Tunable and bandwidth programmable multi-element filter system

• US 5,783,977 A
• Filed: 02/07/1996
• Issued: 07/21/1998
• Est. Priority Date: 02/07/1996
• Status: Expired due to Term

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Loral Aerospace Corp. (MHR Holdings LLC (New York))
• Inventors: Chethik, Frank
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Bettendorf, Justin P.
• Application Number:
  US08/597,844
• Time in Patent Office: 895 Days
• Field of Search: 333/17.1, 333/139, 333/167, 333/156, 333/174, 455/125, 455/266, 455/340, 324/616
• US Class Current: 333/174
• CPC Class Codes: H03H 11/04 Frequency selective two-port networks
• CPC Class Codes: H03H 17/0294 Variable filters; Programmable filters

Abstract

A method and apparatus which realize an RF bandpass filter having a tunable bandwidth and center frequency over a large range of the radio frequency spectrum. The filter is comprised of a cascade of filter elements. Each element is comprised of a signal splitter which simultaneously feeds a signal to a delay line and a phase shifter in a parallel path. The delayed and phase shifted signals are summed. Each filter element exhibits periodic passbands and alternating transmission zeroes. Each successive filter element has a decreased delay by an incremental factor (e.g., typically 1.2), and exhibits an increase in the frequency intervals between stopband zeroes. The bandwidth is controlled by selecting a number of "activated" ones of the successive elements. A center frequency tuning is performed by adjusting a phase shift coefficient of at least one filter element. Such adjustments are performed by applying a test signal at each desired stopband zero frequency and adjusting the phase shift quantity of the element that is required to create the desired transmission zero. The residual power at the test frequency and transmission zero is measured at the filter output at the test signal frequency and the element'"'"'s phase shifter is adjusted until a minimum output power is observed. Per the continuous sequential applications of the test signal at each of the stopband zero frequencies, and readjustments of each of the corresponding phase shifts, the filter parameters can be maintained as desired. The test sequencing and parameter controls are performed by digital computations.

Claims

1. A method for locating stopband transmission zeroes exhibited by a filter system at desired locations on a frequency spectrum, comprising the steps of:

applying a test signal having a predetermined frequency to an input port of a plurality of serially-coupled filter elements, each of the plurality of filter elements having associated stopband transmission zeroes of adjustable frequencies, an input of a first one of the plurality of filtering elements being coupled to the input port;

detecting a magnitude of a signal that is output from a last one of the plurality of filter elements, wherein the output signal exhibits transmission stopband zeroes having frequencies that are a function of the frequencies of the transmission stopband zeroes of each of the plurality of filter elements; and

recursively adjusting the frequencies of the stopband transmission zeroes associated with at least one of the plurality of filter elements to minimize a magnitude of a signal that is output from the last one of the plurality of filter elements at the frequency of the test signal.

2. A method as set forth in claim 1, wherein each of the plurality of filter elements has an input and an output, further comprising the steps of:

delaying within each one of the plurality of filter elements a signal received at the input by a fixed amount to produce a delayed signal, phase shifting the signal received at the input by an adjustable amount to produce an adjustably-phase shifted signal, combining the delayed and adjustably-phase shifted signals, and supplying a combined signal to the output.

3. A method as set forth in claim 2, wherein in each of the plurality of filter elements the amount of fixed delay is a function of, and differs from, the amount of fixed delay of others of said plurality of filter elements.

4. A method as set forth in claim 2, wherein the frequencies of the stopband transmission zeroes associated with each filter element are a function of the amount of adjustable phase shift of the filtering element.

5. A method as set forth in claim 4, wherein the step of recursively adjusting varies the adjustable phase shift of at least one of the plurality of filter elements.

6. A method as set forth in claim 2, wherein the step of phase shifting the signal by an adjustable amount is accomplished by multiplying the signal by at least one complex coefficient having a magnitude nominally equal to one.

7. A method as set forth in claim 1, wherein the plurality of filter elements comprise at least five filter elements.

8. A method as set forth in claim 1, wherein the step of applying is performed by:

summing the test signal with a received RF signal to produce a summation signal; and

applying the summation signal to the input port of the plurality of filtering elements.

9. A method for tuning a bandpass filter exhibiting a plurality of stopband transmission zeroes for RF energy within a band of frequencies, comprising the steps of:

applying a received RF signal to an input port of a plurality of serially-coupled filter elements, each of the plurality of filter elements having an input and an output, the input of a first one of the plurality of filter elements being coupled to the input port;

time delaying within each one of the plurality of filter elements a signal received at the input by a fixed amount to produce a delayed signal, phase shifting the signal received at the input by an adjustable amount to produce an adjustably-phase shifted signal, combining the delayed and adjustably-phase shifted signals, and supplying a combined signal to the output; and

outputting from a last one of the plurality of filter elements a minimum RF signal energy at a predetermined frequency, which RF signal energy is a function of an amount by which a signal is adjustably phase shifted within at least one filter element;

wherein

the step of time delaying performed within each of the plurality of filter elements delays a signal received at the input by an amount of fixed delay that is a function of, and differs from, the amount of fixed delay of others of said plurality of filter elements.

10. A method as set forth in claim 9, wherein within each of the plurality of filter elements the amount of fixed delay differs from the amount of fixed delay of each of the other ones of said plurality of filter elements by a fixed ratio.

11. A method for adjusting the bandwidth of at least one passband of a filter system, comprising the steps of:

applying a received RF signal to an input port of a plurality of serially-coupled ρ

sections, each of the plurality of ρ

sections having an input, an output, and a characteristic delay value that differs from that of others of the plurality of ρ

sections, the input of a first one of the plurality of ρ

sections being coupled to the input port, each of the plurality of ρ

sections being controllable to perform either a first operation or a second operation, a signal being output from a last one of the plurality of ρ

sections exhibiting at least one passband of the filter system; and

controlling at least one of the plurality of ρ

sections to perform either the first operation or the second operation until the at least one passband of the filter system has a desired bandwidth and center frequency;

wherein to perform the first operation, a ρ

section chase shifts a signal received at the input by a fixed phase shift amount and supplies a resultant first phase shifted signal to the output, and to perform the second operation, the ρ

section phase shifts the signal received at the input by an amount that is a combination of the fixed phase shift amount and a variable phase shift amount, and supplies a resultant second phase shifted signal to the output.

12. A method for adjusting the bandwidth of at least one passband of a filter system, comprising the steps of:

applying a received RF signal to an input port of a plurality of serially-coupled ρ

sections, each of the plurality of ρ

sections having an input, an output, and a characteristic delay value that differs from that of others of the plurality of ρ

sections, the input of a first one of the plurality of ρ

sections being coupled to the input port, each of the plurality of ρ

sections being controllable to perform either a first operation or a second operation, a signal being output from a last one of the plurality of ρ

sections exhibits at least one passband of the filter system; and

controlling at least one of the plurality of ρ

sections to perform either the first operation or the second operation until the at least one passband of the filter system has a desired bandwidth and center frequency;

wherein to perform the first operation, a ρ

section forwards a signal received at the input to the output, and to perform the second operation, the ρ

section filters the signal received at the input and supplies a filtered signal to the output, wherein the step of forwarding performed by the ρ

section during the first operation includes the steps of;

delaying the signal received at the input by a fixed amount to produce a delayed signal; and

multiplying the signal received at the input by a multiplication vector magnitude to produce a multiplied signal having a substantially zero amplitude;

combining the delayed and multiplied signals to provide a combination signal to the output.

13. A method for adjusting the bandwidth of at least one passband of a filter system, comprising the steps of:

applying a received RF signal to an input port of a plurality of serially-coupled ρ

sections, each of the plurality of ρ

sections having an input, an output, and a characteristic delay value that differs from that of others of the plurality of ρ

sections, the input of a first one of the plurality of ρ

sections being coupled to the input port, each of the plurality of ρ

sections being controllable to perform either a first operation or a second operation, a signal being output from a last one of the plurality of ρ

sections exhibits at least one passband of the filter system; and

controlling at least one of the plurality of ρ

sections to perform either the first operation or the second operation until the at least one passband of the filter system has a desired bandwidth and center frequency;

wherein to perform the first operation, a ρ

section forwards a signal received at the input to the output, and to perform the second operation, the ρ

section filters the signal received at the input and supplies a filtered signal to the output, wherein the steps of filtering and supplying performed by the ρ

section during the second operation include the steps of;

delaying the signal received at the input by a fixed amount to produce a delayed signal;

multiplying the signal received at the input by a programmable multiplication vector magnitude to produce a multiplied signal exhibiting a phase shift; and

combining the delayed and multiplied signals to provide a filtered signal to the output, wherein the filtered signal exhibits at least one passband bandwidth that is a function of the programmable multiplication vector magnitude.

14. A tunable bandpass filter exhibiting a plurality of stopband zeroes for RF energy within a band of frequencies, comprising:

an input port for receiving an RF signal to be filtered;

an output port for outputting a filtered RF signal;

a plurality of serially-coupled filtering means interposed between said input port and said output port, each of said plurality of filtering means having an input and an output, an input of a first one of said plurality of filtering means being coupled to said input port, each of said plurality of filtering means comprising;

fixed phase shift means;

programmable variable phase shift means;

splitter means coupled to said input and providing a first output and a second output, said first output being coupled to said fixed phase shift means and said second output being coupled to said programmable variable phase shift means;

means for combining an output of said fixed phase shift means with an output of said programmable variable phase shift means, said combining means of a last one of said plurality of filtering means being coupled to said output port and providing a minimum RF signal energy at a predetermined frequency to said output port, which minimum RF signal energy is a function of a phase shift within said programmable variable phase shift means;

wherein

in each of said plurality of filtering means said fixed phase shift means has a predetermined phase shift that is a function of, and that differs from, the predetermined phase shift of the fixed phase shift means of other ones of said plurality of filtering means.

15. A tunable filter as set forth in claim 14 wherein the programmable variable phase shift means is a programmable multiplying means.

16. A tunable filter as set forth in claim 14, wherein said fixed phase shift means of each of said plurality of filtering means has a predetermined phase shift that differs by a constant amount from the predetermined phase shift of the fixed phase shift means of each of the other ones of said plurality of filtering means.

17. A tunable filter exhibiting a plurality of stopband zeroes for RF energy within a band of frequencies, comprising:

an input port for receiving an RF signal to be filtered;

an output port for outputting a filtered RF signal;

a plurality of serially-coupled filtering means interposed between said input port and said output port, each of said plurality of filtering means having an input and an output, an input of a first one of said plurality of filtering means being coupled to said input port, each of said plurality of filtering means comprising;

fixed phase shift means;

programmable variable phase shift means;

splitter means coupled to said input and providing a first output and a second output, said first output being coupled to said fixed phase shift means and said second output being coupled to said programmable variable phase shift means;

means for combining an output of said fixed phase shift means with an output of said programmable variable phase shift means, said combining means of a last one of said plurality of filtering means being coupled to said output port and providing a minimum RF signal energy at a predetermined frequency to said output port, which minimum RF signal energy is a function of a phase shift within said programmable variable phase shift means;

wherein

in each of said plurality of filtering means said fixed phase shift means has a predetermined phase shift that is a function of, and that differs from, the predetermined phase shift of the fixed phase shift means of other ones of said plurality of filtering means, and wherein said tunable filter further comprises;

programmable frequency generation means, for generating a test signal at a predetermined frequency;

means for summing said test signal with said received RF signal and providing a summation signal to said input port;

means for detecting an amount of RF signal energy that is emitted from said output port; and

means for recursively programming said programmable variable phase shift means of at least one of said plurality of filtering means to minimize the RF signal energy at said test signal frequency that is emitted from said output port.

18. A tunable filter exhibiting a plurality of stopband zeroes for RF energy within a band of frequencies, comprising:

an input port for receiving an RF signal to be filtered;

an output port for outputting a filtered RF signal;

a plurality of serially-coupled filtering means interposed between said input port and said output port, each of said plurality of filtering means having an input and an output, an input of a first one of said plurality of filtering means being coupled to said input port, each of said plurality of filtering means comprising;

fixed phase shift means;

programmable variable phase shift means;

splitter means coupled to said input and providing a first output and a second output, said first output being coupled to said fixed phase shift means and said second output being coupled to said programmable variable phase shift means;

means for combining an output of said fixed phase shift means with an output of said programmable variable phase shift means, said combining means of a last one of said plurality of filtering means being coupled to said output port and providing a minimum RF signal energy at a predetermined frequency to said output port, which minimum RF signal energy is a function of a phase shift within said programmable variable phase shift means;

wherein

in each of said plurality of filtering means said fixed phase shift means has a predetermined phase shift that is a function of, and that differs from, the predetermined phase shift of the fixed phase shift means of other ones of said plurality of filtering means, and wherein said tunable filter further comprises;

means for detecting an amount of RF signal energy that is emitted from said output port; and

means for recursively programming said programmable variable phase shift means of at least one of the plurality of filtering means to minimize the RF signal energy emitted from said output port at a predetermined frequency. means.

19. A tunable filter exhibiting a plurality of stopband zeroes for RF energy within a band of frequencies, comprising:

an input port for receiving an RF signal to be filtered;

an output port for outputting a filtered RF signal;

a plurality of serially-coupled filtering means interposed between said input port and said output port, each of said plurality of filtering means having an input and an output, an input of a first one of said plurality of filtering means being coupled to said input port, each of said plurality of filtering means comprising;

fixed phase shift means;

programmable variable phase shift means;

splitter means coupled to said input and providing a first output and a second output, said first output being coupled to said fixed phase shift means and said second output being coupled to said programmable variable phase shift means;

means for combining an output of said fixed phase shift means with an output of said programmable variable phase shift means, said combining means of a last one of said plurality of filtering means being coupled to said output port and providing a minimum RF signal energy at a predetermined frequency to said output port, which minimum RF signal energy is a function of a phase shift within said programmable variable phase shift means;

wherein

in each of said plurality of filtering means said fixed phase shift means has a predetermined phase shift that is a function of, and that differs from, the predetermined phase shift of the fixed phase shift means of other ones of said plurality of filtering means, and wherein the plurality of filtering means comprise at least five filtering means.

20. An RF bandpass filter having a tunable bandwidth and center frequency, said bandpass filter comprising a cascade of individual filter elements, each filter element comprising a signal splitter which simultaneously feeds a signal to a delay line and to an adjustable phase shifter in parallel with said delay line, and means for summing the delayed and phase shifted signals;

each filter element exhibiting periodic passbands and alternating transmission zeroes, and each successive filter element of said cascade of filter elements having a decreased delay in said delay line from a preceding filter element and exhibiting an increase in the frequency intervals between stopband zeroes.

21. An RF bandpass filter as set forth in claim 20, and further comprising means for controlling the bandwidth of said passband filter by selecting a number of activated ones of the successive filter elements.

22. An RF bandpass filter as set forth in claim 20, and further comprising means for tuning a center frequency of said filter, said tuning means comprising means for adjusting a phase shift coefficient of at least one filter element.

23. An RF bandpass filter having a tunable bandwidth and center frequency, said bandpass filter comprising a cascade of individual filter elements, each filter element comprising a signal splitter which simultaneously feeds a signal to a delay line and to an adjustable phase shifter in parallel with said delay line, and means for summing the delayed and phase shifted signals;

each filter element exhibiting periodic passbands and alternating transmission zeroes, and each successive filter element of said cascade of filter elements having a decreased delay in said delay line from a preceding filter element and exhibiting an increase in the frequency intervals between stopband zeroes;

wherein said RF bandpass filter further comprises means for tuning a center frequency of said filter, said tuning means comprising means for adjusting a phase shift coefficient of at least one filter element;

wherein said adjusting means is comprised of;

means for applying a test signal at each desired stopband zero frequency; and

means for controllably adjusting the phase shift quantity of a filter element that is required to create a desired one of the transmission zeroes.

24. An RF bandpass filter as set forth in claim 23, wherein said controllably adjusting means is comprised of:

means for measuring a residual power at the test frequency and transmission zero at an output of said bandpass filter; and

wherein said controllably adjusting means adjusts at least one of a frequency of the test signal and the filter element'"'"'s phase shifter until a minimum output power is observed at the output of said bandpass filter.

25. An RF bandpass filter as set forth in claim 23, wherein said controllably adjusting means is comprised of:

means for measuring a residual power at the test frequency and transmission zero at an output of said bandpass filter; and

wherein said controllably adjusting means recursively applies the test signal at a plurality of said stopband zero frequencies, and readjusts a phase shift of a corresponding filter element'"'"'s phase shifter until a minimal residual power at the test frequency is measured at said output of said bandpass filter.