The authors have constructed and tested an oscillator using a grid amplifier with external feedback from a twist reflector. The twist reflector serves two functions; it changes the output polarization to match the input, and its position sets the feedback phase. This permits a wider tuning range than has been possible with previous grid oscillators. The source could be continuously tuned from 8.2 GHz to 11.0 GHz by moving the twist reflector. By moving the polarizer and mirror in the twist reflector independently, a 1.8-to-1 frequency range from 6.5 GHz to 11.5 GHz was achieved. The peak effective radiated power was 6.3 W at 9.9 GHz

De Lisio, Michael P.

Hacker, Jonathan B.

Kim, Moonil

Rosenberg, James J.

Rutledge, David B.

Sovero, Emilio A.

English

Caltech Authors - Main

1772 
a 
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 41, NO. 10, OCTOBER 1993 
dielectric Physical Electrical length at 
constant ( e ? )  length (mm) lOGHz, degrees 
1 3 36 
A 6.5 GHz-11.5 GHz Source Using a Grid 
Amplifier with a Twist Reflector 
b 
C 
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Moonil Kim, Member, IEEE, Emilio A. Sovero, Jonathan B. Hacker, Student Member, IEEE, Michael 
P. De Lisio, Member, IEEE, James J. Rosenberg, and David B. Rutledge, Fellow, IEEE, 
1 15 180 
1 10 120 
10.5 3.81 148 
Abstract- We have constructed and tested an oscillator using 
a grid amplifier with external feedback from a twist reflector. 
The twist reflector serves two functions-it changes the output 
polarization to match the input, and its position sets the feedback 
phase. This permits a wider tuning range than has been possible 
with previous grid oscillators. The source could be continuously 
tuned from 8.2 GHz to 11.0 GHz by moving the twist reflector. 
By moving the polarizer and mirror in the twist reflector inde- 
pendently, a 1.8-to-1 frequency range from 6.5 GHz to 11.5 GHz 
was achieved. The peak effective radiated power was 6.3 W at 
9.9 GHz. 
I. INTRODUCTION 
N the grid oscillators that have been reported [1]-[3], much I of the feedback is from the metal structure that contains the 
transistors. This makes it difficult to vary the feedback phase, 
and has limited the tuning range to 13% [l]. On the other 
hand, in grid amplifiers [4], the input and output are isolated 
by being cross-polarized. In addition, a 100-element HBT grid 
amplifier has shown a 27% gain bandwidth [5]. This suggests 
that we could make an oscillator with a wider tuning range by 
providing a grid amplifier with external feedback, and varying 
the phase. Fig. 1 shows the approach. Power from the output 
of the grid amplifier is fed back to the input by the twist 
reflector to sustain the oscillation. The output beam radiates 
from the other side. 
11. FEEDBACK 
Obtaining sustained oscillations from a grid ampli- 
fier-which is open-loop stable-requires the introduction 
of appropriate feedback from output to input. The feedback 
circuit must serve two functions. First, it must convert 
some fraction of the vertically-polarized output beam into a 
horizontally-polarized input beam. Second, it should satisfy 
Manuscript received December 1, 1992, revised May 5, 1993. 
This work was supported by the Army Research Office, the Northrop 
Corporation, the Naval Weapons Center in China Lake, CA, and a fellowship 
from the Rockwell International Trust. 
M. Kim was with the Department of Electrical Engineering, Califomia 
Institute of Technology, Pasadena, CA 91125. He is now with the Central Re- 
search Laboratory, Gold Star, 16 Woomyen-Dong, Seocho-Gu, Seoul 137-40, 
Korea. 
E. A. Sovero is with the Science Center, Rockwell International Corpora- 
tion, 1049 Camino Dos Rios, Thousand Oaks, CA 91385. 
J. J. Rosenberg is with the Engineering Department, Harvey Mudd College, 
Claremont, CA 91711. 
J. B. Hacker, M. P. De Lido, and D. B. Rutledge are with the Department 
of Electrical Engineering, California Institute of Technology, Pasadena, CA 
91125. 
IEEE Log Number 9211933. 
i E  
output beam 
I n I 2.2 I 3.18 I 57 I 
Fig. 1. The extemal feedback oscillator. The twist reflector that provides 
the feedback replaces the input polarizer in a normal grid amplifier [5]. The 
distance between the amplifier surface and the twist reflector, d,  was varied 
to change the oscillation frequency. 
the oscillation criteria. That is, the feedback signal must have 
zero phase and should be large enough to sustain oscillations. 
A twist reflector meets these requirements. A twist reflector 
reflects a wave in a different polarization [6], [7]. The feedback 
phase can be set to any value by adjusting the spacing between 
the grid amplifier and the twist reflector. 
The twist reflector consists of a metal-strip polarizer and 
a mirror separated by a quarter wavelength. The metal strips 
of the polarizer are inclined at a 45" angle to the electric 
field of a normally incident plane wave. We can resolve the 
incident field into a component that is parallel to the strips, and 
an equal component perpendicular to the strips. The parallel 
component is reflected at the strip plane, and the perpendicular 
component is reflected at the mirror plane, with an extra 
half-wavelength path delay. The resultant reflected beam is 
polarized orthogonal to the incoming beam. The effect is 
similar to a transmission half-wave plate in optical systems [SI. 
The twist reflector can be modelled for any incident polar- 
ization. An ideal polarizer can be described in terms of a four- 
port scattering matrix. The ports are 1) vertical polarization 
front side, 2) vertical back side, 3) horizontal front, and 4) 
0018-9480/93$03.00 0 1993 IEEE 
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KIM et al.: 6.5 GHz-11.5 GHz SOURCE USING GRID AMPLIFIER 1773 
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Phase 
Loss 
...... 
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0 
-2 
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6 7 8 9 10 1 1  12 
Frequency, GHz 
( b) 
Fig. 2. (a) Representing the twist reflector by a transmission-line equivalent 
circuit. The polarizer scattering matrix is given in (1) The 254-0 transmission 
line represents the Duroid substrate, and the 377-0 transmission line is for 
the air gap between the substrate and the mirror. (b) Conversion loss and 
phase for the twist reflector. 
horizontal back. 
S =  
- cos2 0 sin2 o - sin o cos o - sin o cos o 
sin2 o - cos2 O - sin O cos O - sin O cos O 
- sin o cos o - sin o cos o - sin2 o cos2 0 
- sin O cos O - sin O cos O cos2 O - sin2 o 
(1) 
( 
where 0 is the angle between the strips and the incident 
polarization. The strip polarizer is on a 3.1%" thick Duroid 
substrate ( E .  = 2.2). An air gap of 3 mm separated the polar- 
izer and the mirror. Fig. 2(a) shows a transmission-line model, 
and Fig. 2( b) shows the predicted polarization conversion loss. 
It is less than 1 dB over a 1.6-to-1 frequency range. 
111. TUNING 
The oscillator tuned smoothly between 8.2 GHz and 
11.0 GHz by changing the spacing of the grid amplifier 
and the twist reflector (Fig. 3(a)). This is close to the gain 
bandwidth of the amplifier grid, 8.5 GHz to 11.2 GHz [5]. 
The frequency shows a half-wavelength period from higher- 
t 
10 20 30 40 50 60 
Air gap spacing, mm 
10 L 
10 
 
20 30 40 50 60 
Air gap spacing, mm 
( b) 
Fig. 3. Tuning the oscillator by changing the distance between the grid 
amplifier and the twist reflector (dimension d in Fig. l(a). (a) Frequency. 
(b) ERP. 
order longitudinal modes. The effective radiated power (ERP) 
varied from -5 dBW to 8 dBW (Fig. 3(b)). The ERP also 
shows a half-wavelength period. The maximum ERP was 
6.3 W at 9.9 GHz. We did not measure the radiation pattern, 
so we do not know the total radiated power. However, we 
can estimate it by assuming that the directivity is given by 
D = 47rA/X2, where A is the grid area, 64 cm2. With this 
assumption, the directivity is 19.4 dB, and the total radiated 
power is 72 mW. The grid was biased at 3.5 V and 530 mA, 
so that the efficiency would be 3.9%. 
Fig. 4 shows the effect of changing the angle of the polarizer 
between 25' and 75'. The frequency changes by less than 3%, 
but the power changes over a 13 dB range. This makes sense, 
because changing the polarizer angle changes the strength 
of the feedback much more than the phase. We also tried 
different spacings between the polarizer and the mirror. This 
1774 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 41, NO. 10, OCTOBER 1993 
10.2 + I I I I (  8 1 ,  I l l 5 8  I 8  ( I I I 8 I  I (  I I " ' I I ' " I " I I 1 " "  
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REFERENCES 
[ l ]  Z.B. PopoviC, R.M. Weikle, 11, M. Kim, and D.B. Rutledge, "A 100- 
MESFET planar grid oscillator," IEEE Trans. Microwave Theory Tech., 
vol. 39, pp. 193-200, 1991. 
[2] R.M. Weikle, 11, M. Kim, J.B. Hacker, M.P. De Lisio, and D.B. 
Rutledge, "Planar MESFET grid oscillators using gate feedback," IEEE 
Trans. Microwave Theory Tech., vol. 40, pp. 1997-2003, 1992. 
[3] M. Kim, E.A. Sovero, R. M. Weikle, 11, J.B. Hacker, M.P. De Lisio, 
and D. B. Rutledge, "A 35-GHz monolithic HBT grid oscillator," in Dig. 
(a) pp. 1423-1426, 1992. 
[8] E. Hecht, Optics, 2nd. ed. Reading, M A  Addison-Wesley, 1988, 
[9] T. B. Mader, S. Bundy, and Z. B. PopoviC, "Quasi-optical array VCO's," 
pp. 301-303. 
this issue. 
C 
Q 
3 
U 
9.8 I 
9.6 " ~ ~ ~ ~ ' ~ " ~ ~ ' ~ " ~ ~ ~ ~ ~ ~ " ~ " ~ ~ ~ " ' " ~ ' ~ ' ~ ~ " '  
.. ...... .. 
17th Int. Con$ on Infrared and Millimeter Waves, R. 5. Temkin, Ed., Proc. 
SPIE 1929, pp. 282-283, 1992. 
[4] M. Kim, J. J. Rosenberg, R.P. Smith, R.M. Weikle, 11, J.B. Hacker, 
M. P. De Lisio, and D. B. Rutledge, "A grid amplifier," IEEE Microwave 
Guided Wave Len., vol. 1, pp. 322-324, 1991. 
[5] M. Kim, E. A. Sovero, J. B. Hacker, M. P. De Lisio, J. C. Chiao, S. J. Li, 
D. R. Gagnon, J. J. Rosenberg, and D. B. Rutledge, "A 100-element HBT 
grid amplifier," this issue. 
[6] Samuel Silver, Ed., Microwave Antenna Theory and Design, vol. 12, MIT 
Radiation Laboratory Series, New York: McGraw-Hill, 1949; Repub- 
lished in 1984, London: Peter Peregrinus Ltd., p. 447. 
5 -  
3 
m 
a- 
u -  
0 -  
w 
w -  
-5 
-10 
Emilio A. Sovero, for a photograph and biography, see this issue, p. 1771. 
Moonil Kim, (S'91-M'93) for a photograph and biography, see this issue, 
p. 1770. 
. 
- 
~ 1 1 " " " 1 ' " " ' ' ' 1 " ~ ' ' 1  * ~ c " , L ~ ' l ~ ' ' " ' L L '  
Jonathan B. Hacker (S'84) for a photograph and biography, see this issue, 
p. 1771. 
( b) 
Fig. 4. Varying the polarizer angle. (a) Frequency. (b) Power. The distance 
between the amplifier grid and the twist reflector was 24 mm. 
changes the optimum frequency for the twist reflector. We saw 
oscillation frequencies Over the entire range from 6*5 GHz to Michael p. DeLisio (S'90-M'92) for a photograph and biography, see this 
11.5 GHz. issue, p. 1771. 
IV. CONCLUSION 
We have demonstrated a new oscillator based on a grid 
amplifier with external feedback from a twist reflector. This 
source is tunable from 6.5 GHz to 11.5 GHz, with a peak 
ERP of 6.3 W. The wide tuning range results from the broad 
gain bandwidth of the amplifier and the large frequency range 
of the twist reflector. In this study, the frequency was tuned 
by adjusting the twist reflector mechanically, but it would be 
desirable to have a means of electronic tuning for frequency 
modulation. Mader et al. have used a varactor diode grid for 
tuning an oscillator grid [9]. This Suggests that a diode grid 
be incorporated into the twist reflector for electronic tuning. 
James J. RosenbeQ, for a photograph and biography, see this issue, p. 1771. 
David B. Rutledge (M'75-SM789-F'93) for a photograph and biography, 
see this issue, p. 1662. 
r m  


A 6.5 GHz-11.5 GHz source using a grid amplifier with a twist reflector

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