диафрагмированные волноводные фильтры / 3dad5948-d90e-423c-8402-157a28ef5de8
.pdfDIELECTRIC FILTERS FOR SHORT MILLIMETRE WAVE INTEGRATED CIRCUITS
G.B.Morgan |
TH3.5 |
University of Wales Institute of Science and Technology, |
Cardiff, Wales |
Abstract
Characteristics are presented for narrow.band pass (~3%) filters designed with temperature compensated high permittivity dielectrics (barium nonatitanate). AtW band both duroid and Z cut quartz microstrip substrates have been investigated and the insertkn losses due to the various coupling fields are estimated to be ~ 2dB for 2 resonator filters and ~ 3.5 dB for 3 resonator filters.
Introduction
The concept of temperature compensating dielectric resonant devices is independent of frequency and this paper shows that millimetre wave systems requiring filters could possibly benefit from using high permittivity, temperature compensated dielectric resonators in low order modes, since they are inexpensive, small and
compatible with most of |
the circuit |
technologies1 • |
|
At the short millimetre |
wavelengths |
machined |
|
circuits are expensive so that printed |
circuits |
||
would seem to offer the |
best solutions |
for most |
system requirements. Open microstrip circuits have been used at 600 GHz and are very attractive from the fabrication and cost point of view, so that data are presented for W band dielectric band
pass filters on microstrip with particular |
atten- |
||||||
tion being given to the |
insertion |
loss of the filter. |
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The high |
permittivity |
dielectric |
was barium nona- |
||||
titanate |
provided by Plessey, |
which at |
5.75 |
GHz |
had |
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a permittivity of 39, |
a tan |
8 of < 2x |
10-4 |
and |
a |
temperature coefficient of resonant frequency of 6 ppm;oC.
Microstrip Substrates and Dielectric Spacers
for W Band
AtW band the main microstrip substrates are 5880 duroid, water free fuzed quartz and Z cut crystal quartz which have respective permittivities
of |
2.2, 3.8 |
and 4.4, with corresponding values of |
|||
tan |
8 of ~ |
0.9 mrad, 80 ~rad |
and |
50. ~rad. |
The |
lowest measured values of total |
loss figures 2 for |
||||
50 |
n lines |
are respectively |
0.07, 0.045 |
and ~ |
0.055 dB/mm, but, when only the dielectric filter is considered it is necessary to know the individual values of the conduction loss and the dielectric loss. The ease of circuit fabrication and the effects of temperatures must also be considered, as well as the technique for determining the insertion loss of the filter. Previous experience has shown that the determination of the errors in the value of the filter loss must take into account variations in microstrip losses, microstrip to waveguide transformer loss, and 'bolting up' losses. These last three losses are eliminated in the present work by fabricating the filter in situ.
In comparing substrates, the maximum thickness
of substrate for |
a given impedance and |
frequency |
|
should, strictly speaking, be taken as |
the basis |
||
for comparison. |
But |
as will be seen |
from the |
values of the losses given in Table 1, |
this was |
||
not really necessary |
in the present case, so that |
the |
comparison is at 100 GHz for 50 n lines |
with |
125 |
~ thick substrates propagating the quasi |
TEM |
mode. In the calculation of the conductor loss a correction must bemade for surface roughness and the skin depth of the conductor. The quartz circuits use gold metallizations which are 5 ~ thick and which for good adhesion require a surface finish of ~ 0 .15 ~ CLA and a chrome keying layer of
~0.04~. It is estimated that the finish of the
copper clad 5880 duroid is about 0.75 ~ CLA. Assuming the classical skin depths of chromium, gold and copper, which are respectively 0.26, 0.24
and 0.21 ~, the loss data of Table 1 were calculated.
|
Material |
|
Duroid |
Fused |
Z cut |
|
|
|
|
QuartzQuartz |
|
relative permittivity, |
Er |
2.2 |
3.8 |
4.4 |
|
tan 8 x 10- 4 |
|
9 |
0.8 |
0.5 |
|
Dielectric loss, |
(dB/A) |
.023 |
.002 |
.001 |
|
Smooth |
conductor loss, |
g |
.082 |
.094 |
.104 |
|
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Rough |
|
|
.160 |
.124 |
.135 |
Total |
rough ~ strip loss |
.183 |
.126 |
.136 |
|
|
n |
(dB/l'1Ill) .083 |
.074 |
.101 |
Table 1. Theoretical estimation of conductor and dielectric losses for 50 n microstrip at 100 GHz for various 125 ~ thick substrates.
It maybe seen that the filter should have a minimum amount of conductor in its design. Such designs are given in Figures 1 and 3, where both microstrip and dielectric losses are minimised.
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Two Resonator Filters |
A plan of atwo resonator filter is given in |
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Figure lA. There is RF coupling from the 50 n |
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microstrip |
input line to resonator 1, from resonator |
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1 to resonator 2 and then from resonator 2 to the |
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o~tpu~ |
line. For measurement purposes themicrostrip |
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Clrcult was |
transformed to waveguide WG27 (WR10) at |
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the input and output. |
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Rather than use a substitution method for in- |
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sertion loss, the loss was measured in situ, by |
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taking |
a50 |
n line and removing a section of the |
line, |
without removing any part of the filter |
assembly from the test bench. Thus, after measuring the loss of the 50 n line assembly the section AB
was |
re~oved, |
the resonators |
were put |
in position |
||
(see |
Flgure |
lB) |
and the new |
insertion |
loss |
measured |
thus |
giving |
the |
contribution of the filter |
section ' |
directly3. Two sets of cuboid resonators were used
in this studY;3 one |
set had dimensions of |
0.30 x |
|
|||||
0.60 |
x |
0.60 mm |
and |
had |
an absorption Q of ~ |
660 |
at |
|
82GHz, |
whi.lst3the other had dimensions of |
0.20 x |
|
|||||
0.80 |
x |
0.80 mm |
and had |
an absorption Q of ~ |
420 |
at |
||
78GHz. |
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|
|
|
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|
It was found that the smallness of the gap AB resulted in direct coupling (~-15 dB) between the
input and output microstrip. Figure 2 shows the
CH1917-4/83/0000- TH35 $0.75 C 1983 IEEE
Authorized licensed use limited to: UNIVERSITY OF ROCHESTER. Downloaded on September 22,2020 at 04:42:16 UTC from IEEE Xplore. Restrictions apply.
response of a filter using the 0.8 mm resonators
which had |
an insertion loss |
of ~ 2dB at a centre |
frequency |
of ~ 78.2 GHz.The |
-10 dB bandwidth was |
3.1 GHz (the ripple was due to the sweep oscillator
system) and the signal |
level in |
the band stop was |
|
~-15 |
dB below the value |
at the |
centre frequency. The |
increase of band stop attenuation requires an increase in the distance AB,. and more resonators.
Three Resonator Filters
In the three resonator filter the question of s~pporting the central resonator must be considered,
s~nce, to cut down dielectric losses in the duroid it was decided to remove a section of the 50 n lin~,
and the underlying substrate and ground plane. The
resonators |
were |
thus |
supported ona |
bridge |
of |
Z cut |
||||
quartz |
3 mm |
long, |
1 mm wide |
and 40 |
~ thick*, |
and a |
||||
section through the filter is given in Figure 3. |
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With an air gap of ~ |
1.8 mm, the insertion loss of |
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the |
filter |
was |
~ |
3.5 |
dB and the centre frequency |
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was |
~ |
81.5 GHz |
with a |
-10 dB bandwidth of ~ |
2.6 GHz |
|||||
The skirt levels are much lower than for the two |
||||||||||
resonator design, being ~-35 |
dB on |
the low |
frequ- |
|||||||
ency |
side, |
see |
Figure |
4. |
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|
|
Conclusion
Whilst the performance of these narrow band filters is not as good as metal E plane waveguide filters· they are small and directly compatible with millimetre wave integrated circuits both in technology and size. It would seem that they are comparable to coplanar filters 5
*As the resonator is raised above the microstrip line the coupling will change. In this filter the
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~ "., |
I |
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F~g. |
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lA. |
Plan of 2 Iresonator filters |
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.~ / . |
,1 &l :. |
duroid ' |
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.us |
t.rLp |
a~r |
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T113.5
0.6 mm square resonators were used and the effect of spacer height on the absorption frequency and Q of the TEll mode is given below.
Resonator |
height, ~m |
0 |
40 |
80 |
125 |
Resonant |
frequency, GHz |
82.0 |
81.1 |
79.2 |
78.8 |
Q of absorption dip |
660 |
540 |
500 |
440 |
Acknowledgement
I wish to thank Plessey, Caswell, for the resonators and the use of their facilities.
References
1. MORGAN, 'Temperature compensated high permittivity dielectric resonators for millimetre wave systems', Int. J. of IR and MM Waves,~,Jan
1984.
2.SEASHORE and SINGH, 'mm wave component trade-
offs for tactical systems, M.J., 25, |
41-62, June 82. |
3. MORGAN and DAVIDSON, 'Low ins~tion |
loss, temp- |
erature compensated dielectric filters for microwave
integrated circuits', |
Electron. Lett.19, 545-546,1983. |
4. BORNEMANN, et al, |
'Optimised low-insertion-loss |
millimetre-wave fin line and metal insert filters'. Radio and Electronic Eng., 52,513-521, 1982.
5. |
WILLIAMS and SCHWARZ, |
'Design and |
performance |
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of coplanar waveguide bandpass filters', MTT-31, |
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558-566, |
1983. |
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Quartz spacer |
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50 Q |
Re |
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~strip |
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air |
duroid |
\\\\\\\\ |
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\~p~e~b~se |
\\\\\\\ |
'\ " \' |
Fig. |
3.X |
Section of 3 resonator filter |
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NOT TO SCALE |
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Fig. 4 Response of 3 resonator filter.~ertical
scale 5 dB!cm(div»)
I I I I I
Fig. 2 Response of 2 resonator filter
CH1917-4/83/0000-TH35 $0.75 C 1983 IEEE
Authorized licensed use limited to: UNIVERSITY OF ROCHESTER. Downloaded on September 22,2020 at 04:42:16 UTC from IEEE Xplore. Restrictions apply.