Tunable microwave notch filter

• US 3,895,304 A
• Filed: 03/20/1974
• Issued: 07/15/1975
• Est. Priority Date: 03/20/1974
• Status: Expired due to Term

• Current Assignee: Gerald I. Klein
• Original Assignee: Gerald I. Klein
• Inventors: Klein, Gerald I.
• Primary Examiner(s): Lawrence, James W.
• Assistant Examiner(s): Nussbaum, Marvin
• Application Number:
  US05/452,863
• Time in Patent Office: 482 Days
• Field of Search: 333/31R,73R,76,73W,10,11 328/109,167 325/124,164
• US Class Current: 327/556
• CPC Class Codes: G01S 13/24 using frequency agility of carrier wave
• CPC Class Codes: H01P 1/20 Frequency-selective devices, e.g. filters

Abstract

A recursive microwave notch filter wherein each section is comprised of a pair of power dividers e.g. 3-db couplers joined by a short and a long microwave transmission line. Tuning of the frequency rejection notch is achieved by inserting an electrically controlled phase shifter having 360* of adjustment in the short transmission line. Two identical cascaded sections of the subject notch filter are required for coupling to the input of a saturated RF power amplifier, such as the output amplifier of an amplifier chain utilized in present day radar transmitters, in order to obtain the desired frequency rejection notch, due to the gain compression characteristic present in an amplifier when driven into its saturation region.

Claims

1. A microwave notch filter, comprising at least one filter section including:

a first and second microwave four port signal power splitter each having a pair of input ports and a pair of output ports;

means coupled to one input port of said first power splitter for coupling a microwave signal thereto;

a first transmission line having a selected signal path length coupled between one output port of said first power splitter and one input port of said second power splitter;

a second transmission line, having a sIgnal path length of a predetermined length longer than said first transmission line, coupled between the other output port of said first power splitter and the other input port of said second power splitter whereby an output signal is provided at one output port of said second power splitter, said output signal having a waveform which includes a step in the time domain and a recursive frequency attenuation notch in the frequency domain;

termination means respectively coupled to the other input port of said first power splitter and the other output port of said second power splitter;

means coupled to said other output port of said second power splitter for coupling said output therefrom, additionally including a second filter section coupled in series to said at least one filter section wherein said second section comprises;

third and fourth microwave signal power splitters each having a pair of input ports and a pair of output ports;

said one input port of said third power splitter being coupled to said means coupled to said other output port of said second power splitter;

a third transmission line having a selected signal path length coupled between one output port of said third power splitter and one input port of said fourth power splitter;

a fourth transmission line having a single path length relatively longer than the signal path length of said third transmission line, coupled between the other output port of said third power splitter and the other input port of said fourth power splitter;

termination means respectively coupled to the other input port of said third power splitter and one output port of fourth power splitter;

output signal means coupled to the other output port of said fourth power splitter; and

additionally including a power amplifier operated in the saturation region of its power transfer characteristic and having an input coupled to said output signal means whereby the gain compression of said amplifier provided by the operation thereof in said saturation region effectively removes one step in the time domain and reduces the recursive frequency attenuation notch to that of a single filter section at the output of said amplifier.

2. The filter as defined by claim 1 wherein said output amplifier comprises the output power amplifier of an RF amplifier chain in a radar transmitter including a diverse frequency pulse source.

3. The filter as defined by claim 2 and additionally including first and second electrically controlled phase shifter means coupled in series with said first and third transmission lines, and additionally including control means for varying the phase shift characteristics of said phase shifter in time relationship with the operating frequency of said diverse frequency pulse source.

4. The filter as defined by claim 3 wherein all of said power splitters are each comprised of four port, 3-db directional couplers.