GB2189942A

GB2189942A - Transmission-line bias T

Info

Publication number: GB2189942A
Application number: GB08610563A
Authority: GB
Inventors: Brian John Minnis
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1986-04-30
Filing date: 1986-04-30
Publication date: 1987-11-04
Also published as: GB8610563D0

Abstract

A TEM-mode transmission line comprising an elongate conductor (3) and at least one ground plane (e.g. microstrip) has a short-circuited shunt stub (4) connected to the line for supplying a bias voltage, the stub (4) having a characteristic impedance which is substantially greater than that of the line at the point where the stub (4) is connected. To provide a broad operating bandwidth, the line has a substantial decrease in characteristic impedance in one or both senses along the line towards said point provided by one or a succession of progressively lower impedance-transforming sections (5, 6); the stub and the or each section have the same effective electrical length which suitably is a quarter wavelength at the centre of the operating frequency range. The bias T may be designed as a band-pass filter, and with a succession (5) of three impedance-transforming sections can provide a bandwidth of 10:1. <IMAGE>

Description

SPECIFICATION Transmission-line bias T The invention relates to a bias T comprising a substantially TEM-mode transmission line having an elongate conductor and at least one ground conductor, and further comprising a short-circuited shunt stub connected to the line for supplying a bias voltage to the line, the stub having a characteristic impedance which is substantially greater than that of the line at the point where the stub is connected thereto.

The relavant kind of transmission line comprises microstrip, strip transmission line comprising a strip conductor disposed between and spaced normal to a pair of ground planes, and coaxial line.

Bias Ts are widely used in microwave circuitry for supplying a direct bias voltage or current to one or more active devices included in the microwave circuit, for example to obtain optimum performance from detector diodes in a mixer. The bias T should enable the direct voltage to be applied without substantially affecting the circuit in its operating frequency range. For this purpose, it is common practice to put in shunt with a strip transmission line a high impedance stub which is short-circuited in the operating frequency range of the circuit at an effective electrical distance from the line of a quarter wavelength, the DC bias being applied at the RF short-circuit. The stub ideally presents substantially an open-circuit to the line and consequently does not then affect microwave energy propagating along the line.In strip transmission iines (including microstrip), it is however generally difficult to make a stub with a characteristic impedance much greater than 100 ohms in view of the very narrow strip conductor that would be required; this impedance compares wth a typical characteristic impedance of 50 ohms for the strip transmission line. Consequently, at frequencies which differ substantially from the mid-band design frequency at which the effective electrical length of the stub is a quarter wavelength, the stub may present a relatively low impedance to the transmission line and substantially affect the microwave performance.

An alternative approach which can enable broad operating bandwidths with satisfactory performance to be obtained involves the use of lumped elements such as resistors and/or inductors for supplying the bias voltage; however, such as bias T may be expensive to make and/or may have a restricted bias current capacity which may make it unsuitable for applications in which a large bias current may be required, such as travelling-wave or distributed amplifiers.

According to the invention, a bias T as set forth in the opening paragraph of this specification is characterised in that in one or both senses along the line towards said point, the line has a matched substantial decrease in characteristic impedance provided by an impedance-transforming section, or a plurality of successive impedance-transforming sections that have progressively smaller respective characteristic impedances, extending to said point, and in that the stub and the or each said section have the same effective electrical length.

The invention involves the recognition that instead of merely concentrating on the impedance of the path along which the bias voltage is supplied to the transmission line, an improved bias T can be obtained by providing an impedance transformation along the transmission line, typically from 50 ohms, to a relatively low value at the point where the stub is connected. The bias T may suitably be designed as a band-pass filter by modern network synthesis techniques. Using an appropriate number of sections, very broad bandwidths, for example 10:1 or more, may readily be achieved.

Suitably, said effective electrical length is substantially a quarter of a wavelength at the centre of the operating frequency range of the bias T. This has the advantage of simplifying the design of the bias T.

Suitably, the stub has a characteristic impedance substantially greater than the highest value from which the characteristic impedance of the transmission line is transformed by said section(s). This is particularly suited to obtaining very broad bandwidths.

The ratio of the characteristic impedance of said section or the section of smallest impedance to the highest value from which the characteristic impedance of the transmission line is transformed by said section(s) may be not substantially less than 3:5. The lowest characteristic impedance of said section or the section of smallest impedance may be not substantially greater than 30 ohms. The ratio of the characteristic impedance of the stub to the characteristic impedance of said section or the section of smallest impedance may be not substantially less than 3:1; in a conventional strip transmission line bias T with a 50 ohm line, the required impedance for the stub would be very difficult to achieve.

Where sakid substantial decrease in impedance occurs in both senses along the line, the line suitably is electrically substantially symmetrical about said point.

An embodiment of the invention will now be described, by way of example, with reference to the diagrammatic drawing, the sole Figure of which is a plan view of a microstrip bias T embodying the invention. The drawing shows the strip conductor pattern on the upper surface of a dielectric substrate 1, the lower surface of which is covered with a conductive ground plane (not shown). A microstrip transmission line which comprises a strip conductor 3 and the ground plane has connected thereto a shunt stub which comprises a very narrow strip conductor 4 and the ground plane. The line comprising conductor 3 has a principal characteristic impedance of 50 ohms, in this case at each end, but in order that the bias T may have a broad bandwidth, the impedance is transformed to a much lower value at the point where the stub is connected.Between that point and each end portion of the conductor 3 is an impedance transformer, in this case comprising a respective cascade 5,6 of three successive sections of transmission line each having an effective electrical length of a quarter of a wavelength at the centre of the operating frequency range of the bias T and having progressively smaller respective widths. There is thus a progressive decrease in characteristic impedance along the line towards the point where the stub is connected, in this case in both senses along the line. The shunt stub has a high characteristic impedance, and also has an effective electrical length of a quarter of a wavelength at the centre of the operating frequency range of the bias T. The stub is terminated by a chip capacitor 7 which is disposed in an aperture in the substrate 1.The capacitor has a thickness equal to that of the substrate, and its major surfaces are metallised. The lower surface is connected to the ground plane on the substrate, and its upper surface is connected by a wire bond at 8 to the conductor 4 and by a further wire bond at 9 to a large patch conductor 10 to which a DC bias conductor may readily be connected.

The transmission line forming the shunt stub suitably has a characteristic impedance of about 100 ohms. The number of sections in each impedance transformer in the main transmission line depends on the operatng bandwidth required : the larger the number, the broader the bandwidth. Three sections can enable a bandwidth of about 10:1 to be obtained. The lowest-impedance section of the impedance transformer may have a characteristic impedance in the range 20-30 ohms. A bias T embodying the invention may be designed as a Tchebychev bandpass filter by modern network synthesis, specifying for example the band edges, the magnitude of ripple which can be accepted in the response, and a trial number of sections in each cascade. This can be used to compute a necessary value for the impedance of the shunt stub.If this value is too high to be practicable, the number of sections can be increased.

An embodiment of the form shown in the drawings was designed to operate over the band of 4.5-45.5 GHz, and was constructed on RT/duroid 5880 comprising a substrate of thickness 0.125mm and dieectric constant E, of 2.2, with 1/2 ounce copper cladding. The narrow conductor 4 had a width of slightly less than 0.01 mum, giving a characteristic impedance of about 100 ohms, while the lowest-impedance sections of the impedance transformers had a width somewhat greater than 0.8mm, giving a characteristic impedance of about 28 ohms. The capacitance of the capacitor 7 was about 30 pF.

In ascertaining what physical length corresponds to a desired effective electrical length, it will generally be necessary to take junction effects into account. This particularly applies to the shunt stub, for which there is typically a large change in conductor width at the junction with the main transmission line. It may be suitable to initialiy use a design of stub which allows the point along the stub at which the RF short-circuiting capacitor is connected to be altered, so that the correct physical length of the stub may be determined empirically (by ascertaining whether the edges of the passband of the bias T correspond to the design range). Once the correct dimensions have been settled, however, bias Ts with a specified performance may readily be manufactured at relatively low cost.

In a bias T embodying the invention, there may be an decrease in the characteristic impedance of the line towards the point where the stub is connected in only one sense along the line, for applications in which a relatively low impedance is required for the transmission line adjacent the stub; this may for example be required in a travelling-wave amplifier.

For applications in which it is required to supply a direct current to only that part of the transmission line which lies to one side of the shunt stub, a DC break with RF coupling means such as a series capacitor may be incorporated in the end portion of line on the other side of the stub. Such a capacitor may for example have a value of 50pF.

Since a bias T embodying the invention does not require a substantial series resistance for isolating the source of the direct voltage, it may be used in applications which require a large bias current. Constructed embodiments have for example been used to carry currents of 3 amps.

Claims

1. A bias T comprisng a substantially TEM-mode transmission line having an elongate conductor and at least one ground conductor, and further comprising a short-circuited shunt stub connected to the line for supplying a bias voltage to the line, the stub having a characteristic impedance which is substantially greater than that of the line at the point where the stub is connected thereto, characterised in that in one or both senses along the line towards said point, the line has a matched substantial decrease in characteristic impedance provided by an impedance-transforming section, or a plurality of successive impedance-transforming sections that have progressively smaller respective characteristic impedances, extending to said point, and in that the stub and the or each said section have the same effective electrical length.

2. A bias T as claimed in Claim 1 wherein said effective electrical length is substantially a quarter of a wavelength at the centre of the operating frequency range of the bias T.

3. A bias T as claimed in Claim 1 or 2 wherein the stub has a characteristic impedance substantially greater than the highest value from which the characteristic impedance of the transmission line is transformed by said section(s).

4. A bias T as claimed in any preceding claim wherein the ratio of the characteristic impedance of said section or the section of smallest impedance to the highest value from which the characteristic impedance of the transmission line is transformed by said section(s) is not substantially less than 3:5.

5. A bias T as claimed in any preceding claim wherein the characteristic impedance of said section or the section of smallest impedance is not substantially greater than 30 ohms.

6. A bias T as claimed in any preceding claim wherein the ratio of the characteristic impedance of the stub to the characteristic impedance of said section or the section of smallest impedance is not substantially less than 3:1.

7. A bias T as claimed in any preceding claim wherein there is said substantial decrease in characteristic impedance in both senses along the line and wherein the line is electrically substantially symmetrical about said point.

8. A bias T as claimed in any preceding claim comprising three said sections.

9. A bias T substantially as herein described with reference to the drawing.