE Trans. Electron Devices, 
ED-35, pp. 724-730, June 1988. 
[4] W. R. McGrath and V. M. Lubecke, “RF tuning element,” U.S. patent 
no. 5,115,217, granted May 19, 1992. 
[5] V. M. Lubecke, W. R. McGrath, and D. B. Rutledge, “Sliding backshorts 
for planar circuits,” Int. J .  Infrared and Millimeter Waves, vol. 12, pp. 
1387-1397, Dec. 1991. 
[6] P. A. Stimson, R. J. Dengler, H. G. LeDuc, S. R. Cypher, and P. H. 
Siegel, “A planar quasi-optical SIS receiver,” IEEE Trans. Microwave 
Theory and Techniques, MTT-41, pp. 609-615, Apr. 1993. 
[7] K. C. Gupta, R. Garg, and I. J. Bahl, Microstrip Lines and Slotlines. 
We would like to thank Dr. P. A. Stimson for his encour- Dedham, MA: Artech House, 1979. 
[SI M. Y. Frankel, S. Gupta, J. A. Valdmanis, and G. M. Mourou, “Tera- 
lines,” IEEE Trans. Microwave Theory and Techniques, MlT-39, pp. 
agement and assistance in the fabrication Of the he* attenuation and dispersion characte&ics of coplanar transmission 
test circuit and Dr. P. H. Siegel for generously providing the 
dielectric parabola and mount. 910-916, June 1991. 


A 100 GHz coplanar strip circuit tuned with a sliding planar backshort

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main