Claims:1. A harmonic suppression switched filter bank, the filter bank comprising:
a input transmission line (A) to transfer the RF signal in required path to input switch (D);
a input (SP7T (D) ) switch constructed using PIN diodes (G), RF chokes (F) and DC block capacitors coupled to the input transmission line to receive RF signal, wherein the input switch is for activating and deactivating the function of filter using diode;
a plurality of band selection (low pass) filters (C) are designed independently and coupled to an electronically controllable input (SP7T (D)) and an output (SP7T (E) ) switches on the same planar layer, wherein the band selection filters are designed using lumped inductors and capacitors to meet the requirement of miniaturization and desired performance;
the output (SP7T (E) ) switch constructed using PIN diodes, RF chokes and DC block capacitors coupled to plurality of band filters;
a output transmission line (B) coupled to the output switch, wherein the output transmission is to receive the output RF signal.

2. The filter bank as claimed in claim 1, further comprises a control mechanism to the switches for selecting the particular channel or filter.

3. The filter bank as claimed in claim 1, wherein the RF chokes of the switches are tuned and optimized by rigorous study to block RF completely and allow DC with high DC current handling capability.

4. The filter bank as claimed in claim 1, wherein the isolation of filter bank between “ON” and “OFF” channels is better than 60dB.

5. The filter bank as claimed in claim 1, wherein the filter bank comprises seven low pass filters and tested before assembling the complete structure.

6. The filter bank as claimed in claim 1, wherein the switched filter bank is designed on six layer printed circuit board to achieve the miniaturization.

7. The filter bank as claimed in claim 1, wherein the achieved size of the switched filter bank is 127mm X 82mm.

8. The filter bank as claimed in claim 1, wherein the filter bank further constructed with multi-layer printed circuit board (H), input connector (I), output connector (J) and control line connector (k), where the RF signals are transmitted via input connector (I) to input transmission line (A) and collected via output connector (J) at output transmission line (B).

9. The filter bank as claimed in claim 1, wherein the Integration of plurality of low pass filters and SP7T switches is to make compact harmonic suppression switched filter bank and reduces large real estate area for building the filter bank.
, Description:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
“A Compact High Power Harmonic Suppression Switched filter bank”
By
Bharat Electronics Limited,
Central Research Laboratory
Jalahalli P.O., Bangalore – 560013

The following specification particularly describes the invention and the manner in which it is to be performed.

Field of the Invention
The present invention mainly relates to radio frequency electronic circuits and more particularly to an integrated switched filter bank which has high power handling and good harmonic rejection characteristics.
Background of the invention
Radio frequency electronic circuits are well known in the art which is concerned with electronic circuits which receive or transmit radio signals. Typically, such circuits must operate at radio frequency and power levels which impose special constraints on their design. These constraints increase in their importance with higher frequencies. At microwave frequencies, the reactance of signal traces becomes a crucial part of the physical layout of the circuit.
Selection of radio frequency filters calls for integrated switched filter banks in communication transceivers. Basically, the filter bank is an array of band-pass filters that separates the input signal into multiple components, each one carrying a single frequency sub-band of the original signal. One application of a filter bank is a graphic equalizer, which can attenuate the components differently and recombine them into a modified version of the original signal.
Switched filter banks are commonly used in frequency hopping radios and multi-band communication systems where high isolation between each filter and harmonic rejection are vital requirements. Filter elements play an important role in the design of switchable filter banks. The switched filter bank is mostly constructed with a bank of filtering circuits with a switch at the input and output to select a particular filtering circuit of choice. An input and output switching network, which is controlled by a controller card, selects the desired filter. Filters are typically wide in pass band and used to reject the unwanted signals like image frequencies and other spurious frequencies in the receiver units. In transmitter units, filters are used to reject unwanted spurious and harmonics.
Planar transmission medium using microstrip and striplines for the implementation of filters employ resonators which have dimensions in the range of sub wavelengths at the centre frequency of operation. These filters are easy to design but it occupies more space for the required pass band and harmonic suppression (better than 60dB) when compared with the lumped component filters.
Cavity and waveguide filters offer low loss and better harmonic rejection characteristics. On the other hand, these filters require large real estate area for building the switched filter bank when compared to planar and lumped element filters in practical transceivers.
For example, in WO 2006022932 A1 titled “Switched filter bank and method of making the same” describes about a switched filter bank and method of forming filter bank using stripline filters with various pass bands and active assembly circuits. The limitation of this product is the larger size due to the formation of stripline circuits.
Further, in CA 2539776 A1 titled “Miniature broadband switched filter bank” discloses about stacking of substrates for designing broadband switched filter bank using stripline configurations. Microelectro mechanical switches are used for the selection of filters. Stripline filters offer larger size at ultra high frequency (UHF) and very high frequency (VHF) bands.
Further, in US 6414570 B1 titled “Low profile, high isolation and rejection x-band switched filter assembly” discusses a multi-channel microwave switched filter bank, wherein the input circuitry and output circuitry are mounted on opposite sides of the filter bank, providing a compact structure with excellent isolation. This structure requires accurate rack structure and fabrication procedure.
Furthermore, in US 4763089 A titled “Microwave multiband filter” reports the multiband microwave filter that can selectively transmit a broadband signal or combination of predefined frequency bands. Switch is used to selectively transmit an input signal to the output.
All of the above mentioned prior art have their own limitations. In view of the above, there is a need in the art for an integrated switched filter bank to solve the above mentioned limitations.

Objective of the Invention
The main objective of the present invention is to construct a bank of compact radio frequency filters which is controlled to select a particular desired filter for transmission and unselect other filters at a time.
Another objective of the present invention provides the details of optimizing the RF chokes for blocking RF completely and allowing DC with high DC current handling capability. These chokes offer very low DC resistance in the switched filter circuits.
Further objective of the present invention is to provide compact harmonic suppression switched filter bank which reduces large real estate area for building the filter bank.
Summary of the Invention
An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below.
Accordingly, in one aspect of the present invention relates to a harmonic suppression switched filter bank, the filter bank comprising: a input transmission line (A) to transfer the RF signal in required path to input switch (D), a input (SP7T (D)) switch constructed using PIN diodes (G), RF chokes (F) and DC block capacitors coupled to the input transmission line to receive RF signal, wherein the input switch is for activating and deactivating the function of filter using diode, a plurality of band selection (low pass) filters (C) are designed independently and coupled to an electronically controllable input (SP7T (D)) and an output (SP7T (E) ) switches on the same planar layer, wherein the band selection filters are designed using lumped inductors and capacitors to meet the requirement of miniaturization and desired performance, the output (SP7T (E) ) switch constructed using PIN diodes, RF chokes and DC block capacitors coupled to plurality of band filters, a output transmission line (B) coupled to the output switch, wherein the output transmission is to receive the output RF signal.
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
Brief description of the drawings
The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:
Figure 1 shows the circuit level representation of switched filter bank (SFB) with seven low pass filters and two SP7T switches in its electronic circuitry according to one embodiment of the present invention.
Figure 2 shows the block level representation of proposed switched filter bank designed according to one embodiment of the present invention.
Figure 3 shows the topology of the low pass filter used in the design of the Switched filter bank structure according to one embodiment of the present invention.
Figure 4 shows the circuit level representation for the construction of SP7T switch for each channel path which is used for controlling the transmit band in the switched filter bank according to one embodiment of the present invention.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
Detailed description of the invention
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
Figs. 1 through 4, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way that would limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged communications system. The terms used to describe various embodiments are exemplary. It should be understood that these are provided to merely aid the understanding of the description, and that their use and definitions, in no way limit the scope of the invention. Terms first, second, and the like are used to differentiate between objects having the same terminology and are in no way intended to represent a chronological order, unless where explicitly stated otherwise. A set is defined as a non-empty set including at least one element.
A Radio Frequency (RF) filter can selectively pass a band of radio frequency signals and reject the other unwanted frequency bands. This invention discloses the design of a compact and high power integrated switched filter bank and method of forming an integrated switched filter bank product.
The present invention is a compact switched filter bank having seven band selection filters and two SP7T switches at the input and output. SP7T Switches are incorporated at the input and output transmission lines where signals are fed and taken out. Switch performs selective transmission of input signal to the output. There are seven low pass filters which are band selected by controlling the switches. The electronic control mechanism is done by applying appropriate bias voltages to each of the PIN diodes in the SP7T switch circuit configurations. Multi-layer printed circuit board design of switched filter bank has been carried out in a way to miniaturize the overall product.
Each band selection filter is designed independently and connected to an electronically controllable switch on the same planar layer. Filters are designed using lumped inductors and capacitors to meet the requirement of miniaturization and desired performance. Switches are constructed using PIN diodes, RF chokes and DC block Capacitors.
The proposed switched filter bank circuit comprises of two active SP7T switch circuits and series of filters on top surface of the printed circuit board. The bottom surface of the printed circuit board is a uniform ground. RF chokes in control voltage paths of the SP7T switches have been thoroughly studied and optimized in this invention to block RF completely and allow DC with high DC current handling capability. These chokes offer very low resistance to DC in the switched filter circuits. An experimental switched filter bank has been designed and developed to illustrate the present innovation. This integrated switch filter bank offers high harmonic rejection of the order of 60dB and isolation between ON and OFF better than 60dB.
Yet another aspect of the invention relates to a method of printed circuit board designing of switched filter bank. Board design of Switched filter bank has been done in way to miniaturize the overall circuit. This switched filter bank offers miniaturization, high power handling and good harmonic rejection characteristics.
Further, the proposed high power, miniaturized switched filter circuit involves multilayer printed circuit board design and appropriate selection of inductors and capacitors to achieve the required filtering characteristics. Further PIN diodes have been used to design SP7T switches at the input and output side of the switched filter bank. Frequency band of each channel is determined to achieve better harmonic suppression and insertion loss characteristics.
Figure 1 shows the circuit level representation of switched filter bank (SFB) with seven low pass filters and two SP7T switches in its electronic circuitry according to one embodiment of the present invention.
The figure shows the circuit level representation of switched filter bank (SFB) with seven low pass filters and two SP7T switches in its electronic circuitry. The filter bank comprising of input transmission line (A), output transmission line (B), Filtering circuit C, input switching circuit D, output switching circuit (E), RF chokes (F), PIN diodes (G), multi-layer printed circuit board (H), input connector (I), output connector (J) and control line connector (K).
The input transmission line (A) is to transfer the RF signal in required path to input switch (D), a input (SP7T (D)) switch is constructed using PIN diodes (G), RF chokes (F) and DC block capacitors coupled to the input transmission line to receive RF signal, wherein the input switch is for activating and deactivating the function of filter using diode, a plurality of band selection (low pass) filters (C) are designed independently and coupled to an electronically controllable input (SP7T (D)) and an output (SP7T (E) ) switches on the same planar layer, wherein the band selection filters are designed using lumped inductors and capacitors to meet the requirement of miniaturization and desired performance, the output (SP7T (E) ) switch constructed using PIN diodes, RF chokes and DC block capacitors coupled to plurality of band filters, a output transmission line (B) coupled to the output switch, wherein the output transmission is to receive the output RF signal. The output RF signal is tapped out from the output switch circuitry through the transmission line B. Seven filter circuits (as in C) are connected between the input switch D and output switch E.
Further, the present invention is a radio frequency switched filter bank (SFB) product designed over 30MHz to 512MHz for wireless communication systems. The switched filter bank is comprised of a circuits constructed on multi layer printed circuit board. The multi-layer board consists of active switch circuits and plurality of lumped element filter circuits. Miniaturization is achieved using multi-layer substrate and lumped element components in the design of filters. The lumped element filter circuits are constructed one below and another on the top layer of the printed circuit board. The filters cover the frequency band of 30MHz to 520MHz. The complete proposed product is compact in size and provides higher isolation (more than 70dB) characteristics.
Figure 2 shows the block level representation of proposed switched filter bank designed according to one embodiment of the present invention.
The figure shows the block level representation of proposed switched filter. The filter bank is cheaper and can be manufactured easily for mass production. The new switched filter has seven low pass filters, switch circuit and the corresponding control circuitry.
Figure 3 shows the topology of the low pass filter used in the design of the Switched filter bank structure according to one embodiment of the present invention.
The figure shows the topology of the low pass filter used in the design of the Switched filter bank structure. The topology used for the construction of filter is shown in Figure 3. The values of inductors and capacitors have been determined based on the design and the parasitic effects of mounting pads which are part of the circuit. These seven filters have been independently tested before assembling the complete structure. The filter structure is ninth order Cauer-Chebyshev type and Offers better selectivity/Sharp rejection in the characteristics of the low pass filter.
Figure 4 shows the circuit level representation for the construction of SP7T switch for each channel path which is used for controlling the transmit band in the switched filter bank according to one embodiment of the present invention.
The figure shows the circuit level representation for the construction of SP7T switch for each channel path which is used for controlling the transmit band in the switched filter bank. The switch circuitry for each filter is shown in Figure 4. This circuit has RF chokes, DC block Capacitors and PIN diodes in the series and shunt configurations. RF chokes have been optimally selected to achieve the desired characteristics such completely blocking RF and allowing DC only. These chokes offer very low resistance to DC current and have high DC current carrying capability. The value of the RF chokes range from 1µH to 10µH. Seven such circuits are embedded along with the circuit of the filter shown in Figure 2 to form single-pole-7-throw (SP7T) switches at the input and output. Co-planar waveguide (CPW) transmission line with 50? characteristic impedance is used to carry the RF signal into the SFB circuit and has been used in this PCB design.
The present invention “switched filter bank” is realized on six layer FR4 substrate with copper metal signal pads, copper metallic transmission lines and metallic grounding pads. The required control signals for the switching operation are routed along the bottom and middle layers. This product should be mounted on a proper heat sink plate. This switch filter bank can be integrated with high power amplifiers using high power co-axial cables. The switched filter bank is a low loss, high power, harmonic suppression switched filter bank.
Operation of the Circuit
The present invention provides a high power switched filter bank with low loss and miniaturized size and harmonic suppressions in its electrical characteristics. The switched filter bank has input A and output B transmission lines, Filtering circuit C, input switching circuit D, output switching circuit E, RF chokes F, PIN diodes G, multi-layer printed circuit board H, input connector I, output connector J and control line connector K. When the RF signal is fed into the input connector I and then input transmission line A, it travels to the switching circuitry. The switching circuitries D, E which are embedded in each channel/filter will select the particular channel or filter C using the control lines fed into the control pin connector. The corresponding filtering circuit will be enabled and the filtered signal will be available at the output connector J through the output switching circuit E. The inductors and the capacitors have been optimized in the filter C to achieve insertion loss less than 3dB and the harmonic rejections better than 60dB upto fifth harmonics. The value of inductor varies from 14nH to 220nH. The values of the capacitor varies from 0.5pF to 150Pf The transmission lines, power lines and circuits have been optimized and designed in six layer RF4 printed circuit board with the size of 127mm X 82mm. The invented compact switched filter can handle the power upto 60 watts in the ratio of 1:5, ON: OFF time. The filter is designed with the specifications listed in Table 1 and 2 as per the following steps.
• Deciding upon Filter Architecture (Cauer Chebyshev type)
• Deciding upon the Filter Order for desired sharpness (Order is 9)
• Determining the lumped element prototype values for the required band of operation (The filter bands are listed in Table 1).
• Simulation with element parasitic model substitution
• PCB Fabrication, practical assembly and testing
The measured characteristics of switched filter bank are listed in Table 3. It is understood that this harmonic suppression filters offer rejection better than 60dB. The measured insertion loss is less than 3dB. Return loss is better than 12dB in all the filters. Isolation is mainly determined by SP7T Switch. If one switch “ON” one of the channels corresponding to the input frequency and Switch “OFF” all other channels, the amount of signal leakage form “ON” channel to “OFF” channel determines the isolation. Measured isolation is better than 60dB. The proposed SFB is compact and measures in the size of 127mm X 82mm.

Table I: Low Pass Filter Frequency Bands
Bands Pass band
Band 1 DC-45MHz
Band 2 DC-65MHz
Band 3 DC-95MHz
Band 4 DC-140MHz
Band 5 DC-220MHz
Band 6 DC-330MHz
Band 7 DC-520MHz
Table II: Switched filter bank specifications
Electrical parameters Values
Insertion loss Less than 3dB
Return loss Better than 10dB
Power handling Upto 60Watts
Higher order harmonic rejections More than 60dB

Table III: measured characteristics of switched filter bank
Bands Harmonic rejections(2fo onwards) Maximum Insertion Loss
Band 1 DC-45MHz > 60dB 2.9dB
Band 2 DC-65MHz > 60dB 2.5dB
Band 3 DC-95MHz > 60dB 2.0dB
Band 4 DC-140MHz > 60dB 1.9dB
Band 5 DC-220MHz > 60dB 2.4dB
Band 6 DC-330MHz > 60dB 3dB
Band 7 DC-520MHz > 60dB 3dB

The features of the present invention
The present invention provides miniaturization of the switched filter product through multi-layer printed circuit board H.
The usage of lumped element capacitors and inductors for constructing of filter C is for the purpose of miniaturization at VHF and UHF.
Optimized values of RF chokes E in switch configuration to block RF and allow DC.
The series and shunt PIN diodes F are used for the construction of switching circuits for handling high power.
The harmonic rejection characteristic is better than 60dB (up to 5th order).
Those skilled in this technology can make various alterations and modifications without departing from the scope and spirit of the invention. Therefore, the scope of the invention shall be defined and protected by the following claims and their equivalents.
Figs. 1-4 are merely representational and are not drawn to scale. Certain portions thereof may be exaggerated, while others may be minimized. Figs. 1-4 illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.
In the foregoing detailed description of embodiments of the invention, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description of embodiments of the invention, with each claim standing on its own as a separate embodiment.
It is understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein,” respectively.

We claim:

1. A harmonic suppression switched filter bank, the filter bank comprising:
a input transmission line (A) to transfer the RF signal in required path to input switch (D);
a input (SP7T (D) ) switch constructed using PIN diodes (G), RF chokes (F) and DC block capacitors coupled to the input transmission line to receive RF signal, wherein the input switch is for activating and deactivating the function of filter using diode;
a plurality of band selection (low pass) filters (C) are designed independently and coupled to an electronically controllable input (SP7T (D)) and an output (SP7T (E) ) switches on the same planar layer, wherein the band selection filters are designed using lumped inductors and capacitors to meet the requirement of miniaturization and desired performance;
the output (SP7T (E) ) switch constructed using PIN diodes, RF chokes and DC block capacitors coupled to plurality of band filters;
a output transmission line (B) coupled to the output switch, wherein the output transmission is to receive the output RF signal.

2. The filter bank as claimed in claim 1, further comprises a control mechanism to the switches for selecting the particular channel or filter.

3. The filter bank as claimed in claim 1, wherein the RF chokes of the switches are tuned and optimized by rigorous study to block RF completely and allow DC with high DC current handling capability.

4. The filter bank as claimed in claim 1, wherein the isolation of filter bank between “ON” and “OFF” channels is better than 60dB.

5. The filter bank as claimed in claim 1, wherein the filter bank comprises seven low pass filters and tested before assembling the complete structure.

6. The filter bank as claimed in claim 1, wherein the switched filter bank is designed on six layer printed circuit board to achieve the miniaturization.

7. The filter bank as claimed in claim 1, wherein the achieved size of the switched filter bank is 127mm X 82mm.

8. The filter bank as claimed in claim 1, wherein the filter bank further constructed with multi-layer printed circuit board (H), input connector (I), output connector (J) and control line connector (k), where the RF signals are transmitted via input connector (I) to input transmission line (A) and collected via output connector (J) at output transmission line (B).

9. The filter bank as claimed in claim 1, wherein the Integration of plurality of low pass filters and SP7T switches is to make compact harmonic suppression switched filter bank and reduces large real estate area for building the filter bank.

Abstract
The present invention mainly relates to radio frequency electronic circuits and more particularly to an integrated switched filter bank which has high power handling and good harmonic rejection characteristics. In one embodiment, the the filter bank comprising: a input transmission line (A) to transfer the RF signal in required path to input switch (D), a input (SP7T (D)) switch constructed using PIN diodes (G), RF chokes (F) and DC block capacitors coupled to the input transmission line to receive RF signal, wherein the input switch is for activating and deactivating the function of filter using diode, a plurality of band selection (low pass) filters (C) are designed independently and coupled to an electronically controllable input (SP7T (D)) and an output (SP7T (E) ) switches on the same planar layer, wherein the band selection filters are designed using lumped inductors and capacitors to meet the requirement of miniaturization and desired performance, the output (SP7T (E) ) switch constructed using PIN diodes, RF chokes and DC block capacitors coupled to plurality of band filters, a output transmission line (B) coupled to the output switch, wherein the output transmission is to receive the output RF signal.