Helical filter with a removable tap housing

A high frequency filter kit in which resonating first and second electrical circuits are enclosed between proximal and distal ends of a filter case. Partitioning the inside of the enclosed resonant circuits may be performed by a user to form at least a first cavity and a second cavity. The first resonating circuit is then disposed inside the first cavity of the filter case extending from the proximal end towards the distal end, and the second resonating circuit is disposed inside the second cavity also extending from the proximal end towards the distal end. Electrical signals are coupled into the resonating circuits by an encased signal coupler which is removably mounted by a coupling housing for supporting the signal coupler at the proximal end of the filter case for positioning in the vicinity of the resonating circuits. The kit thus facilitates enhanced turnout time and communication of design specifications for manufacture by specifying the basic components required to build the specific high frequency filter, allowing the user to build prototype filters that may be used for manufacturing a RF/microwave system or be provided as a sample to the filter manufacturers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to RF and microwave filters, and more 
particularly to simplifying the filter design and prototype processes. 
2. Description of the Related Art 
Presently, RF and microwave filters (RFMF) are used extensively in most 
communication devices, radar and RF/microwave systems. They are used to 
create the desired RF or microwave output signal--free of unwanted 
spurious signals and with the proper output characteristics. RF/microwave 
telecommunication equipment manufacturers use millions of these filters 
per year. These filters are used in cellular basestations, satellite 
communication systems and microwave communication links to name a few 
typical applications. RFMF components are either made internally by the 
equipment manufacturer or procured externally. Most of the time these 
filters are procured because the required filter specifications are often 
difficult to manufacture, and thus many companies specialize in making 
RFMF designs. Such filters range in frequency from .about.5 MHz to 100 
GHz, usually in the 200 MHz to 4 GHz range. Some companies focus a great 
deal into military systems while, others focus on commercial pro/ducts. 
Many different types of filters are made by these companies including 
dielectric filters (using conductivity coated ceramic blocks), LC filters, 
comb filters, notch filters, helical filters, coupled cavity filters and 
the like. Most companies make custom filters but have a catalog of 
standard filters. Some companies, but not many, have many standard 
filters. Most companies and their distributors do not stock standard 
filters. 
Engineers using filters usually write their own specifications so that a 
company can submit a design proposal. Some companies have software to help 
engineers specify and define filters. If the engineer likes the proposal 
they request or buy samples from the manufacturers they prefer. This 
process generally takes four to twelve weeks. When the engineer gets the 
RFMF, he tests it and sometimes makes changes to the requirement and the 
process continues, thus sometimes the system requirements change as the 
design progresses. Spurious signals become apparent and they have to be 
reduced, e.g., by RF emission testing (per FCC criteria) which may require 
different filter characteristics etc. Accordingly the process may require 
about one to six months to complete. If the filters, however, are not too 
difficult to make and the cost is a major consideration the filters are 
sometimes made internally using standard inductors and capacitors, or by 
on board techniques such as microstrip coupled lines. Some companies sell 
variable filters which can tune over a wide range of frequencies, however 
these filters are expensive, large, connectorized and thus for most 
situations can not be used in prototype systems. 
There are numerous shortcomings associated with existing filter design 
practices, such as design time, lack of flexibility, difficulty in 
communicating needs, and various difficulties associated with simulating 
and building prototypes. First, as discussed above this process can take 
up to six months or more to build and test a desired filter design. 
Alternatively, the circuit designer may use commercially available parts, 
but must then contend with the attendant lack of flexibility and 
availability of a particular filter characteristic. Thus the engineer must 
modify their circuit design to accommodate the use of the limited number 
of readily available filters. To this end, one must take what is given and 
can not change many times because of the cost and time constraints 
associated with standard and custom filters. 
Secondly, many times difficulty arises in communicating the engineers exact 
filter requirements because the systems are often so complex that it is 
difficult to communicate every specification which is required. For 
example, the filter manufacturing company might build the filter for a 50 
ohm load but what is actually needed is a different impedance. Often the 
engineer does not know exactly what he really wants until the system is 
put together. As a result the filter maybe incorrectly specified. 
Furthermore, difficulty occurs in simulating a circuit or system because of 
the lack of exact information on the filter. Many other components such as 
amplifiers, attenuators, switches are well characterized by the 
manufacturers and their S-parameters can be put into computer programs 
that simulate the circuit or system accurately. Filters also present a 
design problem because many times the engineer does not know the exact 
response or impedance requirement until the engineer receives the actual 
part from which components are characterized to extract the S-parameters. 
Some system simulators only require the passband, rejection and group 
delay of the filter, but more detailed circuit simulators require 
S-parameters or an equivalent circuit. 
Finally, filters are often the rate determining step when building a 
RF/microwave system and many times present the most significant difficulty 
to building the system quickly. Other components such as amplifiers, 
attenuators, switches and mixers are broadband such that standard product 
will be available in short notice from many manufacturers and 
distributors. Filters are generally not broadband and are by definition 
frequency specific. With the exception of some standard telecommunications 
frequency filters, most are typically not held in stock because of their 
specialized nature. Many times engineers desire to modify a standard 
filter's characteristics such as bandwidth, rejection, ripple, impedance, 
etc. 
Numerous problems are associated with building experimental high frequency 
filters on test boards. They include a lack of performance due to low Q 
components and board type restrictions, tuning requirements, as well as 
the time required to build and test the filter design. Generally a test 
board must be created, components must be characterized at required 
frequencies, and finally the filter must then be tested and tuned. 
SUMMARY OF THE INVENTION 
It is an object of the invention to overcome the existing filter problems 
of the prior art. 
It is an object of the present invention to provide circuits and methods of 
making high frequency filters which may be designed and assembled in 
minutes instead of months. 
It is another object of the invention to provide filters which can be 
optimized and well characterized before they are ever built. 
It is yet another object of the present invention to provide filters that 
can be optimized in the real system for maximum performance and control. 
It is a further object of the invention to provide cost effective filter 
designs through the use of readily available competitive components. 
It is a still further object of the invention to provide for manufacture 
with enhanced turn around time and communication of design specifications 
that may use filter design software which specifies the basic components 
required to build the specific high frequency filter. Thus allowing the 
user to build prototype filters that may be provided as a sample to a 
filter manufacturer or given in the form of specifications of the existing 
filter. 
In a described embodiment, a kit for assembling a high frequency filter 
includes a filter case having side walls, a generally open proximal end 
and a generally closed distal end. A partition within said filter case 
separates the inside of the filter case into at least a first cavity and a 
second cavity, the partition having an aperture for coupling the first and 
second cavities. A first helical resonator coil is disposed inside the 
first cavity of the filter case extending from the proximal end towards 
the distal end of the filter case, and a second helical resonator coil is 
disposed inside the second cavity of the filter case extending from the 
proximal end towards the distal end of the filter case. 
A first tap coil is then provided as being connectable in series with the 
first helical resonator coil at the proximal end of the filter case, the 
series connection between the first helical resonator coil and the first 
tap coil providing an input tap for coupling electrical signals to the 
high frequency filter. A second tap coil is further connectable in series 
with the second helical resonator coil at the proximal end of the filter 
case, the series connection between the second helical resonator coil and 
the second tap coil providing an output tap for coupling electrical 
signals from the high frequency filter. A removable tap housing is 
provided for supporting the first tap coil at the proximal end of the 
filter case. 
A method of assembling the high frequency filter thus provides a first coil 
for resonating first electrical signals, and a second coil for resonating 
second electrical signals. The first and the second coils are enclosed 
between a generally open proximal end and a generally closed distal end. 
Partitioning of the enclosed first and second coils provides a first 
cavity and a second cavity respectively. The first coil is disposed inside 
the first cavity extending from the proximal end towards the distal end, 
and the second coil is disposed inside the second cavity extending from 
the proximal end towards the distal end of the enclosure. A removable 
signal coupler provides coupling of electrical signals into the first 
coil, with the coupling tap being supported by a housing at the proximal 
end. 
Briefly summarized, the present invention relates to filters and methods 
wherein resonating first and second electrical circuits are enclosed 
between proximal and distal ends of a filter case. Partitioning the inside 
of the enclosed resonant circuits may be performed by a user to form at 
least a first cavity and a second cavity. The first resonating circuit is 
then disposed inside the first cavity of the filter case extending from 
the proximal end towards the distal end, and the second resonating circuit 
is disposed inside the second cavity also extending from the proximal end 
towards the distal end. Electrical signals are coupled into the resonating 
circuits by an encased signal coupler which is removably mounted by a 
coupling housing for supporting the signal coupler at the proximal end of 
the filter case for positioning in the vicinity of the resonating circuits 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the present preferred embodiments 
of the invention, examples of which are illustrated in the accompanying 
drawings relating to circuit design techniques that may be employed in RF 
and microwave filter (RFMF) prototype kits. The preferred embodiment for a 
high frequency helical filter 10 is depicted in FIGS. 1 and 2. As 
discussed further below, a filter case 12 provides an external enclosure 
having side walls 14, a generally open proximal end 16 and a generally 
closed distal end 18. A partition 20, herein divider plates, within the 
filter case 12 separates the inside of the filter case 12 into at least a 
first cavity 22 and a second cavity 24. The partition has an aperture 26 
for coupling the first and second cavities 22 and 24. A first helical 
resonator coil 28 is disposed inside the first cavity 22 of the filter 
case 12 extending from the proximal end 16 towards the distal end 18 of 
said filter case 12, and a second helical resonator coil 30 is disposed 
inside the second cavity 24 of the filter case 12 which also extends from 
the proximal end 16 towards the distal end 18 of the filter case 12. The 
high frequency filter may employ a plurality of removable tuning screws 56 
for insertion at the distal end of filter case 12. The tuning screws 56 at 
the distal end of the filter case 12 at the first cavity and the second 
cavity respectively provide for tuning of the helical resonator coils. A 
final shield 58 is provided to cover the open proximal end to minimize the 
effects of any stray radio frequency radiation or electromagnetic 
interference (EMI) effects. 
As shown in FIGS. 3a, 3b and 3c, a first tap coil 32 is advantageously 
provided as being connectable in series with the first helical resonator 
coil 28 at the proximal end 16 of the filter case 12, the series 
connection 34 between the first helical resonator coil 28 and the first 
tap coil 32 providing an input tap 36 for coupling electrical signals to 
the high frequency filter 10. The tap coil 32 is provided with a tap 
housing 44 having electrical connection pins 48 and 50. A second tap coil 
38 (FIG. 2) is also provided as being connectable in series with the 
second helical resonator coil 30 at the proximal end 16 of the filter case 
12, with a second series connection 40 between the second helical 
resonator coil 30 and the second tap coil 38 providing an output tap 42 
for coupling electrical signals from the high frequency filter 10. A first 
removable tap housing 44 supports the first tap coil 32 at the proximal 
end 16 of the filter case 12, while a second removable tap 46 housing may 
be provided for supporting the second tap coil 38 at the proximal end 16 
of the filter case 12. Removable tap housings 44 or the like may be used 
in an intermediate position to support the filter case on a printed 
circuit board, or for coupling additional electrical signals to the filter 
e.g., FIG. 7 shows a housing 54 for support and/or for a center tap). This 
center tap if connected properly could be used for example to couple in a 
local oscillator signal in addition to merely supporting the center tap 
portion of the filter on the circuit board. 
The filter case 12 is formed of a metal such as aluminum which can be made 
as a single elongated can, or several smaller cans soldered together. The 
case 12 has ground conductors provided as part of the metal can housing 
which can be soldered onto a printed circuit board. The partition 20 may 
be provided as a permanent part or integral with the case, as where cans 
are placed together. Alternately, Beryllium copper (BeCu) divider pieces 
may be employed as partitions 20 instead of multiple cans or cases, which 
provides multiple possibilities for the partition 20 and the associated 
aperture 26 separating the inside of the filter case 12 into at least a 
first cavity 22 and a second cavity 24. The partition has the aperture 26 
for coupling the first and second cavities 22 and 24. The combination of 
varying the helical coils 28, 52, 30, tap coils 32,38 and apertures 26 
allows the engineer to achieve the desired filter characteristic provided 
it is physically achievable. The partitions 20 may be provided as 
removable partition walls defining the aperture 26 therein, and a kit of 
multiple partition walls 20 can be provided with each having different 
sized apertures 26 for varying the signal coupling characteristics between 
the first cavity 22 and the second cavity 24. Characteristics such as 
center frequency, bandwidth, input and output impedance, ripple, rejection 
and others may be varied with the various filter pieces available in the 
kit. From a relatively small number of pieces a large number of filter 
permutations may be achieved. Although many filters may not be suitable, 
the ultimate number of filters which may be achieved will be the 
multiplication of the number of helical coils by the number of tap coils 
by the number of apertures in the kit. 
The individual filter elements or coils may be provided as helical 
resonators which may be made using a low loss target material such as 
polystyrene. A helical cross coupled cavity type filter(60), e.g., FIG. 9, 
can be produced as well to achieve superior filter characteristics via 
crosscoupling of resonators cavities. 
The kit technique may be extended to other types of RFMF devices. For 
example, higher frequency combline and waveguide filter kits could be 
achieved. Also, low frequency simple LC filters can be put into a kit 
format. Utilizing similar methods of precharacterized filter elements that 
will correspond to quickly make the filter prototypes discussed herein. 
The high frequency class of filters may operate to 100 GHz, although most 
will only operate to 2-3 GHz. 
As shown in the presently described embodiment, the first helical resonator 
coil 28 is disposed inside the first cavity 22 of the filter case 12 
extending from the proximal end 16 towards the distal end 18 of the filter 
case 12, and a second helical resonator coil 30 is disposed inside the 
second cavity 24 of said filter case 12 which also extends from the 
proximal end 16 towards the distal end 18 of the filter case 12. Slits are 
provided in the side of the polystyrene target material of the helical 
resonators used to form the target material, upon which the helix is wound 
with slight tension for improved microphonic performance. 
Several coupling techniques may be employed for coupling electrical signals 
into and between the resonant cavities of the RF filters described herein. 
With reference to FIG. 4 is a schematic diagram illustrates a multiple 
pole helical coil filter providing an input tap 36 and an output tap 42 
configuration. FIG. 5 is a schematic diagram illustrating a multiple pole 
helical coil filter providing loop coupling an input coupling coil and an 
output coupling coil configuration. The loop should be physically close to 
the helical coil to facilitate the loop coupling. FIG. 6 is a schematic 
diagram illustrating a multiple pole helical coil filter providing an 
input capacitive probe and an output capacitive probe configuration. Probe 
coupling may be achieved via a microstrip circuit board placed at the 
proximal end of the case 12 with a mechanical coupling arrangement of the 
case 12 to the printed circuit board (PCB) which provides the microstrip 
circuitry. The PCB employing probe coupling may also be used to match 
impedance's to the circuitry outside the filter. Other known signal 
coupling techniques also may be used, depending upon the type of 
resonators being employed in the filter designs. 
The high frequency filter shown in FIGS. 3b and 3c provides the tap housing 
as including a potting material for encasing the tap coils. The tap 
housing 44 may then position the respective tap coils inside the 
respective helical resonator coils to facilitate signal coupling. The 
potting material or plastic should be formed from a low loss tangent 
material, such as polyethylene, which also is capable of withstanding the 
heat dissipation of soldier applications. FIG. 7 shows an exploded 
perspective view showing assembly of the filter case, the partitions, the 
helical resonator coils and the tap coils of a helical filter embodiment. 
When the described tap housing 44 is provided as a plastic material for 
encasing the tap coils, color coding of the plastic housing potting 
materials may be used as indicia for indicating inductance values and the 
like. Other indicia such as printed text or symbols also may be employed 
to show and identify the values associated with the various resonant 
elements. As described, the housing electrically couples or connects the 
first tap coil with the first helical resonator coil at the series 
connection between the first tap coil and the first helical resonator 
respectively to facilitate the desired coil tap function. The tap housing 
44 may include a metallic coupling, such as a BeCu socket having a 
brushing action, for electrically connecting the tap coils with the 
helical resonator coils at the series connection between the tap coil and 
the helical resonator respectively, while providing a good electrical 
contact for the tap connection. No soldering is required because the tap 
point uses the BeCu brushed socket, and the coupling between helical coils 
may be achieved through the use of capacitive coupling, as discussed. 
Samtec USA surface mount sockets SC/SK/SP series were acceptable for this 
purpose, although any known sockets may be employed for use with the 
described tap housing connection. Thus the tap housing provides an 
electrical socket for electrically connecting the tap coils with the 
helical resonator coils at the series connection between the tap coil and 
the helical resonator. Use of the sockets allows for rapid prototyping of 
various filter designs, and since no soldering is required, filter 
configurations may be modified until the correct response is achieved. 
As illustrated in the exploded view of FIG. 7 and the assembly shown in 
FIG. 8, a first tap coil 32 is advantageously provided as being 
connectable in series with the first helical resonator coil 28 at the 
proximal end 16 of the filter case 12, the series connection 34 between 
the first helical resonator coil 28 and the first tap coil 32 providing an 
input tap 36 for coupling electrical signals to the high frequency filter 
10. FIG. 9 is a perspective view of a cross coupled cavity resonator 
embodiment, whereas FIG. 8 shows a multipole helical filter embodiment. 
FIG. 8 shows a alternate embodiment of the invention in the form of a 
vertical surfacemount filter. The cross coupled cavity filter of FIG. 9 
can expand to 4, 6, 8,10 . . . poles, etc. The plastic material for the 
tap housing 44 of the tap coils may be made with pins for surface mounting 
or through pins may be provided, as required for specific applications. 
The connector pins may thus include surface mount connector pads. 
The second tap coil 38 is also provided as being connectable in series with 
the second helical resonator coil 30 at the proximal end 16 of the filter 
case 12, with a second series connection 40 between the second helical 
resonator coil 30 and the second tap coil 38 providing an output tap 42 
for coupling electrical signals from the high frequency filter 10. 
Removable tap housings 44 and 46 support the first tap coil 32 and the 
second tap coil 38 at the proximal end 16 of the filter case 12. The 
second removable tap 46 housing may be provided for supporting the second 
tap coil 38 at the proximal end 16 of the filter case 12. The removable 
tap housings may be provided with internal BeCu brushes or socket pins for 
good electrical contacts. 
Various filter kits with the numerous standardized and characterized 
components as discussed herein may be provided to include a multiplicity 
of the first tap coils encased in the tap housings for varying signal 
coupling characteristics between the first tap coil 32 and the first 
helical resonator coil 28. Filters may be created from about 5 MHz to 100 
GHz although most will be from 50 MHz to 3 GHz. Helical filters generally 
operate from about 50 MHz to 3 GHz. Various kits will address 
characteristics of various bands. Such as one kit from 100 MHz to 500 MHz 
another from 500 MHz to 1000 MHz, and so on. Kits with various taps and 
partitions (e.g., 3 to 10 pieces) may be provided for various bandwidth, 
e.g., 5% to 20%. As shown in FIG. 10, the kit may include several (e.g., 
20 to 100) helical coils to cover a wide range of frequencies, e.g., 50 
MHz to 1600 MHz. 
The sub-component parts of filter kits, may include: 
1) Rectangular metal shield of various sizes; 
2) Helical Coil and or Inductor pieces; 
3) Coupled Cavity divider pieces; 
4) Inductive and Capacitively couple end pieces; 
5) Various tuning pieces; and 
6) Test boards. 
Software may be used which corresponds with the components of the kits 
which allows the designer to take a filter from frequency characteristics 
to a matrix of required physical components. Software also may be provided 
for generating the filter characteristic information from the filter 
component data with a very close approximation to the actual prototype. 
This can be done verses other existing filter software because the piece 
parts will be very well characterized. Thus software output may be 
accurate for building and simulation purposes. This software could be 
accessable via a web site on the internet. A manual may also be included 
which would contain various filters characteristics corresponding to 
various combinations of kit pieces. 
As described above, the kit which is shown in FIG. 10 may be used by the 
circuit designer to provide a quick method of assembling a high frequency 
filter prototypes, by providing coils for resonating electrical, and 
enclosing at least first and the second coils between a generally open 
proximal end and a generally closed distal end. Additional coils may be 
used for additional filter poles in multiple pole filter applications. The 
designer then partitions the enclosed first and second coils into a first 
cavity and a second cavity respectively. The first coil inside the first 
cavity extends from the proximal end towards the distal end, and the 
second coil inside the second cavity extends from the proximal end towards 
the distal end of the enclosure. Then a signal coupler such as the 
described tap coil is provided for coupling electrical signals into the 
coils. The tap coil may encase the signal coupler in a coupler housing 
such as the tap housing discussed above for removably positioning the 
signal coupler in the vicinity of the resonant coils. The coupler housing 
is thus supported at the proximal end of the filter case. By providing 
various combinations of helical resonators in the embodiment of FIG. 10, 
e.g., the helical coils 28, the partitions 20, the tap coils 44, tuning 
screws 56, enclosure 12, testboard 66, numerous filter combinations may be 
rapidly assembled. Through the appropriate choice of component parts, a 
kit may be made to cover a wide range of frequencies, e.g., 50 MHz to 1600 
MHz with bandwidths of approximately 5% to 20%. This is useful for the 
prototyping, experimentation and production for a wide variety of RF and 
Microwave system designs. 
It will be appreciated by those skilled in the art the modifications to the 
foregoing preferred embodiment may be made in various aspects. The present 
invention is set forth with particularity in the appended claims. It is 
deemed that the spirit and scope of that invention encompasses such 
modifications and alterations to the preferred embodiment as would be 
apparent to one of ordinary skill in the art and familiar with the 
teachings of the present application.