By properly selecting the characteristic impedance and electrical length of the linking line between the coupled lines, phase velocity compensation for microstrip coupled-line phase shifters can be achieved. This technique does not introduce extra element or modification to the coupling in the main coupled lines. It is simple to implement and it has the potential to be used at mm-wave frequencies

Leong, Y. C.

Weinreb, S.

Caltech Authors - Main

TH2B-5 
NOVEL TECHNIQUE OF PHASE VELOCITY EQUALIZATION FOR 
Y.C. Leong' and S. Weinreb2 
'U. of Massachusetts, 619 Lederle GRC, Amherst, MA 01003 
Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109-8099 
MICROSTRIP COUPLED-LINE PHASE SHIFTERS 
2 
this may not be desirable at mm-wave 
frequencies. 
ABSTRACT 
By properly selecting the characteristic 
impedance and electrical length of the linking 
line between the coupled lines, phase velocity 
compensation for microstrip coupled-line 
phase shifters can be achieved. This 
technique does not introduce extra element or 
modification to the coupling in the main 
coupled lines. It is simple to implement and it 
has potential to be used in the mm-wave 
frequencies. 
z, 0L 
mi: 
w1: - 
INTRODUCTION 
Broadband phase and amplitude balance can be 
achieved by using various configurations of 
coupled-line phase shifters [ 11. One interesting 
configuration compares the insertion phases of 
two different pairs of coupled-line structures to 
achieve broadband differential phase shift. These 
coupled-line structures are similar to the one 
illustrated in Figure 1. This type of phase shifter 
can be used to synthesize arbitrary differential 
phase shift that is broadband, compact and totally 
planar. Full W-band, 1 SO-degree differential 
phase shift has been demonstrated with this type 
of structure [2]. However, when the coupled- 
line structures are implemented in microstrip 
form, the match of the coupled lines becomes 
frequency dependent and is a fhction of the odd 
and even mode phase velocities [3]. 
Schiek and Kohler [4] had proposed a solution to 
this problem by using multiple cascaded sections 
of coupled lines in place of the original coupled 
lines. As the number of cascaded sections 
increases, they introduce step discontinuities and 
Figure 1. Layout and model for one of the two 
pairs of coupled lines used in the compensated 
microstrip coupled-line phase shifter. 
In this paper, the influence of the linking 
transmission line connecting the coupled lines 
will be analyzed. This effect has often been 
neglected in the previous works. From the 
analysis, it is shown that by properly selecting the 
characteristic impedance and length of the linking 
transmission line, phase velocity compensation 
can be achieved at a designed frequency. This 
technique is simple to implement and it does not 
introduce extra element or modification to the 
coupling in the main coupled lines. The 
compensated microstrip coupled-line phase 
shifters are broadband, well matched, compact 
and totally planar. They will be suitable to be 
implemented as phase bits in switched-type, 
monolithic mm-wave phase shifters. As 
compared to the more common reflection-type 
phase shifters which rely on DC bias to tune for 
the phase shift in the mm-wave frequencies, the 
coupled-line phase shifters should provide better 
temperature stability on the phase and amplitude 
response. 
0-7803-5687-X/00/$10.00 0 2000 IEEE 
1453 
2000 IEEE M7T-S Digest 
To validate the theory of phase velocity 
compensation, a 4-8 GHz, 90-degree microstrip 
coupled-line phase shifter had been designed and 
fabricated on E ~ 1 0 . 2  substrate using this 
technique. 
THEORY 
Figure 1 shows the configuration for one of the 
two pairs of coupled lines used in the 
compensated, coupled-line phase shifter. Under 
even mode excitation at the designed frequency, 
the linking transmission line (coupled and 
uncoupled) can be represented as an equivalent 
transmission line with characteristic impedance 
equals to 2, of the main coupled lines and an 
electrical length of Beqe: 
where 8c is the electrical length of the coupled 
linking transmission line calculated using the 
average of the odd and even mode phase 
velocities. 
Similarly, under odd mode excitation: 
When the self-coupling of the linking line is 
small, an important property can be derived fiom 
equations ( 1 )  and (2). It can be shown that if the 
characteristic impedance of the linking line is 
higher than the geometric mean of the odd and 
even mode characteristic impedances of the main 
coupled lines, the odd mode equivalent length 
will be longer than the even mode equivalent 
length. This property could be used to equalize 
the difference in length between the even and 
odd mode of the main coupled lines. 
The difference in length (8eqo-8eqe) has been 
analyzed for GaAs substrate with 100 urn of 
wafer thickness. Up to 35 degrees of 
compensating length is achievable for practical 
values of line width and gap spacing. It is also 
showed that the range is a function of both the 
characteristic impedance and coupling of' the 
linking line. Higher characteristic impedancc and 
smaller coupling will give a higher range of 
compensating length. 
DESIGN EQUATIONS 
To exactly compensate for the difference 
between odd and even mode lengths and to 
preserve the transmission phase of the coupled 
lines, the total length of the compensated 
coupled transmission lines has to be equal to the 
target length, Oset: 
'set = 'oo i- 'eqo 
'set = 'oe i- 'eqe 
(3) 
(4) 
For coupled-line phase shifters, Zoe, Z, and 8,
for each of the two pairs of coupled lines are a 
function of differential phase shift, bandwidth and 
phase errors. These values can be synthwized 
using the relations described in [2 ] .  
Equation (3) and (4) can be simplified to: 
By setting 8, as the only variable in the above 
equation, it can easily be solved graphically or 
numerically. Other variables can be as2;igned 
with some practical values based on 1,ayout 
constraints. 
Several 90-degree maximally flat phase slhifters 
are designed at 90 GHz and tabulated in Table 1 .  
The initial design parameters used for the phase 
shifters are described in [2]  and the compensating 
linking lines are designed using equations ('1)-(5). 
1454 
Three different types of substrate with relative 
permittivity of 10.0, 12.4 and 13.0 are used. The 
thickness of the three substrates is assumed to be 
100 um and the thickness of the metal is assumed 
to be 3 um for all cases. LineCalc (by HP) is 
used to calculate the microstrip-coupled 
transmission line parameters at 90 GHz. I 
Free’s expression [3] had been used to calculate 
the match of the compensated coupled lines over 
frequency by assuming that the frequency 
dispersion of the phase velocities to be negligible. 
The frequency response of return loss for all the 
cases listed in table 1 is similar to that of a low- 
pass filter and it starts to degrade after passing 
the designed frequency. The bandwidth 
performance can be characterized by a corner 
frequency at which the mismatch becomes 
significant. A 20 dB return loss had been used as 
criteria for evaluating the comer frequencies of 
the compensated phase shifters. In all designs, 
the corner frequencies were computed by 
normalizing them with the designed frequency at 
90 GHz and they are better than 1.37. 
EXPERIMENTAL VERIFICATION 
A 4-8 GHz, non-maximally flat 90-degree 
microstrip coupled-line phase shifter was 
fabricated using a substrate of relative 
permittivity of 10.2 and of height 1.27 mm. The 
layout was simulated using 3D planar 
electromagnetic simulator to veri@ the design. 
Figure 2 shows the measured return and insertion 
loss of the long and short paths. Figure 3 shows 
the amplitude and phase balance of the phase 
shifter pair. The return loss is better than 18 dB 
for both the long and short paths across 1-8 
GHz. The measured phase balance is 93+6 
degrees across 4.3-8.8 GHz and the amplitude 
balance is better than M.5 dB across the same 
range. 
The good return loss that is achieved across a 
wide bandwidth using a substrate of high 
dielectric constant shows that the compensation 
technique is suitable for use in this application. 
50 
40 
30 
20 
8 10 
Y O  
5 -10 
-20 
-30 
-40 
-50 
1 2 3 4 5 6 7 8 9 1 0 1 1  
Freq ( G W  
Figure 2. Measured return and insertion loss of 
the long and short paths of the C-band 90-degree 
phase shifter. 
105 1 
103 0.5 
101 0 -  
99 -0.5 8 
-1 E 
-2 ; 
- 
97 
5 95 -1.5 
i g 91 - 
93 
-2.5 E 
U 
89 -3 
87 -3.5 
85 -4 
I 2 3 4 5 6 7 8 9 1 0 1 1  
FW ( G W  
I - Phase balance -0- Amplitude balance) 
Figure 3. Measured phase and amplitude balance 
of the long and short paths of the C-band 90- 
degree phase shifter. 
The low phase and amplitude balances of the 
phase shifter indicate that the phase and 
amplitude errors introduced by the compensation 
are- within reasonable 
phase balance of an 
phase shifter is 9033 
narrower bandwidth. 
balance using 3D 
range: The theoretical 
ideal TEM coupled-line 
degrees, with a slightly 
The simulated phase 
planar electromagnetic 
1455 
simulator is 90k5 degrees. This increased in 
phase error and bandwidth is believed to be due 
to the contribution of both the compensating line 
and the microstrip bends. 
CONCLUSION 
The 'effect of the linking transmission line on the 
microstrip coupled-line phase shifters has been 
analyzed. It has been shown that the linking line 
can be used to equalize the phase velocities at a 
designed fiequency and thereby improving the 
match of the coupled-line phase shifters. A 4-8 
GHz, 90-degree coupled-line phase shifter had 
been designed and fabricated using this 
technique. The return loss for both pairs of 
coupled lines is better than 18 dB from 1 to 8 
GHz. The amplitude balance is f0.5 dB the 
phase balance is 93f6 degrees across 4.3-8.8 
GHZ. 
REFERENCES 
JosC Luis Ramos Quirarte and J. Piotr Starski, "Novel 
Schiffman Phase Shifters," IEEE Trans. Microwave 
Theory Tech., vo1.41, no.1, pp. 9-14, Jan. 1993. 
Y.C. Leong and S. Weinreb, "Synthesis of Broadband 
TEM Coupled Line Phase Shifters," submitted 
Charles E. Free and Colin S. Aitchison, "Improved 
Analysis and Design of Coupled-Line ]Phase 
Shifters," IEEE Trans. Microwave Theory Tech., 
vo1.43, no.9,pp. 2126-2131, Sep. 1995. 
Burkhard Schiek and Jurgen KWer, "A Method for 
Broad-Band Matching of Microstrip Differential 
Phase Shifters," IEEE Trans. Microwave Theory 
Tech., vol. 25, n0.8, pp. 666-671, Aug. 1977. 
Table 1. Design parameters for 90-degree maximally-flat coupled-line phase shifters with 
compensation. The designed frequency is centered at 90 GHz. 
1456 


Novel technique of phase velocity equalization for microstrip coupled-line phase shifters

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Novel technique of phase velocity equalization for microstrip coupled-line phase shifters

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