Microstrip Phase Shifter Having Switchable Path Lengths

Abstract

A microstrip phase shifter is inserted in a waveguide aperture, the phase shifting network consists of lightweight transmission line sections which can be switched by microwave diodes to provide the required phase shift. The diodes are switched in pairs, forward or back biased, so that the wave will traverse through a given path to complete the phase shifting according to the switched pair of the diodes.

Description

BACKGROUND OF THE INVENTION

In general this invention relates to microstrip phase shifters, and more particularly to transmissive-type phase shifters.

Transmissive phase shifters provide a fixed-length transmission line path through the phase shifter which has active switching junctions at intervals along its length where additional line lengths may be added. The addition of one incremental length does not depend in any way upon the addition of any length since the wave does not traverse unused paths. All phase shift increments are truly independent. A small number of different increments having a binary relationship may be combined in various ways to obtain many different values of phase shift in binary steps. Transmissive phase shifters use fewer diodes, use them at lower stresses, and require less closely held tolerances than tandem reflective-type phase shifters. Transmissive phase shifters also utilize diodes under conditions of good match during the steady state.

Since there is a need for improved transmissive phase shifters, there is provided according to the invention a new two-diode per section transmissive phase shifter as hereinafter described.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a microstrip phase shifter.

It is another object of this invention to provide a two-diode per section transmissive phase shifter.

A feature of this invention is to provide a binary phase shifting network, the phase shifting network consisting of lightweight printed transmission line sections which will be switched by microwave diodes to provide 0 to 360.degree.of phase shift, a typical phase shifting section consisting of two diodes, one capacitor, and a biasing means.

A microstrip phase shifter is provided in a waveguide aperture, the phase shifting network consisting of lightweight printed transmission line sections which are switched by microwave diodes to provide the proper phase shift; the diodes are shifted in pairs, forward or back-biased, whereby when the diodes are forward biased the wave provides straight through fixed length paths and does not traverse an extra length of transmission line, and whereby when a reverse bias is applied the wave will traverse the extra length of transmission line and provide the proper phase shift through the network.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and the following detailed description will be better understood as reference is made to the following drawings in which:

FIG. 1 shows a preferred embodiment of a two-diode per section s phase shifter;

FIG. 2 illustrates a microstrip phase shifter positioned in a waveguide aperture;

FIG. 3 illustrates a typical array element microstrip phase shifter incorporating the features of the invention; and

FIG. 4 illustrates a binary phase shifter network utilizing the invention which will provide 0 to 360 of phase shift in increments of 22.5.degree..

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a two diode junction phase shifter. Junctions A & B are a portion of a straight-through fixed length path 11, 12. An extra length path 13 of a transmission line in the form of a loop is added by proper switching of the diodes 14, 15. Both junctions A, B employ quarter waves stubs 16, 17, and capacitors 18, 19 in the standard manner so that biasing voltages may be applied at 16 and the diodes operated in proper sequence. The extra length path 13 is the phase shift in electrical degrees and includes a matching taper section 20 coupling the signal to fixed length path 12.

IN this embodiment, both diodes 14, 15 are biased forward or backward simultaneously. When the diodes are forward biased by a bias voltage applied to stub 16, the wave proceeds down the fixed length path 11, 12 and does not traverse the loop 13, because an incremental short-circuited line section 21 which is added to halves of the loop CA & CB to form quarter wave lines EDCA and EDCB. This isolates these lines even though they are conductively connected to the straight through line at points A & B. Since points A & B are effectively equipotential and equiphase when diode 14 is conducting, the phase shift loop appears as an open circuit, and is isolated from the main line. The two-diode phase shift section shown requires only one bias voltage, two diodes, and two capacitors per phase shift section. This array is placed on an insulating microstrip 22, the components may be integrated with the array or placed at another position and coupled by appropriate leads through a waveguide as shown in FIG. 2.

Referring now to FIG. 2, a section 23 of a waveguide containing a microstrip phase shifter 24 is shown. The microstrip elements are held in position within the rectangular guide by the retaining means 25 (shown in dashed-lines) and terminal means 26 are added to the guide 23 to provide suitable connections for the bias voltages and the components.

The received energy, propagating within the rectangular waveguide, will couple into a microstrip binary phase shifter through a printed dipole-to-microstrip transducer as shown in FIG. 3, wherein one side 24a forms the ground plane of the dipole, and the other side 24b forms the phase shifting array. The two sides 24 a, b of the microstrip form the dipole transducer so that the energy may be intercepted and phase shifted according to the diodes switched in the array. The retaining means 25 may also serve as the shorting plane for the array.

A typical binary phase shifting network utilizing the concept of FIG. 1 is placed on a microstrip board as shown in detail in FIG. 4. This phase shift network consists of printed transmission line sections having microwave diodes to provide 0 to 360 360.degree.phase shift in increments of 22.5.degree.. This network provides four discrete phase shifts of 22.5.degree., 45, 90, and 180.degree. which can be added in any combination through switching of the diodes CR1 to CR8 to achieve the required phase shift. Each of the phase shifting sections consists of two diodes i.e. CR.sub.4,CR.sub.5, two capacitors i.e. C.sub.4,C.sub.6, a bias quarter wave stub and a grounded quarter wave stub. For example when diode CR.sub.8 is forwarded biased to present a low impedance, energy will follow path AB. Conversely, when diode CR.sub.8 is reverse biased to present high impedance, the RF energy will follow path ACB and a 180.degree. phase shift equal to the difference in path lengths will be obtained at point B. When diode CR8 is forward biased it is shunted by the loop of path ACB. Some of the energy will flow through this loop and cause a high standing wave ratio unless proper circuit compensation is provided. To reduce adverse effects on the transmission line, when diode CR.sub.8 is conducting, a high impedance at points A & B is provided by an electrical RF ground placed a quarter wave length one-quarter from both points A & B.

Diode CR.sub.1 placed between points C & D is biased in the forward direction to create quarter-wavelength paths ACD and BCD. With both diodes biased in the forward direction, the phase shift section together with the switched in line sections will appear appear as a high impedance at points A & B. The biasing network will not change the RF characteristics of the phasing line. The bias for the diodes is supplied through quarter wavelength stubs, the capacitors are required to isolate the terminals of the diodes from DC ground and provide an RF bypass. The complete phase shift network of FIG. 4 consists of eight diodes and nine capacitors. The effect of the insertion loss of these components on the complete phase network will be small because only five of the 17 components are in series with an antenna at a given time. The base material (i.e. teflon fiberglass laminate, tellon) for the baseboard circuit can keep this loss at a minimum.

By properly selecting the bias to each section, the phase shift through the array of FIG. 4 can be controlled in the illustrated increments.

Accordingly, there has been described a microstrip phase shifter which is inserted in a waveguide aperture, the phase shifting network comprises printed transmission lines sections which are switched by microwave diodes to provide a desired phase shift. Diodes in a particular section are switched in pairs, such that they are both forward or back biased. When the diodes are forward biased, the wave proceeds straight through a fixed length path and does not transverse a shunted loop path since the section emulates quarter-wave lines. When the diodes are reversed biased the waves will follow the longer shunted path to provide a corresponding electrical shift in degrees in accordance with the length and shape of the path. A complete phase shift network is thus provided in this manner illustrated in FIGS. 1 through 4.

Although I have described above a microstrip phase shifter which is designed to be inserted into a wave guide aperture, it should be clearly understood that this novel type phase shifter is clearly suitable in any phase shifting application.

Claims

I claim:

1. A phase shifter comprising:

first and second fixed length paths forming a first junction;

a shunting path across said first junction;

a quarter-wave stub coupled to each of said first and second fixed length paths;

a first switchable device located at said first junction;

a second switchable device coupled between said shunting path and a shorting plane; and

means coupled for switching said first and second devices from one state to another, said means including a source of bias voltage coupled to one of said stubs, a first capacitor coupled to the bias voltage stub, and a second capacitor located in said shunting path, whereby RF energy is caused to follow a fixed length path or an extra length path formed by said shunting path.

2. A phase shifter according to claim 1 wherein said first and second switchable devices are diodes.

3. A phase shifter according to claim 2, wherein said bias voltage applied to said bias stub causes said first and second diodes to be simultaneously forward or backbiased.

4. A phase shifter according to claim 3, wherein said bias stub is coupled to said shorting plane by said first capacitor, and said stub connected to said second fixed path is coupled directly to said shorting plane, whereby when said diodes are forward biased said phase shifter emulates quarter-wave lines.

5. A phase shifting array, each section of the array comprising:

a first junction formed by first and second fixed length paths;

a shunting path across said first junction to provide the phase shift in electrical degrees for each section;

a pair of quarter-wave stubs, each coupled to one of said first and second fixed length paths;

a first solid state device located at said first junction;

a second solid state device coupled between said shunting path and a common shorting plane; and

means for switching said first and second devices from one biased state to another, said means includes a source of bias voltage coupled to one of said stubs, a first capacitor coupled to the bias voltage stub, and a second capacitor positioned within said shunting path, whereby RF energy is caused to follow in response to said switching a fixed length path or an extra length path formed by said shunting path.

6. A phase shifting array according to claim 5 wherein said first and second solid-state devices are diodes.

7. A phase shifting array according to claim 6 wherein said bias voltage applied to said bias stub causes said first and second diodes to be simultaneously forward or backbiased.

8. A phase shifting array according to claim 7 wherein said bias stub is coupled to said common shorting plane by said first capacitor, and said stub connected to said second fixed path is directly coupled to said common shorting plane, whereby when said diodes are forward biased this phase shifting section emulates quarter-wave lines.