GB1573300A - Microwave oscillators - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO MICROWAVE OSCILLATORS (71) We AEI SEMICONDUCTORS LIMITED, a British Company of Carholme Road, Lincoln LN1 1SG do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to microwave oscillators, and is primarily intended for use with microwave intruder detectors. In such detectors, the maximum permissible power radiated is limited by official regulations and in addition to the level of energy radiated at the fundamental frequency, limits are also imposed on the permissible levels of the harmonic frequencies radiated. Whilst it is relatively straightforward to limit the power of the fundamental frequency, it is more difficult to restrict the power of the harmonics to particular levels. The present invention seeks to provide a microwave oscillator in which the level of one or more harmonics can readily be limited.

According to this invention a microwave oscillator includes a first cavity tuned to a fundamental frequency of oscillation, a radiator coupled to said cavity to radiate the fundamental frequency, a semiconductor oscillator diode coupled to said cavity to stimulate oscillations at the fundamental frequency of oscillation, and a second cavity tuned to the second harmonic and coupled to said first cavity which is positioned between said second cavity and the radiator so as to inhibit radiation of the second harmonic frequency.

The second harmonic is usually the most troublesome, but it is desirable to also limit the third harmonic, and preferably a third cavity is positioned between the first cavity and the radiator and the electrical impedance of the third cavity is dimensioned so as to reflect energy at the third harmonic back into the microwave oscillator.

The first and second cavities are, preferably, designated regions of a common waveguide cavity and which are separated by a semiconductor oscillator diode mounted at one end of a conductive post which extends between opposite walls of the cavities.

Typically said semiconductor diode oscillator is a Gunn diode.

Preferably, a wall is provided between the first and third cavities, the wall being relatively thin and having a central aperture which provides communication between the two cavities.

Preferably, the aperture is circular.

Preferably again, the height of the third cavity is equal to and aligned with the diameter of the circular aperture, and is less than the width of the third cavity.

The radiator is, preferably, a horn radiator.

Preferably, each cavity is provided with an adjustable conductive member, which provides fine tuning of the electrical reactance of that cavity.

Preferably again, each conductive member is an elongate post provided with a screw thread to facilitate adjustment of the amount by which it projects into a respective cavity.

The invention is further described, by way of example, with reference to the drawings accompanying the Provisional specification in which, Figure 1 shows in diagrammatic form a mirowave oscillator in accordance with the present invention, and Figure 2 shows in greater detail a portion of the same microwave oscillator.

The microwave oscillator is particularly suitable for use in an intruder detection system, in which case it is mounted adjacent to a microwave receiver. A small fraction of the energy generated within the microwave oscillator is fed directly to the receiver, and the frequency of this energy is compared with the frequency of signals reflected from a moving target and thence received by the microwave receiver. Any movement occuring in front of the radiator (as would be caused by an intruder) produces a change in the frequency of the signal received by the receiver which is detected when it is compared with the fraction of the energy fed directly from the oscillator.

Referring to the drawings, a rectangular hollow waveguide section 1 is provided with a vertical conductive post 2 having a Gunn diode 3 mounted between the lower end of the post and the adjacent floor of the waveguide. The post 2 serves to electrically divide the waveguide 1 into two cavities 4 and 5. The cavity 4 is of length B and is bounded at its far end by a thin wall 6 having a central circular aperture 7 which provides communication to a third cavity 8 and a horn radiator 9. The length B determines the fundamental frequency of oscillation, which to quote a figure by way of example is typically 101/2GHz. The second cavity 5, which is closed by an end wall 10, is of length A and is dimensioned to resonate at the second harmonic of the fundamental frequency. The third cavity 8 is defined by a relatively thick plate 11 and has a height equal to the diameter of the aperture 7, but is much wider. The cross-section of cavity 8 is best seen in Figure 2, which is a section view of XY of Figure 1. The cavity 8 is directly connected to the horn 9.

Each of the three cavities 4, 5 and 8 is provided with a tuning member in the form of a conductive post 12, 13, 14 respectively.

The post is screw threaded to facilitate adjustment, and each cavity is tuned by adjusting the post length projecting into it.

When setting up the oscillator, the tuning post 12 is adjusted to tune the Gunn diode oscillation frequency to the fundamental frequency. Although this frequency is primarily dependent on the dimension B, some degree of fine tuning is required to allow for normal manufacturing tolerances.

The second cavity 5 is then tuned to the second harmonic frequency by means of the tuning post 13. Again the tuning post 13 is responsible only for fine tuning of the resonant frequency of the cavity which is largely dependent on the length A. Both cavities 4 and 5 are tuned to resonance, and when cavity 5 is resonant at the second harmonic frequency very little energy at this frequency is passed through the aperture 7.

The tuning post 14 in cavity 8 is adjusted to control the reactance at the aperture 7.

By creating an impedance mis-match for energy at the third harmonic, this energy is largely reflected back into the oscillator and is absorbed bv the walls of the waveguide 1.

When post 14 has been adjusted to produce minimum radiation of the third harmonic frequency, the sequence of adjusting the first tuning post 12, and then tuning post 13 is repeated since the act of tuning the cavity 8 may slightly de-tune the other two cavities.

Correct tuning permits the energy radiated at the second and third harmonics to be kept within predetermined limits, whilst allowing satisfactory radiation of energy at the fundamental frequency, which in this example is about 1Of/2GHz.

WHAT WE CLAIM IS: 1. A microwave oscillator including a first cavity tuned to a fundamental frequency of oscillation, a radiator coupled to said cavity to radiate the fundamental frequency, a semiconductor oscillator diode coupled to said cavity to stimulate oscillations at the fundamental frequency of oscillation, and a second cavity tuned to the second harmonic and coupled to said first cavity which is positioned between said second cavity and the radiator so as to inhibit radiators of the second harmonic frequency.

2. A microwave oscillator as claimed in claim 1 and wherein a third cavity is positioned between the first cavity and the radiator and the electrical impedance of the third cavity is dimensioned so as to reflect energy at the third harmonic back into the microwave oscillator.

3. A microwave oscillator as claimed in claim 1 or 2 and wherein the first and second cavities are designated regions of a common waveguide cavity and which are separated by a semiconductor oscillator diode mounted at one end of a conductive post which extends between opposite walls of the cavities.

4. A microwave oscillator as claimed in claim 3 and wherein said semiconductor diode oscillator is a Gunn diode.

5. A microwave oscillator as claimed in claim 2 and wherein a wall is provided between the first and third cavities, the wall being relatively thin and having a central aperture which provides communication between the two cavities.

6. A microwave oscillator as claimed in claim 5 and wherein the aperture is circular.

7. A microwave oscillator as claimed in claim 6 and wherein again the height of the third cavity is equal to and aligned with the diameter of the circular aperture, and is less than the width of the third cavity.

8. A microwave oscillator as claimed in any of the preceding claims and wherein each cavity is provided with an adjustable conductive member, which provides fine tuning of the electrical reactance of that cavity.

9. A microwave oscillator as claimed in claim 8 and wherein each conductive member is an elongate post provided with a screw thread to facilitate adjustment of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. with the frequency of signals reflected from a moving target and thence received by the microwave receiver. Any movement occuring in front of the radiator (as would be caused by an intruder) produces a change in the frequency of the signal received by the receiver which is detected when it is compared with the fraction of the energy fed directly from the oscillator. Referring to the drawings, a rectangular hollow waveguide section 1 is provided with a vertical conductive post 2 having a Gunn diode 3 mounted between the lower end of the post and the adjacent floor of the waveguide. The post 2 serves to electrically divide the waveguide 1 into two cavities 4 and 5. The cavity 4 is of length B and is bounded at its far end by a thin wall 6 having a central circular aperture 7 which provides communication to a third cavity 8 and a horn radiator 9. The length B determines the fundamental frequency of oscillation, which to quote a figure by way of example is typically 101/2GHz. The second cavity 5, which is closed by an end wall 10, is of length A and is dimensioned to resonate at the second harmonic of the fundamental frequency. The third cavity 8 is defined by a relatively thick plate 11 and has a height equal to the diameter of the aperture 7, but is much wider. The cross-section of cavity 8 is best seen in Figure 2, which is a section view of XY of Figure 1. The cavity 8 is directly connected to the horn 9. Each of the three cavities 4, 5 and 8 is provided with a tuning member in the form of a conductive post 12, 13, 14 respectively. The post is screw threaded to facilitate adjustment, and each cavity is tuned by adjusting the post length projecting into it. When setting up the oscillator, the tuning post 12 is adjusted to tune the Gunn diode oscillation frequency to the fundamental frequency. Although this frequency is primarily dependent on the dimension B, some degree of fine tuning is required to allow for normal manufacturing tolerances. The second cavity 5 is then tuned to the second harmonic frequency by means of the tuning post 13. Again the tuning post 13 is responsible only for fine tuning of the resonant frequency of the cavity which is largely dependent on the length A. Both cavities 4 and 5 are tuned to resonance, and when cavity 5 is resonant at the second harmonic frequency very little energy at this frequency is passed through the aperture 7. The tuning post 14 in cavity 8 is adjusted to control the reactance at the aperture 7. By creating an impedance mis-match for energy at the third harmonic, this energy is largely reflected back into the oscillator and is absorbed bv the walls of the waveguide 1. When post 14 has been adjusted to produce minimum radiation of the third harmonic frequency, the sequence of adjusting the first tuning post 12, and then tuning post 13 is repeated since the act of tuning the cavity 8 may slightly de-tune the other two cavities. Correct tuning permits the energy radiated at the second and third harmonics to be kept within predetermined limits, whilst allowing satisfactory radiation of energy at the fundamental frequency, which in this example is about 1Of/2GHz. WHAT WE CLAIM IS:

1. A microwave oscillator including a first cavity tuned to a fundamental frequency of oscillation, a radiator coupled to said cavity to radiate the fundamental frequency, a semiconductor oscillator diode coupled to said cavity to stimulate oscillations at the fundamental frequency of oscillation, and a second cavity tuned to the second harmonic and coupled to said first cavity which is positioned between said second cavity and the radiator so as to inhibit radiators of the second harmonic frequency.

2. A microwave oscillator as claimed in claim 1 and wherein a third cavity is positioned between the first cavity and the radiator and the electrical impedance of the third cavity is dimensioned so as to reflect energy at the third harmonic back into the microwave oscillator.

3. A microwave oscillator as claimed in claim 1 or 2 and wherein the first and second cavities are designated regions of a common waveguide cavity and which are separated by a semiconductor oscillator diode mounted at one end of a conductive post which extends between opposite walls of the cavities.

4. A microwave oscillator as claimed in claim 3 and wherein said semiconductor diode oscillator is a Gunn diode.

5. A microwave oscillator as claimed in claim 2 and wherein a wall is provided between the first and third cavities, the wall being relatively thin and having a central aperture which provides communication between the two cavities.

6. A microwave oscillator as claimed in claim 5 and wherein the aperture is circular.

7. A microwave oscillator as claimed in claim 6 and wherein again the height of the third cavity is equal to and aligned with the diameter of the circular aperture, and is less than the width of the third cavity.

8. A microwave oscillator as claimed in any of the preceding claims and wherein each cavity is provided with an adjustable conductive member, which provides fine tuning of the electrical reactance of that cavity.

9. A microwave oscillator as claimed in claim 8 and wherein each conductive member is an elongate post provided with a screw thread to facilitate adjustment of the

amount by which it projects into a respective cavity.

10. A microwave oscillator as claimed in any of the preceding claims and wherein the radiator is a horn radiator.

11. A microwave oscillator substantially as illustrated in and described with reference to the drawings accompanying the Provisional specification.