Abstract:
A low-filter is arranged for attachment to an exterior face of a wall of an electrically conducting screened chamber encasing the magnetron and an associated isolation transformer electrically connected to terminals of the magnetron. Output connections of the filter pass directly through an interface between the electrically conducting screened chamber and the filter to connect electrically, directly or indirectly, with the isolation transformer. There are therefore no electrical leads outside the screened chamber electrically connecting the filter to the isolation transformer.

Description:
[0001]    This invention relates to a filter for reducing stray emissions from a magnetron operating at frequencies in the vicinity of 900 MHz, and particularly in a range 890 to 930 MHz and to a method of filtering such stray emissions. 
       BACKGROUND 
       [0002]    Magnetrons for known domestic ovens are provided with an L-C filter to prevent, as far as is possible, stray radiation generated by the magnetron from passing along the leads which supply power to the cathode heater. Such a filter, which is located at least partially within a screen chamber housing the magnetron terminals, is known from U.S. Pat. No. 4,900,985. 
         [0003]    A typical domestic cooker magnetron has a peak power of a few kilowatts, and an average power of around 1 kW and requires a heater current of around 10 A. However, for industrial RF processing applications, peak powers of several tens of kilowatts are needed, and a correspondingly larger heater supply is needed with typical currents of the order of 100 amps, so that much higher gauge conductors are needed compared with domestic cooker magnetrons. In particular, it would not be practical or economic to wind such high gauge conductors into a choke coil used for a domestic cooker magnetron. 
         [0004]    A basic problem to be addressed is therefore that in a microwave source for industrial applications a magnetron requires a high voltage supply to be applied to the cathode, perhaps as much as −20 kV, together with a heater supply of typically 11 V at 110 A, derived from an isolation transformer (and rectifier if a DC heater is used) connected across heater and cathode terminals of the magnetron. These terminals can be the source of considerable stray radiation in the frequency range 100 MHz to &gt;1 GHz, as illustrated in a first inset  20  in  FIG. 1 , for a magnetron designed to produce an output at around 900 MHz. This stray radiation can be picked up and/or conducted in lead wires from the magnetron to the isolation transformer and lead wires from the isolation transformer to an external heater supply inverter. The isolation transformer, which is designed to hold off 20 kV, provides no significant barrier to currents induced by the stray radiation. 
         [0005]    Because of the high levels of stray radiation, it is usually necessary fully to shield the magnetron and the isolation transformer in a metallic or other electrically conductive screened chamber. If a filter is fitted, its effectiveness may be limited by radiation picked up on its output. Such a filter may provide no attenuation to the stray radiation because the filter itself acts as an antenna and picks up the stray radiation on its output even although the filter may have significant attenuation over the desired frequency band. 
         [0006]    In many applications the drive current from the heater supply inverter is modulated as a high frequency (Fi) square wave, as illustrated in a second insert  21  in  FIG. 1 . Any filter used must be able to pass, without any significant distortion and loss, the heater supply inverter waveform into the screened chamber but significantly attenuate and minimise stray radiation to the outside of the screened chamber. 
         [0007]    It is an objective of the current invention at least to ameliorate some of these difficulties in the prior art. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0008]    In accordance with a first aspect of the present invention there is provided a low-pass filter for reducing stray emissions from a magnetron, wherein the filter is arranged for attachment to an exterior face of a wall of electrically conducting screening means for encasing the magnetron and for encasing an associated isolation transformer means electrically connected to terminals of the magnetron; and wherein an output connection of the filter passes directly through an interface between the electrically conducting screening means and the filter to connect electrically, directly or indirectly, with the isolation transformer. 
         [0009]    Conveniently, the filter comprises a printed circuit board with a ground plane on a first face and at least one capacitor plate on a second face opposed to the ground plane on the first face, wherein the output connection of the filter is connected directly or indirectly to the capacitor plate. 
         [0010]    Advantageously, the output connection is via a through-hole in the printed circuit board directly to the at least one capacitor plate. 
         [0011]    Advantageously, an aperture is provided in the ground plane for passage therethrough of the output connection, for voltage hold off between the output connection and the ground plane. 
         [0012]    Conveniently, the filter comprises a plurality of LC stages between a first line and a ground plane and between a second line and the ground plane. 
         [0013]    Advantageously, inductors in neighbouring stages are orthogonal to each other to minimize coupling between the inductors. 
         [0014]    Advantageously, capacitor plates of the plurality of LC stages have dimensions of substantially 22 mm by 22 mm. 
         [0015]    Conveniently, the filter further comprises a first capacitor and a first resistor in series between the first line and the ground plane and a second capacitor and a second resistor connected in series between the second line and the ground plane to ensure a nominally matched impedance at frequencies of the stray radiation thereby minimizing gain of the filter at frequencies in the desired attenuation band but providing insignificant impedance to a waveform output from the heater supply inverter. 
         [0016]    Advantageously, the filter further comprises filter electrical screening means encasing the filter and arranged for electrical connection to the electrically conducting screening means of the magnetron. 
         [0017]    Conveniently, the ground plane is electrically connected to the filter electrical screening means. 
         [0018]    Advantageously, the filter is arranged to filter stray radiation with frequencies in a range 100 MHz to 1 GHz. 
         [0019]    Alternatively, the filter is arranged to filter stray radiation with frequencies in a range 100 MHz to 2 GHz. 
         [0020]    Conveniently, the filter is arranged to filter stray radiation from a magnetron producing at output at a frequency of substantially 900 MHz. 
         [0021]    According to a second aspect of the invention, there is provided a method for reducing stray emissions from a magnetron, using a low-pass filter attached to an exterior face of a wall of electrically conducting screening means encasing the magnetron and encasing an associated isolation transformer means electrically connected to terminals of the magnetron; wherein an output connection of the filter passes directly through an interface between the electrically conducting screening means and the filter to connect electrically, directly or indirectly, with the isolation transformer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: 
           [0023]      FIG. 1  is a schematic drawing of a magnetron and heater supply for use with the invention; 
           [0024]      FIG. 2  is a circuit diagram of a filter according to the invention; 
           [0025]      FIG. 3  is a schematic layout of the filter of  FIG. 2 ; and 
           [0026]      FIG. 4  is a schematic side view and a base view of the case and connections of the filter of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Referring to  FIG. 1 , a microwave radiation source  10 , suitable for use with the invention, comprises a magnetron  11  with associated solenoid and waveguide launch section as shown, located in an electrically screened chamber  16 . Also within the screened chamber  16  is an isolation transformer  14  connected to heater and cathode connections  12  of the magnetron  11  by output leads  13 . Inputs of the isolation transformer are connected by input leads  15  to outputs  3 ,  4  of a filter  17  located externally on a wall of the screened chamber  16 . Inputs  1 ,  2  of the filter  17  are connected by leads  18  to outputs of a heater supply inverter  19  external of the screened chamber  16 . Locating the filter  17  outside the screened chamber  16  has the advantage of screening the filter components from the stray radiation  23  within the screened chamber  16 . 
         [0028]    A circuit diagram of an embodiment of the filter  17  according to the invention is shown in  FIG. 2 , with a schematic layout of the filter shown in  FIG. 3 . There is provided a simple low cost PCB based filter  17  according to the invention to reduce conducted emissions from a screened chamber  16  screening a magnetron  11 . The filter  17  causes no significant distortion to a 600 V peak (1200 V peak to peak) 15 kHz trapezoidal waveform, illustrated in insert  21  in  FIG. 1 , that is used to provide drive to, and monitor, the current and voltage of an isolation transformer  14  mounted in the screened chamber  16 . Loss due to a primary current of 6 A rms at 15 kHz is similarly kept low, less than 2 W being desirable. 
         [0029]    Referring to  FIG. 2 , the circuit comprises a first line  171  between a first input connection  1  and a first output connection  3 ; a second line  172 , parallel to the first line, between a second input connection  2  and a second output connection  4 ; and an earth plane  173  between the first line  171  and the second line  172 . 
         [0030]    The first line  171  comprises a first inductor L 1  and a third inductor L 3  connected in series. A first resistor R 1  and a first capacitor C 1  are connected in series between the first line  171  and the ground plane  173  at a point between the first input connection  1  and the first inductor L 1 . A third capacitor C 3  is also connected between the first line  171  and the ground plane  173  at a point between the first resistor R 1  with the first capacitor C 1  in series and the first inductor L 1 . A fifth capacitor C 5  is connected between the first line  171  and the ground plane  173  at a point between the first inductor L 1  and the third inductor L 3 . A seventh capacitor C 7  is connected between the first line  171  and the ground plane  173  at a point between the third inductor L 3  and the first output connection  3 . 
         [0031]    The second line  172  comprises a second inductor L 2  and a fourth inductor L 4  connected in series. A second resistor R 2  and a second capacitor C 2  are connected in series between the second line  172  and the ground plane  173  at a point between the second input connection  2  and the second inductor L 2 . A fourth capacitor C 4  is also connected between the second line  172  and the ground plane  173  at a point between the second resistor R 2  with the second capacitor C 2  in series and the second inductor L 2 . A sixth capacitor C 6  is connected between the second line  172  and the ground plane  173  at a point between the second inductor L 2  and the fourth inductor L 4 . An eighth capacitor C 8  is connected between the second line  172  and the ground plane  173  at a point between the fourth inductor L 4  and the second output connection  4 . 
         [0032]    With a suitable choice of component values, at 900 MHz the filter attenuation is around 55 dB or better. Roll off starts at 120 MHz at 3 dB attenuation, that is there is 3 dB attenuation at 120 MHz rising to substantially 55 dB attenuation at 900 MHz. This filter performance is provided for each line of the line drive  18  from the heater supply inverter  19  and filters a noise voltage on each line  18  with respect to earth. For the filter to be effective the third to eighth capacitors C 3  to C 8  have very low inductance and the connections  3  and  4  to the seventh and eight capacitors C 7  and C 8  are directly to the capacitor plates without any leads, as best seen in  FIGS. 3 and 4 . That is, by using a PCB capacitor, the connections are directly to the plates of the capacitors via feedthrough connections  3  and  4  through the printed circuit board. As shown in  FIG. 3 , the first and second input connections  1  and  2  are similarly directly connected to plates of the third and fourth capacitors, C 3  and C 4 , respectively. Although in the presently preferred embodiment the connections  3  and  4  are connected by through holes directly to the capacitor plates, it will be understood that alternatively the through holes may be connected to conductors on the printed circuit board which are connected to the capacitor plates. Moreover, it will be understood that in an alternative arrangement, direct connection to capacitors could be made without the use of a printed circuit board. Moreover, although the isolation transformer is shown in  FIG. 1  connected by input leads  15  to the filter  17  mounted on an external face of the wall of the screened chamber  16 , it will be understood that the isolation transformer may alternatively be mounted on an inner face of the screened chamber  16  opposed to the external face on which the filter is mounted, so that the filter may be directly electrically connected to the isolation transformer without a requirement for the input leads  15 . 
         [0033]    The filter is based upon a double-sided 1.0 mm thick FR4 board  175  with one side a ground plane  173  with all components surface mounted on the upper face opposed to the ground plane. A soldered case  174  bonded to the ground plane  173  provides full screening to the filter unit  17 . 
         [0034]    Also shown in  FIG. 2  is an alternative arrangement of the input connections  1 ′ and  2 ′ which includes additional feed-through capacitances C 9  and C 10  respectively in the walls of the screened case  174  if additional attenuation is required. 
         [0035]    As best seen in  FIG. 3 , the size of the printed circuit board  175  for the filter  17  is determined primarily by the size of the third to eighth capacitors C 3  to C 8 . These capacitors each comprise, for example, a 22 mm by 22 mm square with a 5 mm gap between each capacitor and between the capacitors and side walls of the screened case  174 . 
         [0036]    Each inductor L 1  to L 4  comprises, for example, six equally spaced turns of 1.0 mm tinned copper wire, wound on a 13 mm long 10 mm diameter former. Tinned copper is preferred to enamelled copper because of the greater loss of enamelled wire when the majority of the current is subject to the skin effect at high frequencies. As shown in  FIG. 3 , coils of the first and second inductors L 1  and L 2  are mounted at right angles to the coils of the third and fourth inductors L 3  and L 4 , to minimize coupling. This ensures that the required attenuation is achieved without a need for internal screening that would otherwise increase cost and mechanical complexity. 
         [0037]    The first and second resistors R 1 , R 2 , (e.g. 100 ohm 0.5 W carbon) and first and second capacitors C 1 , C 2  (e.g. 150 pF 1 kV NPO SM (i.e. surface mounted) ceramic) ensure the filter does not have any passband gain by providing low frequency damping and matching. It will be understood that NPO ceramic is a class of ceramic dielectric that is stable over a wide temperature and voltage range. These component values are required because the source and load impedances of the filter are unknown when the components are optimised for their primary filtering purpose. This usually gives undefined impedance at a frequency of the stray radiation  23 . Values of capacitance and resistance respectively of the first and second capacitors C 1  and C 2  connected in series with the matching first and second resistors R 1  and R 2  are chosen to ensure a low reactance at the stray radiation frequencies but to provide insignificant impedance to the waveform output from the heater supply inverter  16 . 
         [0038]    As best shown in  FIG. 4 , filter input connections  1  and  2  pass through the screened case  174  to the PCB with suitable voltage clearance for  600  V. Filter output connections  3  and  4  pass straight through the side wall of the magnetron screened chamber  16  when the filter is externally mounted on a wall of the screened chamber  16 . That is, connections  3  and  4  are mounted on a side wall of the screened chamber  16 . The first and second output connections  3  and  4  pass through the ground plate with suitable clearance for the voltage rating provided by circular apertures  176  in the ground plane. The ground plane  173  is bonded on assembly to the magnetron compartment screen  16  to make electrical connection. 
         [0039]      FIG. 4  shows an overall arrangement of the filter  17 . The ground plane  173  is electrically connected to a top face perimeter of the upper layer of the PCB again with suitable clearance from the components for the voltage rating used. Connection of the ground plate to a perimeter of the opposed face of the PCB is provided by a plurality of plated through holes  177  or as an alternative by fully plating over the edge of the PCB. The spacing of the plated through holes is less than 0.05 of a wavelength to provide effective shielding. For 900 MHz a spacing of 1.0 cm suffices. The screened case  174  of the filter  17  is provided with an outward facing flange  1741  where the walls of the screen case meet the PCB to accommodate the plated through holes  177  and for fixing the filter  17  to the wall of the screened chamber  16  and making electrical connection thereto. 
         [0040]    An advantage of the present invention is therefore that the step-down isolation transformer  14 , as shown in Applicant&#39;s co-pending application GB 0919718.7, is moved into the magnetron enclosure  16 , so that filtering can be carried out on lower currents than would be the case with filtering between the isolation transformer and magnetron, for example, the isolation transformer  14  (and rectifier) might have 240 volt at 6 amps on its input and 12 volt at 120 amps on its output. A suitable heater supply typically operates at 15 kHz but heater supplies with frequencies in the range 10 kHz to 500 kHz are known. 
         [0041]    The filter  17  is positioned outside the magnetron enclosure  16 . If it were within the screened chamber, although its output would be duly filtered, further stray radiation  23  could be picked up on the filtered output which would then be carried on the output leads through the screened magnetron chamber  16 . Also, there are no electrical leads outside the magnetron enclosure leading to the filter, which could pick up stray radiation. 
         [0042]    The filter minimizes stray capacitance on the inductances, and stray inductance on the capacitors, promoted by surface mounting. 
         [0043]    The filter passes the heater supply current with a frequency of 15 kHz, which may be compared with a domestic cooker magnetron, in which the heater supply is at a frequency of only 50 Hz. 
         [0044]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0045]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
         [0046]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.