U.S. patent number 3,634,788 [Application Number 04/671,046] was granted by the patent office on 1972-01-11 for waveguide filter.
This patent grant is currently assigned to International Standard Electric Corporation. Invention is credited to George Frederick Craven.
United States Patent |
3,634,788 |
Craven |
January 11, 1972 |
WAVEGUIDE FILTER
Abstract
This invention relates to waveguide filters wherein waveguide
sections functioning in the evanescent mode for frequencies in the
desired passband are utilized to couple conventional cavity filter
sections together. The evanescent sections operate in their normal
modes at frequencies higher than the passband frequencies.
Suppression devices are then coupled to one or more of the
evanescent sections (instead of to the cavity filter sections) to
suppress the parasitic harmonic waves while having negligible
effect on the evanescent mode passband frequencies.
Inventors: |
Craven; George Frederick
(Sawbridgeworth, EN) |
Assignee: |
International Standard Electric
Corporation (New York, NY)
|
Family
ID: |
24692932 |
Appl.
No.: |
04/671,046 |
Filed: |
September 27, 1967 |
Current U.S.
Class: |
333/210; 333/209;
333/81B; 333/211 |
Current CPC
Class: |
H01P
1/219 (20130101) |
Current International
Class: |
H01P
1/219 (20060101); H01P 1/20 (20060101); H03h
013/00 (); H03h 007/10 () |
Field of
Search: |
;333/73,73C,73W,10,7,9 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Baraff; C.
Claims
I claim:
1. A waveguide band-pass filter comprising:
at least one waveguide section which is evanescent for the passband
frequency of the filter, said waveguide section being dimensioned
to propagate parasitic passband frequencies;
at least a first and second resonator cavity filter section coupled
together through said one evanescent waveguide section;
a narrow-band absorption filter coupled to said evanescent
waveguide section for suppressing parasitic frequencies, said
absorption filter is of the constant resistance type and
includes:
at least two stubs;
means including at least two broadband resonant slots for coupling
said at least two stubs to said evanescent waveguide section;
means coupled to one of said stubs for absorbing reflected energy
from other of said stubs coupled to said evanescent waveguide
section;
at least one tunable screw coupled to the other of said at least
two stubs.
2. A waveguide band-pass filter comprising:
at least one waveguide section which is evanescent for the passband
frequency of the filter, said waveguide section being dimensioned
to propagate parasitic passband frequencies;
at least a first and second resonator cavity filter section coupled
together through said one evanescent waveguide section;
a narrow-band absorption filter coupled to said evanescent
waveguide section for suppressing parasitic frequencies, said
absorption filter includes:
a third cavity resonant at the parasitic frequency;
a fourth cavity;
means including a coupling hole in each of said third and fourth
cavities to transfer energy at the parasitic frequency from said
third cavity to said fourth cavity;
a matched load coupled to said fourth cavity for absorbing said
transfer energy; and
a series rejection filter section coupling said third and fourth
cavity to said evanescent waveguide section.
3. A waveguide band-pass filter comprising:
at least one waveguide section which is evanescent for the passband
frequency of the filter, said waveguide section being dimensioned
to propagate parasitic passband frequencies;
at least a first and second resonator cavity filter section coupled
together through said one evanescent waveguide section;
a narrow-band absorption filter coupled to said evanescent
waveguide section for suppressing parasitic frequencies, said
absorption filter includes:
a third cavity resonant at the parasitic frequency;
an obstruction mounted within said third cavity for absorbing
the
parasitic frequency energy without having any significant effect on
the passband energy; and
a series rejection filter coupling said parasitic frequency cavity
to said evanescent waveguide section.
4. A filter according to claim 3 wherein said obstruction includes
an absorption screw.
Description
BACKGROUND OF THE INVENTION
This invention relates to waveguide band-pass filters.
Conventional direct-coupled and quarter-wave-coupled filters each
have advantages and disadvantages. The advantages of the
quarter-wave-coupled filter lies in its small coupling susceptances
and consequently, the ease with which a paper design can be
realized with practical mechanical tolerances. Initially, this type
of filter was preferred because the cavities could be made
separately and tuned before assembly. However, now that filters are
always made and tuned as a complete unit this advantage is of
little value and general, if not complete, preference has passed to
the direct-coupled filter. The direct-coupled filter eliminates the
frequency-sensitive quarter-wave couplings and, therefore, is
applicable to wider bandwidth designs; it is also shorter (about 75
percent of the length of the quarter-wave filter). This is achieved
by substituting one large susceptance for the two smaller
susceptances and the quarter-wave coupling between cavities. This
makes the direct-coupled filter potentially cheaper although this
is offset by the much stricter tolerances, or additional
adjustments, that are necessary. The difficulties arise from the
highly critical nature of large susceptances which, in the example
of a symmetrical diaphragm, for instance, varies as cot.sup.2
(.pi.d.sub.1 /2a)(d.sub.1 0 for large susceptances). Use has also
been made of multipost arrangements but this alternative leads to
manufacturing difficulties and the thin posts also increase
loss.
SUMMARY OF THE INVENTION
Therefore, the main object of this invention is to provide an
improved waveguide filter wherein the higher order parasitic
frequencies are suppressed while having negligible effects on the
desired passband signals.
According to the invention there is provided a waveguide band-pass
filter having main resonator cavities coupled together by waveguide
sections which are evanescent at the passband frequency of the
filter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(a) is the equivalent circuit of two cavities with a
conventional quarter-wave coupling,
FIG. 1(b) is the equivalent circuit of two cavities with a
conventional direct coupling,
FIG. 1(c) is the equivalent circuit of two cavities with an
evanescent mode coupling,
FIG. 1(d) is the voltage transfer equivalent of an evanescent
section,
FIG. 2(a) is the bisected circuit of FIG. 1(a).
FIG. 2(b) is the bisected circuit of FIG. 1(c).
FIG. 3 shows a typical attenuation/length characteristic for an
evanescent mode coupling measured at 4,000 mc./sec.,
FIG. 4 shows the evanescent mode coupling giving the characteristic
of FIG. 3,
FIGS. 5(a) and 5(b) show a symmetrical evanescent mode coupling its
equivalent circuit, respectively,
FIG. 6 shows the reactance at a junction of a waveguide terminated
in the symmetrical evanescent section of FIG. 5(a).
FIG. 7(a) and 7(b) show an asymmetrical evanescent mode coupling
and its equivalent circuit, respectively,
FIG. 8 shows the reactance at a junction of a waveguide terminated
in the symmetrical evanescent section of FIG. 7(a),
FIGS. 9(a), 9(b) and 9(c) are plan, side and end views,
respectively, of a six-section resonant cavity waveguide band-pass
filter with evanescent mode coupling sections,
FIG. 10 is a perspective view, partially cut away, of a modified
form of an evanescent mode coupling section,
FIG. 11 shows the curves insertion loss and V.S.W.R. vs. frequency
for the filter of FIG. 9,
FIG. 12 shows the measured standing wave pattern in a three-section
filter with evanescent mode coupling sections throughout,
FIG. 13 shows the bisected equivalent circuit of a symmetrical
network of imaginary characteristic impedance terminated for full
energy transfer,
FIG. 14 shows the calculated standing wave pattern for the
symmetrical half-section evanescent coupling between cavities of
FIG. 13,
FIGS. 15(a) and 15(b) are plan and sectioned side views,
respectively, of an evanescent coupling section with a harmonic
rejection filter,
FIGS. 16(a) and 16(b) are plan and sectioned side views
respectively of an evanescent coupling section with a constant
resistance rejection filter,
FIG. 17 shows the transmission response of the filter of FIG.
16,
FIG. 18 is a perspective partially cutaway view of an evanescent
coupling section with a resonant slot hybrid junction rejection
filter,
FIG. 19 shows the attenuation characteristic of the filter of FIG.
18,
FIG. 20 is a perspective partially cutaway view of an evanescent
coupling section with a form of constant resistance rejection
filter,
FIG. 21 shows the attenuation characteristic of the filter of FIG.
20,
FIG. 22 shows the equivalent circuit of the filter of FIG. 20,
FIGS. 23(a) and 23(b) are side and end views, respectively, of an
evanescent coupling section with a series resonant rejection
filter,
FIG. 24 shows the attenuation characteristic of the filter of FIG.
23,
FIG. 25 is a perspective partially cutaway view of an evanescent
coupling section with a form absorption filter,
FIG. 26 shows the standing wave pattern in the coupling section of
FIG. 25,
FIG. 27(a) is an end view, and FIG. 27(b) is a half-sectioned side
view along the line A--A of FIG. 27(a), of an evanescent coupling
section with a low-pass rejection filter,
FIG. 28 shows the attenuation characteristic of the filter of FIG.
27,
FIG. 29 shows a typical frequency characteristic of a waveguide
low-pass filter,
FIG. 30 shows the wide-band rejection characteristics of a
waveguide low-pass filter, and
FIG. 31(a) is a plan view, and FIG. 31(b) a section on the line
B--B of FIG. 31(b), of cascaded low-pass filter evanescent coupling
sections with a dissipative section in between.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Waveguide at frequencies below cutoff exhibits characteristics
common to all nondissipative filter networks in their stop-band
region. The characteristic impedance, which is real in the passband
becomes imaginary in the stop-band. The propagation constant, which
is imaginary in the passband becomes real in the stop-band. The
transmission line analogue then has the properties shown in FIG.
1(c), FIGS. 1(a) and 1(b) represent the corresponding circuits for
quarter-wave and direct-coupling, respectively. The conditions for
virtual identity between the two circuits of FIGS. 1(a) and 1(b)
have been established for example, in "An Improved Design Procedure
of the Multisection Generalized Microwave Filter," Levy R., I.E.E.
Monograph Vol. 232R, Apr. 1957, in which it is shown that if the
insertion loss and phase shift of the two networks are the same,
the circuit of FIG. 1(a) may be replaced by that in FIG. 1(b). In a
similar way the network of FIG. 1(c) may be replaced by that in
FIG. 1(b). The input reactance of the network of FIG. 1(b) is
equivalent to a certain line length in the network of FIG. 1(c) and
thus, the cavity length will be generally less when evanescent
couplings are used. The insertion loss of the two networks may then
be made identical by appropriate choice of the parameters in the
evanescent section.
The conditions for equivalence between the two networks of FIGS.
1(b) and 1(c) can be readily derived from the standard transmission
line equations. As the networks are symmetrical they may be
bisected (FIGS. 2(a) and 2(b) nd the open- and short-circuit
admittances derived. If the respective open circuit and
short-circuit admittances can be equated and solved for physically
realizable values then the two networks may be made identical at a
given frequency. The short-circuit admittances for the networks of
FIGS. 2(a) and 2(b) are, respectively.
Y.sub.scl =-jYo cot .theta. (1)
Y.sub.sc2 =-jB.sub.1 -jYoi coth (.nu.1/ 2 ) (2)
The open-circuit admittances for the same networks are:
Y.sub.ocl =-jYo cot (.nu.+.phi. ) (3)
where .phi. = cot.sup. 1 (B.sub.2/ 2Yo )
Y.sub.oc2 =-jB.sub.1 -jYoi tanh (.nu.1/2 ) (4)
Equating the short-circuit admittances.
Yo cot .theta.=B.sub.1 +Yoi coth (.nu.l/ 2 ) (5)
Equating the open-circuit admittances.
Yo cot (.theta.+.phi.)=B.sub.1 +Yoi tanh (.nu.l/2 ) (6)
Expanding the left hand side (6) and substituting for cot .theta.
(from (5)) we have, solving for cot .theta., ##SPC1##
cot .phi.=(B.sub.1 /Yo) or (B.sub.2 /2 =B.sub.1 which is obviously
true. Equation (5) states simply the second requirement for
equivalence i.e., the relationship between the line length in FIG.
2(a) and the combined lumped susceptance including the junction
susceptance and the (inductive) characteristic admittance of the
line.
The notion of energy propagating freely through a nondissipative
line (or guide) of imaginary characteristic impedance, with the
propagation constant real and assuming large values, may conflict
with the intuitive notions held by many. This concept amounts to
the belief that the ratio of input voltage, E.sub.1, to output
voltage, E.sub.2, is given to good accuracy by the equation:
(E.sub.1 /E.sub.2)=e.sup..nu.l
where
.gamma. = propagation constant
= length
However, this is only true if the network is terminated in its
characteristic impedance (imaginary). With other terminations the
results can be very different. For instance, if the termination is
capacitive (FIG. 1d) the ratio E.sub.1 /E.sub.2, is given by
E.sub.1 /E.sub.2 = cosh .nu.l-(Zo.sub.1 /x.sub.1) sinh .gamma.l
(8)
For large values of .gamma.1, cosh .gamma.l and sinh .gamma.l are
nearly equal. Thus, for example, if Zo.sub.1 .apprxeq.x.sub.1,
E.sub.1 /E.sub.2 is quite small and may be zero. The circuit then
behaves as a series resonant circuit with a large resonant rise in
voltage across the "inductance" and capacitance. Clearly, a wide
range of values including E.sub.1 /E.sub.2 =1 is obtainable. A
general engineering conception of the behavior is simply to say
that the reactive insertion loss of evanescent guide may be "tuned
out" by a suitable termination. Such evanescent waveguide sections
are generally described on my copending application, Ser. No.
643,279. It roughly corresponds to neutralizing the effect of a
shunt susceptance by adding one of equal value (but opposite sign)
in shunt with it.
The design of direct-coupled filters is described in "Direct
Coupled Filters," Cohn S. B., Proc. I.R.E., Feb. 1957. The coupling
parameter between resonant cavities is a common shunt susceptance.
However, the design of a filter using evanescent couplings is more
conveniently handled in terms of the insertion loss of the coupling
section. The insertion loss of a susceptance shunted across a
matched line is given by: ##SPC2##
The insertion loss of a section of evanescent guide interposed
between matched guides will be a function of the guide length and
junction susceptance. The guide length will normally be the
principal factor, the insertion loss being: ##SPC3##
and .lambda.c = cutoff wavelength .lambda.o = free space
wavelength.
The junction susceptance is generally quite large but the insertion
loss from this effect is less than might be expected. The reason
for this is that the field from the junction spreads into the
evanescent section causing the initial rate of decay to be less
than e .sup.l, FIG. 3 shows the insertion loss, as a function of
length, of a typical section shown in FIG. 4 of
1.000.times.0.667-inch waveguide 41 coupling 2.000.times.0.667-inch
waveguide 42. Except when the evanescent section is very short (and
ultimately degenerates into a thin iris) the curve is a straight
line. Thus, only two properly chosen measurements are essential in
order to establish a design curve. Comparing the calculated (eq.
(10)) and measured curves it will be seen that they are
approximately 2 db. apart and parallel within about 1 db. over the
range. Therefore, eq. (10) will give a very accurate guide to the
effect that minor variations in .lambda..sub.c will have on the
measured attenuation curve. It is of interest that the difference
between the two curves is roughly one-half the insertion loss of an
iris of the same dimensions as the cutoff guide. This is a useful
rule of thumb correction, if it is desired to use the theoretical
curve for ratios a'/a (a'/a is defined in FIGS. 5 to 8).
The practical design of a filter is carried out in the following
way. The values of the prototype sections are calculated using the
equations in FIG. 2 of Cohn's paper. The susceptance values are
then determined from FIG. 5 of the same paper. The insertion loss
is then calculated from eq. (9) of this specification. A width
dimension, which is well beyond cutoff at the passband frequency is
chosen, and the input reactance of the section is then determined
(FIG. 6 or 8). The "half-length" of the cavity is then found
from:
.theta..sub.2 =(.pi./2)-.phi. (11)
Where .phi.= tan (-1.sub.x /Zo) (see FIG. 6 or 8 for value of
(x/Zo). If the cavity is physically symmetrical about the
electrical center i.e., the input reactance at each end is
identical) the total length will be twice the value found in eq.
(11); otherwise the second electrical length is calculated. If the
total cavity length is unreasonably short as a consequence of the
end loading, a second trial using a smaller guide width for the
evanescent section will be necessary. The length of the section
which will yield the desired insertion loss is then found from a
curve such as that shown in FIG. 3.
Where the precise value of the filter bandwidth is not very
important an approximate design technique using eq. (10) with an
approximate correction for the discontinuity, can be employed. The
desired characteristic maximally flat or Tchebycheff--will be
obtained even if the theoretical attenuation curve is in error.
This follows because the correct ratio between the insertion loss
values will be realized. The overall error in bandwidth can be
gauged roughly by noting that an error of 0.5 db. in all of the
intercavity coupling sections will produce an error of about 5
percent in the bandwidth of a typical filter.
So far only the intercavity couplings have been discussed as it is
in these sections that evanescent couplings have their chief
application. Susceptances at the end of the filter are usually of a
comparatively low value and may, conveniently, be of the
conventional type. However, where it is desirable to preserve the
same type of construction throughout the filter it is possible to
use evanescent sections at the end.
FIGS. 9(a), 9(b) and 9(c) show a six-section maximally flat
direct-coupled filter with a 3 db. bandwidth of 37 mc./sec. The 3
db. limit frequencies are 3,983 mc./sec. and 4,020 mc./sec. the
construction being in 2=2/3 in guide. There are evanescent mode
coupling sections 1 between the resonant cavities 2. Each cavity 2
has a 2-BA coarse-tuning screw 3 and an 8-BA fine-tuning screw 4,
and each evanescent section 1 has A 0-BA coupling adjustment screws
5. Inductive arises 6 at the end of the filter are
conventional.
The physical dimensions of the filter are given below:
A--16.716 in., B--0.030 in., C--1.728 in., D--1.146 in., E--1.683
in., F--1.394 in., G--1.683 in., H--1.448 in., I--0.500 in.,
J--0.854 in.
The first step in designing the filter was to calculate the values
of the prototype sections using Cohn's paper. For a maximally flat
filter, using the same notation as Cohn, the values are:
g.sub.1 =g.sub.6 =0.518; g.sub.2 =g.sub.5 =1.414;
g.sub.3 =g.sub.4 =1.93
The susceptance connecting the i.sup.th and i+1.sup.th cavities are
given by: ##SPC4##
The guide wavelength terms .lambda..sub.gl and .lambda..sub.g2 are
the values occurring at the 3 db. band limits.
Taking .lambda..sub.g1 =11.207 cm. and .lambda..sub.g2 =10.981 cm.
L=0.0321.
Substituting for L and g in (12) the values of B.sub.i,
.sub.i.sup.+1 are:
B.sub.12 =26.6; b.sub.23 =51.6; B.sub.34 =60.2.
The value of the outer susceptances B.sub.01 and B.sub.60 are
obtained by substituting g.sub.o =L in (12) which gives
B.sub.01 =3.77=B.sub.60.
The insertion loss of B.sub.12, B.sub.13 and B.sub.34 are from
(9):
L.sub.12 =22.49 db=L.sub.56
l.sub.23 =28.23 db=L.sub.45
l.sub.34 =29.57 db.
The dimensions of the evanescent guide section may now be
calculated. Choosing a symmetrical junction with guide widths of 1
in. (evanescent section) and 2 in. (propagating section)
respectively, the value XZ.sub.o /2a.lambda..sub.g may be obtained
from FIG. 6. For a'/a=0.5;.lambda..sub.g =11.1 cm. X(Z.sub.o
normalized) is given by X=0.31. From (11) the electrical
"half-length" of the cavity is:
.theta./2=72.8.degree.
or a total length, for identical junctions at each end of the
cavity, of 145.6.degree.. This gives a value of
l.sub.c = (145.6.times.11.1/360)=4.49 cm. (or 1.77 in.)
Reducing this value by 5 percent to allow for tuning screws we
have:
l'.sub.c =1.682 in.
The details of the inductive irises at the end of the filter remain
to be settled. These are conventional symmetrical irises the
dimensions of which may be obtained from any standard text.
The performance of the filter is shown in FIG. 11.
The field distribution in evanescent modes conveying full power
through the section is of interest. FIG. 12 shows the measured
field distribution in a typical three-section filter using
evanescent couplings throughout. The filter was constructed with a
slotted line section and measurements made with a probe (with very
small insertion) in the usual way. When measuring the field in the
various sections it was necessary to close up the slot in order to
prevent radiation from the asymmetrical end sections. This was
achieved by fitting a number of tongued sections (similar to the
probe section) into the slot. These were linked to each other and
attached to the probe so that they travelled along the guide with
it.
FIG. 14 gives the calculated field along a transmission line of
imaginary characteristic impedance which is terminated for full
energy transfer (FIG. 13). The network is symmetrical and it is,
therefore, sufficient to bisect the network and calculate E.sub.1
/E.sub.2 as a function of l (equation (8)). The values of .gamma.
and x.sub.1 (x.sub.1 is the equivalent reactance presented by the
cavity at the line terminals) are chosen for a typical example. It
will be seen that a standing wave exists on the line reaching zero
in the dissipationless case at the center and comparatively small
values if the loaded Q is high. The general character of results is
the same at that shown in the experimental results of 12.
It is of interest that the magnitude of the electric field behaves
in much the same way as if an iris of very high susceptance were
located at the center of the intercavity coupling section. The end
sections behave differently owing to their asymmetrical nature. The
semiresonant rise in voltage predicted by eq. (8) can be clearly
seen in FIG. 12.
The essential difference between this type of band-pass filter and
conventional types lies in two features: the physical length of the
coupling sections and the nature of the field existing within the
sections. The insertion loss of an evanescent section depends both
on its width and length and, therefore, the length may vary over a
wide range of values. This means that the filter will be somewhat
longer than the conventional direct-coupled type although, in
general, it need not be longer than the quarter-wave coupled
filter. This is a disadvantage where the shortest possible filter
is desired. However, where a modest increase in length can be
tolerated the filter has a number of advantages.
a. The freedom of choice in the length of the coupling sections
means that the overall length of a given type of filter can be
maintained irrespective of its center frequency. This follows,
because of the longer wavelengths the greater cavity length is
obtained by reducing the length (and if necessary, the width) of
the evanescent section. Thus, only one fixed body size, with
appropriate drillings, needs to be retained for a complete
waveguide band. This simplified storage problems in production. The
inserts, which form the evanescent sections, and are considerably
more robust than conventional irises, are then the only
variables.
b. The mechanical tolerances for large values of B, by whatever
method that are obtained, are very severe. For example, if the
coupling sections in the center of the filter of FIG. 9 are
replaced by symmetrical inductive irises of the appropriate value,
the tolerance on the width dimension (i.e., the gap between the
irises) is only one-third of that which may be permitted with the
corresponding evanescent guide dimension. Other types of obstacles,
such as holes and multipost arrangements require even stricter
tolerances. Therefore, filters in which the bandwidth is
significantly narrower than about 1 percent (the above example)
will require considerably less severe tolerances if evanescent
coupling sections are employed.
c. The style of construction considered so far can be produced by
inserting milled blocks in waveguide in order to produce the
evanescent sections. Another method is to spray metal on a suitable
mandrel. The filter can be produced in two halves by milling from
solid material. This method of construction can be expected to
yield very high precision and has the advantage of eliminating
soldered joints.
d. Variable bandwidth filters can be produced by using
variable-cutoff evanescent coupling sections such as that shown in
FIG. 10, which incorporates a movable block 7 locked in any desired
position by a screw 8 and additional to fixed blocks 9 and 10.
e. The most important feature of this filter is its potential for
eliminating the parasitic passbands that are characteristic of all
band-pass filters at microwave frequencies. Normally, suppression
of these passbands requires an additional filter which then affects
the midband insertion loss and reflection. However, with evanescent
coupling sections this disadvantage is avoidable. One or more of
the coupling sections can be so designed that the frequency it is
desired to suppress propagates in the coupling section instead of
being an evanescent wave, like the desired passband. It is then
possible to insert parasitic suppressors in the section concerned
which will have a very large effect on a progressive wave, but none
on an evanescent wave.
Conventional resonant-cavity waveguide band-pass filters suffer
from unwanted transmission bands above the desired passband. These
passbands occur for two reasons. Firstly, in direct-coupled
resonant-cavity filters resonance occurs at frequencies for which
the electrical length of the cavity is n.lambda.g/2 (where n is an
integer and .lambda.g is guide wavelength). In addition to these
multiple resonances, which are approximately in harmonic
relationship, further resonances can be expected as a result of
higher order modes propagating. In general, individual cavities
tend to resonate at slightly different frequencies and interaction
between these resonances occurs. As a result, multiple narrow
passbands occur at unpredictable frequencies and, therefore, there
is no way of knowing whether a filter will provide satisfactory
harmonic rejection, for example, before it is built. If, in
addition, a specific filter design is to be tuned over a wide range
of frequencies the danger of harmonics penetrating the filter
becomes great. In these circumstances, where suppression of
high-power harmonics is essential, it is necessary to cascade a
second filter giving broad band suppression. This may take the form
of a corrugated (or waffle-iron) low-pass filter or a leaky wall
filter. The former is based on the classical lumped circuit
analogue and is a reflection filter; the second permits the
harmonics to leak through the wall of the filter to auxiliary
guides where they are dissipated. The difficulty with both of these
filters is that they affect the passband performance adversely.
Both filters add unavoidable dissipation and reflection that cannot
be matched out (because the phase of the reflection varies rapidly
with frequency).
The behavior of filters using evanescent mode couplings is
completely different from conventional iris-type filters at
frequencies will above type filters at frequencies well above the
main transmission band. Above a critical frequency the coupling
section propagates and the resonant cavities formed by the
evanescent coupling sections virtually disappear. Instead, the
filter behaves, practically, as a straight through device of nearly
zero attenuation and therefore, only has minor internal
reflections. In this form it is worse than the conventional filter,
but the elimination of large internal reflections in the filter is
a prerequisite to providing predictable suppression of unwanted
frequencies. The second necessary condition is that the coupling
waveguide should behave differently for the passband and unwanted
frequencies, respectively. In this way devices which affect the
unwanted frequency will have little or no effect on the desired
frequency. The behavior of evanescent couplings, as described
above, suits them to this application.
In general, the suppression of unwanted frequencies presents itself
in two possible forms. One occurs when the filter functions in a
closed circuit e.g., it provides filtering in the output of an
oscillator. The unwanted frequencies, virtually entirely harmonics,
are known. If the device is a high-power oscillator a very large
amount of suppression may be necessary, but it will be required at
specific frequencies i.e., narrow-band rejection circuits of very
high rejection will be required. The second example occurs when the
filter is located in an open-circuit system i.e., a system
connected, for example, to a wide-band aerial. Signals over a wide
band of frequencies will be likely to be received and, thus,
wide-band suppression is necessary. In general, the degree of
suppression required will be required to be less than the first
example.
FIGS. 15(a) and 15(b) shows an evanescent coupling section 11
between resonant cavities 12 incorporating a narrow-band harmonic
rejection filter. At the desired transmission band the coupling
section is designed to be evanescent but the dimensions are chosen
so that propagation occurs at the frequency to be suppressed. A
three-section rejection filter is shown, the stubs 13 containing
semiconductor tuning pistons 14 being conveniently connected to the
guide by the broadband resonant slots 15. Approximately 35 db.
rejection per section can be obtained in a typical example over a
relatively narrow band. Thus, a three-section filter built in the
coupling sections as shown gives rejection, at a given frequency,
in excess of 100 db. If it is desirable to absorb the undesired
harmonic, rather than reflect it back to the source, the filter can
be converted into a constant resistance filter (British Pat. No.
1,018,923 G. F. Craven 7) and the energy absorbed into the matched
load 16, FIGS. 16(a) and 16(b) in which like references have been
used as for FIG. 15. The rejection of one section is largely lost
but the advantage of being able to absorb the energy is often
significant. The performance of a filter using one of the sections
of FIG. 16 is shown in FIG. 17. The midband insertion loss of this
section (0.4 db.) compared with a section in which no rejection
sections were included was identical within the limits of
measurement. The reason for the absence of additional loss is that
the guide section incorporating the load is beyond cutoff at the
main transmission frequency and can easily be made sufficiently
long to prevent measurable loss. In this respect, this form of the
rejection filter has something in common with leaky wall filters
which dissipate the energy by coupling to a large number of guides
(which are beyond cutoff at the main transmission frequency).
However, this filter avoids the main disadvantage of the leaky wall
filter: the main transmission frequency propagates in the dominant
mode and therefore, harmonics propagate in several modes. Multiple
coupling to all possible modes is then necessary and this produces
measurable insertion loss at the main frequency. In many
applications the present filter is likely to be superior to the
leaky wall filter, because the guide size of the coupling section
may be chosen so that the harmonic to be suppressed propagates in
the dominant mode only (the main frequency propagating in an
evanescent mode). Thus, the present filter will be much simpler,
have less insertion loss and combine the functions of band-pass
filter and harmonics suppression filter.
Since the filter of FIG. 15 is essentially a reflection filter,
only one suppression filter of this type is permissable per
complete band-pass filter. This is because, if additional
evanescent coupling sections are converted into suppression
filters, mutual cancellation can be expected with further parasitic
passbands resulting. Thus, the amount of suppression that can be
obtained is limited to the number of suppression sections that can
be contained in one coupling section.
However, this limitation does not apply if the suppression filters
are dissipative. Ordinarily, dissipation in the main coupling arm
is not acceptable, but if the suppression filter is designed as a
constant resistance type a considerable resistive attenuation is
obtainable at the parasitic frequency. At the desired passband
frequency the coupling guide is evanescent and therefore no
dissipation can occur.
Another form of narrow-band suppression filter is shown in FIG. 18.
Two resonant cavities 17 are coupled by an evanescent coupling
section 18. Resonant coupling slots 19 couple the evanescent
section 18 to a length of circular waveguide 20 terminated in a
matched load 21 to absorb the energy. This type of suppression
filter based on resonant slot hybrid junctions can give isolation
of 20-40 db. over a band which is broader than the filter of FIG.
16. The attenuation characteristic of the filter of FIG. 18 is
shown in FIG. 19.
A further form of absorption narrow band filter is shown in FIG.
20. Here, two resonant cavities 22 are coupled by an evanescent
coupling section 23 in which is a cavity 24 resonant at the
propagated parasitic frequency to transfer the power at the
parasitic frequency via a coupling hole 25 into a second cavity 26
where it is absorbed by a matched load 27. A series rejection
filter 28 provides isolation, in the form of a short circuit across
the guide, from the remainder of the circuits in the complete
filter. At the passband frequency neither of the cavities 24, 26
can resonate because the guide is in the evanescent condition.
The attenuation characteristic and equivalent circuit of the filter
of FIG. 20 are shown in FIGS. 21 and 22 respectively.
A point to be noted in a filter incorporating this type of
parasitic suppressor is that, if power at the spurious response is
to be absorbed and not merely reflected, the filter is no longer a
symmetrical device. Thus input and output ports have to be
specified.
A further type of narrow-band rejection filter of the series
resonance type is shown in FIGS. 23(a) and 23(b). Resonant cavities
28 are coupled by an evanescent coupling action 29 which contains a
central short-circuiting screw 30 across the guide. Since this is a
reflection filter, there can be only one per complete filter for
the reasons already given.
The attenuation characteristic of the filter of FIG. 23 is shown in
FIG. 24.
A further form of narrow-band absorption filter is shown in FIG. 25
which exploits the difference existing between the propagated mode
of the parasitic frequency and the evanescent mode of the passband
frequency. Two resonant cavities 31 are coupled by an evanescent
coupling section 32 which contains a cavity 33 resonant at the
parasitic frequency. There is a cavity tuning screw 34 and an iron
dust slug 35. A series rejection filter 36 provides isolation, in
the form a short circuit across the guide, from the remainder of
the circuits in the filter.
As shown in FIG. 26, the amplitude of the electric field in the
evanescent mode is at a minimum at the center of the coupling
section. However, the amplitude of the electric field in the
propagating mode (the harmonic) is at a maximum in the resonant
cavity 33 built in the coupling section. The relative amplitudes
are in the approximate ratios of Q.sub.1, Q.sub.2 and Q.sub.1 and
Q.sub.2 are the loaded Q's of the main filter cavities 31. Thus an
absorption screw (the slug 35) in the cavity 33 absorbs the
harmonic energy without having any significant effect on the main
transmission frequency.
Wide-band rejection filters are based on lumped circuit low-pass
networks and take the form of corrugated waveguide filters, FIGS.
27(a) and 27(b) which show two resonant cavities 37 coupled by an
evanescent coupling section 38 incorporating a corrugated low-pass
filter 39. A later and more complex version is the waffle-iron
filter. Such networks are characterized by rejection over a very
wide band FIG. 29 and are employed in the following way. The
low-frequency cutoff of the guide containing the suppression
section is set at a much higher frequency than the other coupling
sections in the filter. If this LF cutoff is suitably chosen this
section will propagate freely in the band in which the other
resonant cavities are providing high rejection, FIG. 30. No serious
harm will result from this in a multisection filter (more than 3
cavities) for only the rejection of one cavity will be lost.
However, at higher frequencies where the other coupling sections
are propagating freely the suppression section (if its HF cutoff is
correctly chosen) will be functioning as a low-pass filter and
providing considerable rejection. In this way the usual spurious
passbands can be suppressed over a wide band. A filter has been
constructed confirming the effectiveness of this type of
suppression section. As a general principle it is preferable to use
only one suppression section per filter because of the danger of
two sections interacting and causing a new parasitic passband.
However, where more than one suppression section is necessary in
order to provide the desired rejection, two suppression sections
can be included as shown in FIG. 31(a) and 31(b) in which
reflective interaction between the two suppression sections 38(a)
and 38(b) is prevented by a matched load 40 terminating a stub 41
between the suppression sections absorbing the reflected energy
from either filter.
It is to be understood that the foregoing description of specific
examples of this invention is made by way of example only and is
not to be considered as a limitation on its scope.
* * * * *