Microstrip Filter Comprising A Ferromagnetic Resonant Body

Abstract

A YIG crystal filter, notably for use in mixing circuits, by means of which, in addition to a frequency separation, a separation of even and odd waves on a double line can be realized. According to the invention, the transmission properties are substantially improved by interconnecting points of the double line at the area of the YIG crystal by means of a transmission line having a length of one half wavelength related to the magnetic resonant frequency of the YIG crystal.

Description

The invention relates to an electric band-pass filter comprising at least one body of ferromagnetic material.

Known filters of this kind comprise two transmission lines which cross each other at right angles and which are magnetically interconnected by way of a body of ferromagnetic material, for example, a YIG crystal sphere which is magnetically pre-polarized in the direction perpendicular to the directions of the transmission lines. These two transmission lines which cross each other at right angles are normally not coupled to each other. At the magnetic resonant frequency (dependent of the strength of the pre-polarizing field), however, a precession of the magnetic vector occurs, with the result that coupling takes place. This coupling is frequency-selective.

Also known are filters of this kind comprising a cascade of two or more of such magnetic coupling bodies which are coupled to each other, for example, via an opening in a conductive partition.

Also known is a diode mixing circuit for high frequencies (for example, 2 GHz), in which use is made of such a magnetic filter for filtering out the intermediate frequency and possibly for forming directly therefrom a second intermediate frequency by a means of a second transmission line. In this device a local oscillator is connected to a diode mixing circuit via a first transmission line. On this transmission line odd waves occur (that is to say, waves whose currents are in phase-opposition) which have the frequency of the local oscillator, and even waves (that is to say, waves whose currents are in phase) which correspond to the desired intermediate frequency signal.

This signal is filtered out by means of a YIG crystal filter. Therefore, the two conductors are symmetrically arranged with respect to the pre-polarized magnetic body such that the magnetic field of the even waves on the transmission line is mainly perpendicular to the pre-polarization direction at the area of the body, so that coupling to the precession field takes place, whilst the magnetic field of the odd waves is mainly parallel to the pre-polarization direction, with the result that the coupling is nil or at least insignificant.

Even though the frequency difference between the even and the odd waves can often be comparatively small in practice, the filtering action is very effective as a result of the difference in coupling strength of the even and the odd waves with the magnetic body.

The desired intermediate frequency signal is derived by means of a second transmission line which crosses the first transmission line at right angles and which is magnetically coupled to the magnetic body, possibly via a cascade of inter-connected magnetic bodies. Like the first transmission line, the second transmission line can also consist of a plurality of conductors.

The invention relates notably to an electric band-pass filter in which a first transmission line is magnetically coupled to a second transmission line, extending perpendicular thereto, by way of at least one body of ferromagnetic material which is magnetically pre-polarized in the direction perpendicular to the directions of the transmission lines with a strength such that magnetic resonance occurs at the desired band-pass frequency, the first transmission line being formed by two conductors and an earthed return conductor, it being possible for even and odd waves which can have frequencies of the same order of mangitude to appear on the said two conductors, the said two conductors being symmetrically arranged with respect to the magnetic body such that the magnetic field of the even waves is mainly perpendicular to the pre-polarization direction at the area of the magnetic body, that of the odd waves extending mainly parallel to the prepolarization direction.

The invention is characterized in that the first transmission line is terminated on one end by a detour line having an electrical length of approximately one half wavelength related to the magnetic resonant frequency.

This detour line substantially has a short-circuit between the conductors and the return conductor for the even waves at the area of the magnetic body, whilst for the odd waves a comparatively high impedance is present between the conductors and the return conductor (at this area).

The invention will be described in detail hereinafter with reference to the drawing.

FIG. 1 is a diagrammatic representation of a double mixing circuit of a known kind in which the filter according to the invention is used.

FIGS. 2 and 3 show the shape of the magnetic field generated by currents on the conductors in the same direction and by currents in the opposite direction, respectively.

FIG. 4 is a plan view of a given embodiment of the filter.

FIG. 5 is a cross-sectional view taken along the line V--V, and

FIG. 6 is a perspective view of a detail.

The double mixing circuit shown in FIG. 1 can be used, for example, as the input mixing circuit in a television receiver by means of which all television bands with a frequency range of, for example, 50 MHz to 900 MHz can be covered merely by variation of the frequency of a local oscillator, without the switching over of tuned circuits being necessary. For this purpose, the input signal is first stepped up in a first mixing stage, comprising an oscillator of fixed frequency, to an intermediate frequency of, for example, 2,000 MHz to which the filter is tuned, and is subsequently stepped down in a second mixing stage to the normal intermediate frequency of approximately 35 MHz of the television receiver.

The input signal of the signal source S is applied, via the conductors L1, L2 to the diode bridge R1, the output terminals of which are connected to the YIG crystal filter F via the conductors L3, L4.

The conductors L3, L4, are arranged to be symmetrical with respect to a sphere Y1, made of YIG-crystal, which is magnetically pre-polarized by a magnetic field H.sub.a perpendicular to the plane of the conductors (FIG. 2) such that magnetic resonance (Kittel mode) occurs at 2,000 MHz.

In the points 1 and 2 just below the sphere Y1 the conductors L3, L4, are terminated by a detour line L5, the length of which is equal to one half wavelength at the resonant frequency.

Furthermore, the points 1 and 2 are connected, by way of conductors L6, L7, to the local oscillator G1 whose frequency can be varied between 2050 MHz and 2900 MHz. The signal of the oscillator G1 is thus applied, via the conductors L6, L3 and L7, L4, to the diode bridge R1 such that the currents in the conductors are in phase-opposition (odd waves).

Under the influence of the signal of generator G1 and the input signal, the diode bridge R1 forms mixing products, including the intermediate frequency signal of 2,000 MHz, which is applied in the same phase (even wave) to the conductors L3 and L4.

FIGS. 2 and 3 show a cross-sectional view at the area of the sphere Y1. The conductors L3 and L4 are constructed as microstrips and are provided on one side of a dielectric carrier D. Provided on the other side of the carrier D is an earthed metal plate A having a coupling opening K. The sphere Y1 is situated in a recess in the carrier D and is pre-polarized by a magnetic field H.sub.a.

FIG. 2 also shows lines of force M of the high-frequency magnetic field which is generated by even waves on the conductors L3 and L4. These lines of force enclose the two conductors L3 and L4 and are directed mainly perpendicular to the magnetic field H.sub.a at the area of the sphere Y1. As is known, under these circumstances a precession of the magnetic vector about the direction of the field H.sub.a occurs. This precession is maximum if the frequency of the waves corresponds to the magnetic resonant frequency of the medium which is determined by the strength of the field H.sub.a. The even waves, representing the desired intermediate frequency signal of 2,000 MHz, therefore, as strongly magnetically coupled to the sphere Y1.

FIG. 3 shows lines of force M of the high-frequency magnetic field generated by odd waves on the conductors L3 and L4. These lines of force enclose the conductors L3 and L4 separately and are directed mainly parallel to the field H.sub.a at the area of the sphere Y1. Consequently, the odd waves are very weakly mangetically coupled to the sphere, on the one hand because the lines of force parallel to the field H.sub.a do not initiate the precession, and on the other hand because the generator frequency deviates substantially from the magnetic resonant frequency.

The two wave types are thus very effectively separated.

The sphere Y1 is magnetically coupled to the further circuit, via the coupling opening K, as will be described hereinafter.

The detour line L5 constitutes a short-circuit between the points 1 and 2 and earth for the even waves, with the result that at the area of the YIG crystal sphere Y1 a current loop appears for the intermediate frequency signal. The magnetic coupling between the even waves and the sphere Y1, consequently, is maximum.

In the known mixing circuit the detour line L5 is absent, with the result that the currents of the even waves in the vicinity of the sphere can be much smaller than the maximum value.

Moreover, the enen waves on the conductors L3 and L4 can proceed further over the conductors L6 and L7 in the direction of the generator G1, which causes additional losses and which has an adverse effect on the stability of the generator.

One consequence is that a high insertion loss of, for example, 16 dB can occur in the filter.

However, for the odd waves of the generator signal the detour line has a high impedance between the points 1 and 2. This implies a given loading of the generator, but is of no further importance.

The conductors system L3, L4, L5, L6, L7 is arranged, as is shown in the plan view of FIG. 4, on one side of the filter plate. The diameter of the spheres amounts to, for example, 0.8 mm and that of the coupling opening to 4 mm. As is denoted by broken lines in FIG. 4, a conductor system L8, L9, L10, L11, L12, of the same shape as the first system but rotated through 90.degree. with respect thereto, is provided on the other side. FIG. 5 shows a cross-sectional view. Analogous to the FIGS. 2 and 3, the first condutor system is provided on a dielectric carrier D1 and is coupled to the sphere Y1. Similarly, the second conductor system is provided on a carrier D2 and is coupled to the YIG crystal sphere Y2. Present between the carriers D1 and D2 is a metal plate A having a coupling opening K, a detail of which is shown in FIG. 6.

The spheres Y1 and Y2 are connected to carrier rods H1 and H2 which are inserted into holes in the carriers D1 and D2. By means of the carrier rods H1 and H2, the crystal axes of the spheres Y1 and Y2 can be oriented such that the temperature influence is minimum. The spheres are magnetically coupled to each other via the opening K and are tuned to the same frequency. As is known, a band-filter action is thus obtained.

As appears from FIG. 1, the conductors L11 and L12 are connected to a generator G2 whose frequency has the fixed value 1965 MHz. The generator G2 generates odd waves on the condutor system L8, L11 and L9, L12, respectively.

The magnetic rotary field in the sphere Y2 induces even waves in the conductors L8 and L9, the said waves proceeding at a frequency of 2,000 MHz in the direction of the diode bridge R2 where they are mixed with the generator signal. The intermediate frequency signal having a frequency of 35 MHz appears on the output terminals U1 and U2.

The detour line L10, the electric length of which is equal to one half wavelength at the resonant frequency of 2,000 MHz and which is connected between the points 3 and 4, constitutes a short-circuit for the even waves between the conductors L8 and L9 and the earthed intermediate plate A. It is thus prevented that the even waves can proceed on the conductors L11 and L12 to the generator G2, which would lead to losses.

As a result of the use of the detour lines L5 and L10 according to the invention, it was found to be possible to achieve a drastic reduction of the insertion loss of the filter, for example, from 16 dB to 2.6 dB. Moreover, the filter action was improved by this step because the strength ratio of the even and the odd waves is improved in the vicinity of the YIG crystal spheres.

The filter can be modified in various manners within the scope of the invention.

For example, a single sphere can be used instead of two spheres.

If the filter is used in a single mixing circuit, it is not necessary for the two crossed transmission lines to consist of two conductors each. In such a case one of the transmission lines can consist of a single conductor like in the known filters.

Claims

What is claimed is:

1. An electric band-pass filter in which a first transmission line is magnetically coupled to a second transmission line, extending perpendicular thereto, by way of at least one body of ferro-magnetic material which is magnetically pre-polarized in the direction perpendicular to the directions of the transmission lines with a strength such that magnetic resonance occurs at the desired band-pass frequency, the first transmission line being formed by two conductors and an earthed return conductor, it being possible for even and odd waves to appear on the said two conductors, the said two conductors being symmetrically arranged with respect to the magnetic body such that the magnetic field of the even waves is mainly perpendicular to the pre-polarization direction at the area of the magnetic body, that of the odd waves extending mainly parallel to the pre-polarization direction, characterized in that the first transmission line is terminated on one end by a detour line having an electrical length of approximately one half wavelength related to the magnetic resonant frequency.