U.S. patent number 3,681,652 [Application Number 05/039,836] was granted by the patent office on 1972-08-01 for capacitive filter for suppression of spurious electrical radiation.
This patent grant is currently assigned to Raytheon Company. Invention is credited to Carlo P. Domenichini, Leo Terjenian.
United States Patent |
3,681,652 |
Domenichini , et
al. |
August 1, 1972 |
CAPACITIVE FILTER FOR SUPPRESSION OF SPURIOUS ELECTRICAL
RADIATION
Abstract
A capacitive filter arrangement coupled to the voltage supply
circuit of an electrical energy generator to reduce undesired
radiation of energy to acceptable levels in accordance with Federal
Communications Commission specifications and requirements for such
generators when employed in certain electronic equipment.
Elimination of corona discharge at predetermined levels of
alternating current and no breakdown of insulation material at
predetermined levels of direct current are other characteristics of
the structure of the invention.
Inventors: |
Domenichini; Carlo P.
(Lexington, MA), Terjenian; Leo (Natick, MA) |
Assignee: |
Raytheon Company (Lexington,
MA)
|
Family
ID: |
26716498 |
Appl.
No.: |
05/039,836 |
Filed: |
May 22, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
754641 |
Aug 22, 1968 |
3531613 |
Sep 29, 1970 |
|
|
Current U.S.
Class: |
315/85;
315/39.51; 315/39.53; 331/86; 333/181 |
Current CPC
Class: |
H05B
6/66 (20130101); H01J 23/15 (20130101); H01J
23/14 (20130101); H05B 6/76 (20130101); H04B
15/02 (20130101) |
Current International
Class: |
H01J
23/00 (20060101); H04B 15/02 (20060101); H05B
6/76 (20060101); H01J 23/14 (20060101); H01J
23/15 (20060101); H01j 001/52 () |
Field of
Search: |
;315/39.51,39.53,85
;333/79 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Chatmon, Jr.; Saxfield
Parent Case Text
This is a division of pending application, Ser. No. 754,641 filed
Aug. 22, 1968.
Claims
1. In combination:
a magnetron-type electrical energy generator comprising electrodes
having external electrical leads;
circuit means for supplying operating voltages to said
generator;
electrically conductive enclosure means defining capacitive filter
means coupled to said generator and enclosing said external
leads;
opposing walls of said enclosure means defining a central aperture
enclosed by insulating means;
electrical feedthrough conductors extending through said insulating
means;
means for electrically connecting one end of said electrical
feedthrough conductor to said circuit means;
means for electrically connecting the opposing end of said
conductor to said generator external leads; and
capacitive radio frequency filter means disposed adjacent to
opposing ends of said conductors;
said capacitive means including inner and outer conductive plate
members extending on opposing sides of and parallel to the walls of
said enclosure means with dielectric means disposed
therebetween;
said outer conductive plate members extending beyond the peripheral
edges of said central aperture a distance of approximately
one-quarter of a wavelength of a hormonic frequency of said
generator operating frequency.
2. In combination:
a magnetron-type electrical energy generator comprising electrodes
having external electrical leads;
circuit means for supplying operating voltages to said
generator;
an electrically conductive enclosure defining capacitive filter
means coupled to said generator and enclosing said external
leads;
opposing walls of said enclosure means defining a central aperture
enclosed by insulating means;
electrical feedthrough conductors extending through said insulating
means; and
capacitive radio frequency filter means disposed adjacent to
opposing ends of said conductors;
said capacitive means including inner and outer conductive plate
members extending on opposing sides of and parallel to the walls of
said enclosure means with dielectric means disposed
therebetween;
said dielectric means being dimensioned to extend beyond the outer
peripheral edges of said enclosure;
said outer conductive plate members extending beyond the peripheral
edges of said central aperture a distance of approximately
one-quarter of a wavelength of a harmonic frequency of said
generator operating frequency; and
said inner conductive plate members having a diameter substantially
larger than said outer conductive plate members.
Description
BACKGROUND OF THE INVENTION
The magnetron is an illustrative electrical device for the
generation of energy in the electromagnetic spectrum. The text
"Microwave Magnetrons," Radiation Laboratories Series, Vol. 6, by
G. B. Collins, McGraw-Hill Book Company, Inc., 1948, provides
explicit details on construction and operation of such devices. The
generated microwaves have many uses including radar systems for the
location and detection of distant objects as well as cooking in
electronic ovens. Conventionally, a power supply circuit includes a
high voltage transformer carrying many thousands of volts of
rectified electrical energy along leads to the cathode components
of the microwave energy generator.
In view of the high electrical voltage present in the energy
generation apparatus the inherent safety as well as interference
problems with adjacent communications equipment such as television
and U.H.F. as well as high frequency radio receivers have dictated
the imposition of controls and regulation by the Federal
Communications Commission. Particularly, allocation of frequency
bands of operation in the low and high frequency radio spectrum
have been designated as well as peak and normal voltage handling
destructive tests. For industrial microwave frequency electronic
oven applications the first frequency band allocated is 915
megacycles and many U.H.F. communications links as well as TV
systems may operate at or near such frequencies. The second and
perhaps most commonly employed band is 2,450 megacycles. Police and
government communications operate close to this frequency and
elimination of interference is therefore of paramount importance.
Second harmonic frequencies of the main microwave generator
operating frequency or 4900 megacycles, plus or minus 100
megacycles, also create problems in interference with
communications systems and equipment. The filtering and suppression
of radiation at TV, U.H.F. and the lower portion of the microwave
frequency spectrum as well as second harmonics of these frequencies
from microwave energy generators is therefore required in order to
comply with the aforementioned regulations.
A prior art capacitive filter box for microwave generators of the
magnetron type is disclosed in U.S. Pat. No. 3,020,447 issued Feb.
6, 1962 to Louis H. Schall and Donald L. Winsor and assigned to the
assignee of the present invention. This structure provides an
electrically conductive shield enclosing the cathode electrode
support assembly and an internal condenser plate is defined with
dielectric spacers cooperating with opposite sidewalls of the
shield. Ceramic insulating bushings are provided for feedthrough of
the high voltage electrical leads from the external supply circuit
to the cathode assembly. A low inductance shunt lead between the
condenser plate and the cathode lead is designed to suppress
radiation due to undesired resonances in the oscillation
circuit.
In operation the prior art structure has failed to prevent radio
frequency leakage along paths in many regions of the conductive
enclosure. Additionally, the electrical insulating bushings of
ceramic material for the high voltage connections do not prevent
radio frequency leakage through this structure. While suppressing
some of the radiated noise energy in the frequency spectrum the
structure has not provided the reduction of interference with
adjacent electronic equipment due to the areas of excessive radio
frequency leakage through the filter box construction. The
capacitance of the Schall et al prior art embodiment is only of the
order of approximately 100 micromicrofarads or, as it will be
referred to herein, picofarads. The need arises therefore to
provide a substantially higher value of capacitance to more fully
isolate the microwave generator leads at the operating frequency as
well as the second and higher harmonic frequencies in order to
comply with the standards established by the F.C.C.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention a unique
capacitive filter enclosure means is provided having internal as
well as external conductive plate members to prevent radiation of
the operating and harmonic radio frequencies through all the leads
and/or wires disposed at the back end of the microwave energy
generators. Through a novel structural feature of the outer
conductive plate member the capacitive means provides a leakage
path impedance approaching a short circuit adjacent to an aperture
in the enclosure walls to thereby define a radio frequency choke.
At TV and U.H.F. frequency bands the leakage path impedance is
reduced substantially to limits well within the Federal
Communications Commission regulations. Second and higher harmonic
frequency radiation at high microwave frequencies which previously
exceeded the regulation limits have also been substantially reduced
by a factor of approximately 20 decibels. Plural dielectric plates
sandwiched between the capacitive disc members and enclosure walls
are dimensioned to enclose substantially all the previous radio
frequency leakage areas and comply with the stringent requirements
of no insulating material breakdown at voltages of many thousands
of volts under normal or peak conditions. The dielectric members
may be of any configuration such as round, square or elliptical.
The structure also satisfied the requirement of being corona free
at all peak and operating conditions. In an illustrative embodiment
of the invention a measured capacitance of approximately 600
picofarads or 500 percent higher indicates the order of magnitude
of the improvement of the present invention over the art of
record.
The disclosed apparatus is equally applicable to all energy
generators requiring high voltage supplies including magnetrons,
crossed field devices, traveling wave tubes, klystrons and high
powered triode tubes.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a vertical sectional view of the microwave energy
generator and filter box assembly embodying the invention, removed
from the electronic oven apparatus;
FIG. 2 is a vertical sectional view of an electronic oven apparatus
utilizing the embodiment of the invention;
FIG. 3 is an enlarged fragmentary cross-sectional view of the
illustrative embodiment of the invention;
FIG. 4 is a side elevational view along the line 4--4 in FIG. 1
viewed in the direction of the arrows;
and FIG. 5 is a vertical sectional view along the line 5--5 in FIG.
1 viewed in the direction of the arrows.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, FIG. 1 is illustrative of a complete
power generation assembly for an electronic oven designated
generally by the numeral 10. A microwave power generator 11 with
accompanying magnetic field producing means 12 has appended thereto
an electrically conductive enclosure means 13 defining a capacitive
filter arrangement incorporating the preferred embodiment of the
invention. Before describing in detail the present invention, it
will be of assistance to refer to FIG. 2 illustrative of an
electronic oven apparatus.
A hollow rectangular parallelpiped conductive enclosure 14 provides
a cavity 15 which serves as the oven for the cooking of any
articles of food 16. Access to the interior of the oven is provided
by door closure means which are well known in the art and,
therefore, need not be described in detail herein. A dimpled wall
arrangement 17 defining a sump is disposed in the bottom wall of
the oven enclosure to provide for more efficient cooking of the
articles of food resting on a dielectric member 18 supported by the
shoulders of the wall 17.
The microwave energy generation means and embodiment of the
invention 10 is coupled to a conventional high voltage supply
circuit 19 including a transformer, D.C. rectifier, control means,
capacitor and resistor means together with suitable leads and
interconnections. The electromagnetic energy from the generator is
fed by means of a radiating probe 20 in dielectric dome member 21
to a section of rectangular waveguide 22 appropriately dimensioned
to propagate the energy at the desired frequency into oven cavity
15. The waveguide section is closed at one end by wall 23 and is
open at the inner end 24. Upon energizing of the microwave energy
generator the electromagnetic waves radiating within the oven
cavity are uniformly distributed by a stirrer 25 which is driven by
a fan motor 26. The stirrer means are well known in the art and
have been described in detail in U.S. Pat. No. 2,813,185 issued
Nov. 12, 1957 to Robert V. Smith and assigned to the assignee of
the present invention.
In FIG. 1 the microwave energy generator 11 is of the magnetron
type which includes within an evacuated envelope 27 of a highly
conductive material such as copper, an anode defining plural
resonant cavities and a centrally disposed cathode with an internal
heater, all of well known construction, and explained in detail in
the aforementioned text and Letters Patent to Schall et al. An
electron interaction region is defined between the cathode and
anode with a high electric field normal to the generator axis. The
cathode lead 28 and heater lead 29 are disposed within the
dielectric cathode electrode support assembly 30 axially disposed
at the opposing end of the envelope 27. Cooling fins 31 provide for
the direction of a circulated coolant for the rapid removal of the
heat generated by the high frequency oscillations.
The magnetron device is completed by magnetic field producing means
such as an electromagnet 12 axially disposed adjacent to the device
and providing by a large number of turns of conductive wire 32 and
core member 33 a magnetic field directed transversely to the tube
axis. The electromagnet 12 is coupled to and supports the magnetron
device by housing and pole piece members 34 of a conductive metal.
The oscillations are generated by the crossed electric and magnetic
fields which induce a spoke-like revolving space charge in the
interaction region. A bracket member 35 secured to electromagnet 12
may be utilized for mounting of the entire assembly within the oven
apparatus.
The embodiment of the invention includes an enclosure 13 of a
box-like configuration defining the capacitive filter means
surrounding the cathode and heater leads 28 and 29 of the magnetron
device and provides means for the connection of these leads to the
high voltage supply 19. Other possible configurations include
circular, spherical or irregularly shaped structures. The filter
box enclosure 13 is fabricated of a lightweight electrically
conductive metal such as, for example, aluminum and is secured to
the magnetron magnet housing structure by plural fastening means,
each including clamp 36, threaded stud 37 and nut 38 engaging
bracket 39. The magnetron device 11 and electromagnet assembly 12
are conventionally grounded and the opposing sidewalls 40 of the
filter box enclosure are therefore also at ground potential. Access
to the interior of the filter box enclosure 13 may be had by
opening 41 and a radio frequency gasket 60 of a woven mesh material
such as, for example, commercially available Metex, surrounds this
opening to prevent radiation and leakage of any microwave
energy.
The filter box is enclosed on the remaining three sides by
perforated metallic walls 42 to permit circulation of a coolant
where desired. The dimensions of the perforations are selected to
prevent radiation at the operating frequency of the magnetron. On
opposing sides of each ground plate wall member 40 the capacitive
filter arrangement is disposed to prevent radiation of any spurious
radio frequency energy. An outer and inner capacitive plate member
such as discs 43 and 44, respectively, of a metal such as aluminum,
having deburred and rounded peripheral walls 45 are conductively
coupled to the high voltage feedthrough electrical conductors 51.
Any other shaped plate members such as square or elliptical are
also encompassed by the invention. Dielectric insulating members 46
and 47 are sandwiched between the capacitive plate members and the
ground plate wall members 40. Members 46 and 47 are preferably of a
substantially larger dimension than the companion or corresponding
capacitive plate members so as to extend beyond intersecting point
of the filter box walls 40 and 42. The spacer members prevent
electrical breakdown across the surface of the material to ground,
commonly referred to as creep voltage breakdown, and are desirably
of a material having a minimum dielectric strength of 3,000 volts
per mil such as, for example, Mylar, a commercially available
brand. The dielectric members contact opposing conductive surfaces
of the wall members 40 and overlap all possible areas of radio
frequency radiation through the intersecting corners of the filter
box enclosure of radio frequency energy escaping from the back end
of the magnetron through the cathode support 30 and electrical
leads 28, 29, as well as getter lead 48.
Referring now to FIGS. 3, 4 and 5, it will be noted that each of
the walls 40 has a central aperture 49 completely enclosed by an
insulator disc 50 of any suitable material, such as Mylar. High
voltage feedthrough conductors 51 comprising a rivet are supported
by insulator disc 50 and extend through the capacitive plate
members 43 and 44. Terminal lugs 52 and 53 contact the capacitive
members 43 and 44 and are retained by the conductors 51 to provide
good electrical continuity. Capacitive filter means are therefore
provided on opposing sides of the grounded wall members 40 to
substantially suppress the radiation of low and high radio
frequency energy. The appropriate internal and external electrical
connections between the magnetron components and high voltage
supply circuit are made to the terminal lugs by electrical leads
and connectors 54. Additional lugs may also be provided where
desired on either end. All dielectric components including spacers
46, 47 and insulators 50 are coated with an insulating compound,
illustratively Dow Corning silicon grease, and riveted together
under high pressure to eliminate air pockets to prevent incidence
of corona discharge.
In FIG. 4 the overlapping construction of the outer dielectric
spacer members 46 is clearly shown with the edges extending beyond
the enclosure walls on all four sides. For the sake of clarity the
filter box enclosure has been removed from the adjacent
electromagnet structure to illustrate the salient features of the
invention and the fastening means are not shown. In FIG. 5 the same
overlapping feature is illustrated with the surplus edges bent in a
direction towards the viewer.
With the dimensions of the capacitive plate members 43 and 44
selected to be similar on both sides the filter box capacitance was
measured at a value of approximately 600 picofarads. At this value
the radiation level was substantially reduced at the TV and low
frequency end of the microwave spectrum. Still further improvement
was noted, however, by the provision of structure now to be
described. Referring again to FIG. 3, it is noted that the diameter
of the outer capacitive plate members 43 relative to inner members
44 is reduced and is selected with relation to the dimensions of
central apertures 49 in wall members 40 so as to define a radio
frequency choke arrangement. The distance indicated by the arrow 55
is selected to provide a dimension of approximately one-quarter of
a wavelength in the dielectric medium at the second harmonic of the
magnetron operating frequency or approximately 4,800 - 5,000
megacycles. This radio frequency choke arrangement results in an
open circuit at point 56 defined between plate member 43 and wall
member 40 which effectively reflects a short circuit at the
peripheral edges of the aperture 49 indicated by the numeral 57. At
the higher microwave frequencies then the leakage path of any
energy bypassing the space between the edge of inner capacitive
slate members 44 and the wall members 40 to the point of the
aperture walls will be substantially completely reflected. Through
this arrangement radio frequency interference measurements at the
second harmonic frequencies of the normal magnetron operating
frequency were measurably improved by a factor of 18 to 20 decibels
over prior art structures. The larger internal capacitive plate
members 44 then provide for suppression of most of the radiation at
the lower frequencies and the combined structure defining the radio
frequency choke will successfully prevent the radiation of energy
at the higher frequencies.
In an exemplary embodiment utilized with a magnetron generator in
an oven apparatus, a capacitive filter box enclosure arrangement in
accordance with the teachings of the present invention was measured
to be corona free at 4,000 to 5,000 volts peak AC and no breakdown
of the dielectric insulating material was observed at voltages up
to 24,000 volts DC. Radio frequency interference measurements were
well within the Federal Communications Commission regulations at a
level of approximately 35 decibels above 1 microvolt measured with
a calibrating antenna at 10 feet. The central apertures 49 of the
wall members 40 has a dimension of 1.250 inches and the diameter of
the outer plate members was approximately 1.910 inches. The
increment of approximately 0.660 inches is divided in half to
result in a path dimension of 0.330 inches which is the approximate
one-quarter wavelength dimension at the second harmonic frequency
of the magnetron. In the same embodiment the inner capacitive plate
members had an overall diameter of approximately 3.187 inches. The
disclosed capacitive filter box enclosure has therefore provided an
efficient high voltage feedthrough conductor having low loss
characteristics and providing for suppression of spurious radiation
at TV frequencies and the low and high end of the applicable
microwave frequencies.
The structure disclosed may be modified for example by including
multiple plate members having dielectric material sandwiched in
between to collectively define either smaller outer member or
larger inner member or both. Such a filter would be a multi-stage
filter and provide even greater attenuation over a broad bandwidth.
Other modifications may be practiced in the configuration of the
overall filter box as well as the capacitive plate and dielectric
members. It is desired, therefore, that this description be
considered as illustrative only without limiting in any way the
interpretation of the broadest aspects of the invention as defined
in the appended claims.
* * * * *