GB2040593A

GB2040593A - Microstrip isolator

Info

Publication number: GB2040593A
Application number: GB7943553A
Authority: GB
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1978-12-19
Filing date: 1979-12-18
Publication date: 1980-08-28
Also published as: JPS5583301A; FR2445038A1

Abstract

A microstrip isolator includes a circulating junction 2d between first, second and third microstrips 2a, 2b, 2c on one surface of a substrate 1, a ground conductor 5 on the other surface of the substrate and a terminating resistor 3 on one lateral side of the substrate connected between the ground conductor and one of the microstrips (2c). The substrate may be made from a ferrimagnetic material. The resistor 3 may be attached to or deposited on the side of the substrate. Impedance- matching means 3 may be incorporated in the line 2c. <IMAGE>

Description

SPECIFICATION Microstrip isolator This invention relates to an isolator using a microstrip circulator.

A conventional microstrip circulator comprises a ferrimagnetic substrate, a copper disk and three transmission lines disposed on the upperside of the substrate, a ground plane rested on the underside of the substrate, and a permanent magnet embedded in the ground plane.

A conventional isolator comprises the conventional circulator and an external terminating resistor having one terminal connected to one of the three transmission lines through a connector and the other terminal connected to the ground plane. Use of the external terminating resistor and the connector results in an increased size and cost in the isolator construction.

Another conventional isolator comprises the conventional circulator, and a terminating resistor disposed on the upperside of the substrate and having one terminal connected to one of the three transmission lines (or microstrips) and the other terminal connected to the ground plane through a conductor disposed on one lateral side of the substrate. The isolator has deteriorated isolation characteristics at a frequency higher than 10 GHz because of variation in impedance of the conductor with the frequency.

For better understanding of the present invention, reference is made to the following papers: (1) Y.S. Wu, "X-Band Microstrip-lnserted Puck Circulator Using Arc-Plasma-Sprayed Ferrite", IEEE Transactions on Microwave Theory and Techniques, pp. 504-506, June 1975; (2) D. Masse, "Broadband Microstrip Junction Circulators", Proceedings of the IEEE (Letters), pp.

352-353, March 1968: and (3) B. Hershenov, "X-Band Microstrip Circulator", Proceedings of the IEEE (Letters), vol. 54, pp.

2022-2023, December 1 966.

The object of this invention is to provide a small-size isolator which is free from the defects of conventional isolators and provides good characteristics.

Other objects and advantages of this invention will be apparent by reading the following detailed description of the invention in conjunction with the accompanying drawings, in which: FIG. 1 is a perspective view of the conventional isolator; FIG. 2 is a cross section taken along the line Il-Il of FIG. 1; FIG. 3 is an equivalent circuit of FIG. 1; FIG. 4 is a perspective view of the first embodiment of this invention; FIG. 5 shows isolator vs. frequency characteristics for illustrative embodiments of the conventional isolator and the isolator of this invention; FIG. 6 is a perspective view of the second embodiment of this invention; FIG. 7 is a cross section taken along the line VIl-VIl of FIG. 6; FIG. 8 is a perspective view of the third embodiment of this invention; and FIG. 9 is a cross section taken along the line IX-IX of FIG. 8.

Referring now to FIGS. 1 and 2, on the upperside of a ferrimagnetic substrate 1 serving as both a non-reciprocal element and substrate are provided an internal conductor 2 comprising three transmission microstrips, 2a, 2b and 2c and a junction (or copper disk) 2d. A ground conductor 5 is provided on the underside of the substrate 1. A resistor 3 is also provided on the upperside of the substrate 1. One end of the resistor 3 is connected to the transmission microstrip 2c, and the other end is connected to the ground conductor 5 through a microstrip or conductor 4 disposed on one lateral side of the substrate 1. The resistor 3 acts as a terminating resistor to make an isolator of the circulator. The ferrimagnetic substrate 1 is provided with a magnet 6 for magnetizing the substrate 1 in the ve-rtical direction.

Assuming that the impedance of the transmission microstrip 2c (having the resistor 3) at the junction 2d is Zo, the resistivity of the resistor 3 is R, the electrical length of the transmission microstrip 4 from the resistor 3 to the underside ground conductor 5 is pl, and the impedance of the transmission microstrip 4 isZl; then, the equivalent circuit of the isolation terminal of the isolator can be depicted as shown in FIG. 3.On the other hand, by selecting suitable values for the saturation magnetization of the ferrimagnetic substrate 1 (4xmas), the relative dielectric constant (E5), the magnitude of the working magnetic field and the size of the junction, the reactance component of the impedance Z0 of the microstrip 2c at the junction 2d can be reduced to so low a level that Z0 is made nearly equal to R0 (where R0 is the net resistivity).The impedance of the microstrip 2c at the resistor 3 is represented by the following formula: Z = R + jZ; tan I (1) In this formula, Z and pI cannot be made zero because the size of the ferrimagnetic substrate 1 is finite. Since the electrical length pl will vary depending upon the frequency, it is difficult to keep the value of tan pl zero over a wide frequency range, and as a result, the impedance Z has a reactance component. To realize matching between the impedance Z and the impedance R at the junction 2d, a reactive matching circuit is necessary, but the use of such circuit is deleterious to the isolation frequency characteristics.

FIG. 4 is a perspective view of an isolator using a ferrimagnetic substrate 1 according to the first embodiment of this invention. To eliminate the defects of the conventional technique described above, the isolator disposes a resistor 3 on one lateral side of the substrate 1 so that it comes closest to the underside ground conductor 5. Such arrangement can reduce Zl and pi of the formula (1) to substantially zero, and in the substantial absence of a reactance component, the impedance of the microstrip at the resistor 3 is almost equal to the net resistivity (Z + R).By selecting R0 as the resistivity (R) of the resistor 3 an impedance, matching is realized between the transmission microstrip at the junction 2d and that at the resistor 3 over a wide frequency band, thus offering good isolation characteristics. It is to be noted here that the internal conductor patterns on the upperside of the ferrimagnetic substrate and the underside ground conductor can be formed by metallization, electroless plating or bonding of a metal foil. If the resistivity R of the resistor 3 is predetermined, an impedance transformer 9 may be provided on the microstrip 2c in a position closer to the junction which adjusts the impedance at the junction to R to match the impedance at the junction and the resistor.

FIG. 5 is an isolation characteristic curve for three isolators that operate in a frequency band of from 10.5 to 13.5 GHz; each isolator comprises a junction 2d which is 3.1 mm in diameter and disposed on a 1 O-mm square YlG substrate of 0.6 mm in thickness and to the junction are connected 'input/output microstrips 2a, 2b and 2c each having a strip width (W) of 0.5 mm, and to one end of the microstrip 2c is soldered a square plate (1 mm per side) forming thereon a film resistor (50 ohms) having a square of 0.4 mm. In FIG. 5, a curve 10 represents a conventional isolator wherein the resistor 3 is provided on the upperside of the substrate 1, and the resulting isolation is only 9 to 6 dB.In comparison, one arrangement of this invention wherein the resistor 3 is disposed on one lateral side of the substrate 1 achieves an isolation of 21 to 14 dB as indicated by a curve 11, and another arrangement of this invention which adds a compensator circuit (conductor pattern) 9 in the microstrip 2c produces an isolation of 20 dB or greater as indicated by a curve 12.

FIGS. 6 and 7 are a perspective view and a cross section of the second embodiment of this invention, respectively. As shown, one end of the resistor 3 may be connected to an external conductor 7 that is disposed on the underside of the ferrimagnetic substrate 1 as a ground conductor. Alternatively, a substrate having a ground conductor formed by metallization or other suitable means may be disposed separately from an external conductor which is connected to one end of the resistor.

FIGS. 8 and 9 are a perspective view and a cross section of the third embodiment of this invention, respectively. This embodiment has a ferrimagnetic substrate 1' inserted into a dielectric substrate 8, to which are added internal conductor patterns 2a, 2b, 2c and 2d as well as a back ground conductor 5' which is further covered with an external conductor 7.

It is to be understood that according to this invention, the resistor 3 may be a chip resistor or the like which is externally soldered or provided by thermocompression bonding, or it may be a resistor film formed on the substrate by metallization.

As described in the foregoing, according to the isolator of this invention, a resistor which is disposed on one lateral side of the substrate so that it comes closest to the ground conductor is connected to the isolating end of the transmission microstrips on the substrate. Therefore, the isolator offers good isolation characteristics over a wide frequency range and is particularly useful as a high-frequency isolator.

Claims

1. An isolator comprising a ferrimagnetic substrate; an internal conductor disposed on one surface of the substrate and including a first microstrip for connection with a terminating resistor, a second microstrip for connection to an input terminal, a third microstrip for connection to an output terminal, and a junction connected to the first, second and third microstrips thereat; a ground conductor disposed on the other surface of the substrate; and means for applying a magnetic field to the substrate substantially in the vertical direction thereof, characterized in that said terminating resistor is disposed on one lateral side of said ferrimagnetic substrate to connect said first microstrip to said ground conductor.

2. An isolator as claimed in claim 1 including an impedance transformer arranged to match the impedance between the junction and the terminating resistor.

3. An isolator as claimed in either claim 1 or claim 2 wherein the ground conductor is formed by metallization, including a conductor connected between one end of the said resistor and the metallized ground conductor.

4. An isolator as claimed in any one of the preceding claims wherein the substrate is constituted by a ferrimagnetic portion within a dielectric portion.

5. An isolator as claimed in claim 1 substantially as described herein with reference to Fig. 4, Figs. 6 and 7, or Figs. 8 and 9 of the accompanying drawings.