GB2110025A - Phase modulator - Google Patents

Abstract

A phase modulator in a compact, generally planar form and suitable for fast phase-shift keying comprises a slot transmission line series T (3) having three branches of which one (4) forms one port of the modulator and of which the other two (5, 6) are in series with one another and are coupled by symmetrical microstrip lines (12, 13) to a common line (17) forming another port of the modulator. Two diodes (23, 24) respectively bridge the two series arms (5, 6) at the junction of the branches and are connected in opposite senses to a common terminal (21) to which a modulating signal can be applied so as substantially to short-circuit either one of the branches (5, 6) while leaving the other branch unaffected. Since signals in the two branches (5, 6) are in antiphase, modulation between phases differing by 180 DEG independent of frequency can be obtained; frequency- independent impedance matching at the two ports can also be obtained. <IMAGE>

Description

SPECIFICATION Phase modulator This invention relates to a phase modulator suitable for modulating R.F. signals.

A phase modulator may be used for phase shift keying (PSK) in a digital communications system.

A phase modulator known from "Broadband Binary 180 Diode Phase Modulators" by Robert A. Garver, IEEE Trans. MTT-13, pp.

32-38 (January 1965), is formed in waveguide and and comprises an H-plane T junction joined to an E-plane T junction by two paths whose lengths are equal to one another and are equal to half a wavelength at the centre of the operating frequency band. Midway along each path is a diode switch which can be operated to present substantially a short-circuit at points half a wavelength away, i.e. at each end of its respective path, at the centre frequency. The diode switches can thus be used to cause a signal to be transmitted along one or the other of the two paths; the phase of the output signal relative to the input signal will have one of two values 1 80' apart, independent of frequency, depending on which path the signal is transmitted along.

However, the insertion loss and the impedances presented by the phase modulator at its input and output are dependent on frequency since the spacing of the diode switches from the T junctions is not half a wavelength at frequencies other than the centre frequency, and at these other frequencies a short-circuit in the plane of the diode switch is transformed to a reactive impedance at the T junctions, causing reflections. Moreover, since the phase modulator is formed in waveguide, it is bulky and heavy.

A phase modulator formed in microstrip is known from "A Fast-Switching Low-Loss 12 GHz Microstrip 4-PS K Path Length Modulator" by B. Glance and N. Amitay, IEEE Trans.

COM-28, pp. 1824-1828. This modulator utilises phase changes along sections of transmission line of predetermined length, and its chacteristics are therefore necessarily frequency-dependent.

According to the invention, a phase modulator comprises two ports for use one as an input and the other as an output, a slot transmission line series T having three branches meeting substantially at a common point, a first of the branches constituting a first of said ports and the second and third branches being in series with one another, diode switch means located substantially at the junction of the three branches for selectably subsantailly short-circuiting either one of the second and third branches while leaving the other of those two branches substantially unaffected, and coupling means formed at least partly in strip transmission line for coupling both the second branch and the third branch to a common transmission line constituting the second of said ports.

The phase shifts between a point in the first branch of the slot-line series T and two points equally electrically spaced therefrom respectively in the second and third branches necessarily differ by 180 degrees, and a phase modulator embodying the invention may thus be used to introduced into an R.F. signal a modulation between two phases differing by an amount which is independent of frequency. By locating the diode switch means substantially at the junction of the three branches rather than needing to transform a diode short-circuit along a length of waveguide or transmission line, a further frequency-dependent parameter is obviated.

Suitably, the phase modulator presents a substantially matched load at each of its two ports over its operating frequency range when either one of the second and third branches of the series T is selectably substantially shortcircuited by the diode switch means. To achieve this, it is particularly helpful to employ coupling means comprising a junction device having at least three arms of which two are isolated from one another and are respectively coupled to the second and third branches of the series T and of which a third arm constitutes the second port of the phase modulaor.The junction device may for example be a split-T junction having a resistive load coupled to its two arms which are coupled to the second and third branches of the series T, the resistive load being locted at a distance from the third arm that is substantially a quarter of a wavelength at the centre of the operating frequency range of the phase modulator. As an alternative, the junction device may be a hybrid ring having a fouth arm terminated in a matched load.

The two above-mentioned junction devices are particularly useful when modulation between phases differing by 1 80' is desired. If modulation between phases differing by for example 90 or 270 is desired, a further frequency-independent phase shift of 90 may be introduced into the signal which travels along the second or third branch of the slotline series T by using as the junction device a 90 directional coupler having a fourth arm terminated in a matched load.

In an arrangement which is particularly suitable for use over a narrower frequency range, the coupling means comprise a junction having three arms meeting substantially at a common point, two of the arms being respectively coupled to the second and third branches of the series T and the third arm constituting the second port of the phase modulator, wherein the electrical path-lengths between the common point of said junction and the common point of the series T via the second and third branches thereof respectively are each substantially an odd integral number of quarter wavelengths at the centre of the operating frequency range of the phase modulator.

This has the advantage that matching can be achieved without the use of a resistive load in which R.F. signal power is dissipated.

Suitably, the coupling means are substantially symmetrical. This can obviate a requirement for phase adjustment to obtain optimum performance.

The diode swich means may comprise two adjacent diodes respectively extending across the slots forming the second and third branches of the series T. The two diodes are preferably connected in opposite senses to a common terminal which is isolated at low frequencies from, and coupled at R.F. to, a portion of ground plane bounding the slots forming the second and third branches of the series T and remote from the slot forming the first branch of the series T. Suitably, the common terminal is a strip conductor overlying said portion of ground plane, being separated therefrom by a layer of dielectric.

As an alternative, the diode switch means may comprise a star configuration of four diodes connected to a common terminal substantially overlying the common point of the series T, two of the diodes being connected in opposite senses to a portion of ground plane remote from the first branch at points respectively adjacent the slots forming the second and third branches, and the other two diodes being connected in opposite senses respectively to portions of ground plane on opposite sides of the slot forming the first branch and respectively adjacent the slots forming the second and third branches.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of a phase modulator which embodies the invention and which is mainly formed on a dielectric substrate, and Figure 2 is a view on an enlarged scale of part of the underside of the substrate, showing details which for clarity have been omitted from Fig. 1.

The phase modulator shown in the drawings is formed on a dielectric substrate 1. The underside of the substrate carries a conductive layer 2 in which is defined a slot transmission line series T 3 having three branches 4, 5 and 6 respectively which at their ends remote from the junction of the branches are terminated in open-circuits 7, 8 and 9 respectively.

A microstrip transmission line comprising the conductive layer 2 on the underside of the substrate as a ground plane and a strip conductor 10 on the upper side of the substrate is coupled to branch 4 of theslot-line series T 3 by virtue of strip conductor 10 crossing branch 4 and being connected to the ground plane 2 by a conductive pin 11 extending through the substrate immediately adjacent branch 4 and open-circuit 7. Two further microstrip lines comprising strip conductors 12 and 13 respectively are analogously coupled to branches 5 and 6 respectively of the slot-line series T 3 immediately adjacent opencircuits 8 and 9 respectively.At their ends remote from the slot-line series T 3, these two microstrip lines are combined in a microstrip split-T junction 14 having two arms 15 and 16 respectively connected to conductors 12 and 13 and both connected to a third arm 17, and a resistive load 18 between arms 15 and 16. (Such a junction is also known as a Wilkinson power divider or combiner.) Portions of the upper surface of the substrate not utilised for the microwave circuit are covered with a resistive layer 19 to inhibit undesired coupling between spaced parts of the circuit via surface modes.

In Fig. 1, the widths of the microstrip and slot lines have for clarity been increased relative to the other dimensions of the microwave circuit. Fig. 2 is approximately to scale.

Referring to Fig. 2 which shows on a larger scale the underside of the substrate in the region of the junction of the three branches of the slot-line series T 3, a strip conductor 21 adjacent the junction is supported by the substrate 1, being separated from the ground plane 2 by a thin layer 22 of dielectric so as to be isolated from the ground plane at low frequencies but coupled to the ground plane at R.F. Two matched diodes 23 and 24 extend closely adjacent to one another across the branches 5 and 6 respectively of the slotline series T. One terminal of each diode is connected to the end of the strip conductor 21 nearest the junction, the diodes being connected in opposite electrical senses, and the other terminals of the diodes are respectively connected to portions of the ground plane 2 on opposite sides of branch 4 of the slot-line series T adjacent branches 5 and 6.

The strip conductor 21 comprises two broad portions separated by a narrow portion to form a low-pass filter; thus a connection made to the right-hand end (as drawn) of the strip, for example with a coaxial cable, for applying a modulating signal is isolated from microwave signals in the branches of the slotline series T.

In operation, an R.F. signal to be phase modulated may be applied to the microstrip line comprising strip conductor 10, a modulating signal may be applied to the right-hand end of strip conductor 21, and the modulated R.F. signal may be taken from arm 17 of the microstrip split-T junction 14.

The modulator functions as follows. If the diodes 23 and 24 were absent, an input signal supplied to branch 4 of the slot line series T would be divided between branches 5 and 6 in equal amplitude (provided that, as here, the branches present equal impedances) and in opposite phase, i.e. the phase shifts between a point in branch 4 and two points equally electrically spaced therefrom respectively in branches 5 and 6 would differ by 180 . With the diodes present and a voltage of suitable magnitude applied between strip conductor 21 and the ground plane 2, one of the diodes will be forward-biased so as to present a very low impedance and the other diode will be reverse-biased so as to present a relatively high impedance; which of the diodes is forward-biased will of course depend on whether the strip conductor 21 is positive or negative with respect to the ground plane 2. As the diodes are coupled at R.F. to the portions of ground plane overlapped by their terminals, the diode which presents a very low impedance will substantially short-circuit the branch of the slot-line series T that it bridges, preventing that branch from transmitting the input signal, while the branch bridged by the other diode will be substantially unaffected, and will transmit the input signal with little attenuation.

The signal in branch 5 or 6 is then coupled via strip conductor 12 or 13 respectively and the split-T junction 14 to the outpuy 17 without any further change in phase in one path relative to the other.

Thus a modulating signal which suitably is balanced with respect to the ground plane will cause the phase of the R.F. output signal relative to the phase of the R.F. input signal to have alternately one and the other of two values differing by 180 .

Since the diode switch means are located substantially at the common point at which the three branches of the slot-line series T meet, the diode which at any moment shortcircuits one of the branches 5 and 6 effectively forms a boundary of the direct path between branch 4 and the other branch, so that energy which reaches that other branch after reflection by the short-circuit is substantially in phase with energy that has travelled there directly.

It should be noted that since the signals which are transmitted by branches 5 and 6 of the slot-line series T 3 and combined by the split-T 14 are in phase opposition, the phase of the output signal from the phase modulator can have one of only two values differing by 1 80' and no other value, even if one of those branches is not completely short-circuited by its respective diode; the undesired leakage signal in that branch can only reduce the amplitude of the output signal but cannot affect its phase.

The phase modulator also has the advantage that switching between the two phase states may be very fast, since it is dependent mainly on the time taken to switch a diode from a high resistance under reverse bias to a very low resistance under forward bias, rather than on the slower process of switching from low to high resistance (which is retarded by the effect of junction capacitance). Considering the switching process in more detail, the diode which was substantially open-circuit rapidly attains a low resistance as the modulating signal changes sign, and consequently the amplitude of the R.F. signal transmitted by the branch bridged by that diode rapidly decreases.Therefore, although the amplitude of the signal transmitted by the other branch bridged the diode that is being switched from a low to a high resistance increases at a significantly slower rate, the point at which the amplitudes of the signals transmitted respectively by the two branches are equal is soon reached; at that point, the phase of the output signal changes from one to the other of its two values. A modulator embodying the invention may therefore be particularly suitable for frequency-invariant fast phase shift keying.

In an embodiment of the form shown in Figs. 1 and 2 constructed for operation in Sband, the substrate 1 was alumina 2 mm thick. The centres of the open-circuits 8 and 9 were approximately 3 cm apart. The microstrip input and output connections each had an impedance of 50 ohms at the edge of the substrate, the impedance of the line comprising strip conductor 10 being transformed to 100 ohms by two successive quarter-wave transformers as depicted in Fig. 1. Each slot of the series T 3 had a width of 100 elm giving an impedance of approximately 100 ohms; this width was selected as being commensurate with the dimensions of the diodes used. Near the junction of the branches the widths of the slots were increased to 120 ym so that the diodes could be positioned centrally over the slots.Each broad portion of strip conductor 21 was 1.5 mm > < X 0.70.7 mm and the central narrow portion was 0.05 mm X 2.0 mm. Each of the microstrip lines comprising strip conductors 12 and 13 has an impedance of 100 ohms, as did arms 15 and 16 of the split-T junction 14; the resistive load 18 was 200 ohms. The arms 15 and 16 were each a quarter wavelength long at the centre of the operating frequency band so as, in combination with the resistive load 14, to isolate conductors 12 and 13 from one another. The resistive load 18 and the resistive layer 19 consisted of an evaporated nickel-chromiun layer having a resistance of 100 ohms per square. The conductors comprised a 7 ym thick layer of gold. The dielectric layer 22 on the underside of the substrate consisted of silicon nitride 5 ym thick and was formed by R.F. sputtering.The diodes were Hewlett-Packard type 5082-2716 which are Schottky-barrier beam-lead diodes. (For higher R.F. signal powers, PIN diodes may be used.) With an arrangement such as that of Figs.

1 and 2, using impedances such as those mentioned above, the phase modulator presents a matched load at each of its two ports when either of the branches 5 and 6 of the slot-line series T is selectably short-circuited by its respective diode.

Since the circuit is of a reciprocal form, the input and output could be interchanged.

Embodiments of the invention in which the coupling of the two series branches of the slot-line series T to a common transmission line forming one port of the modulator is symmetrical as in the embodiment of Figs. 1 and 2, should inherently not require any phase adjustment to achieve optimum performance.

For operation over very broad bandwidths, a split-T junction having two arms each with a succession of quarter-wave sections, for example as described in U.K. Patent Specification 1 330 408, may be used. For operation over relatively narrow bandwidths of less than 10%, the split-T junction may be replaced by a simple junction having three arms meeting substantially at a common point, the electrical path lengths between this point and the common point at which the branches of the slotline series T meet via the latter's two series branches being arranged to be an odd integral number of quarter wavelengths at the centre of the operating frequency band; the shortcircuit of one of the branches of the slot-line series T will therefore be transformed to present an open-circuit at the common point of the other junction, and the impedance presented to the third arm of the latter junction will be that of the path along which the signal is being transmitted. The two paths may thus be effecively isolated from each other and an impedance match be obtainable without the use of a resistive load in which signal power is dissipated (3dB in the above-described embodiment).

The microstrip/slot-line mode transducers used in the embodiment of Figs. 1 and 2 are suitable for operation over a bandwidth of at least an octave. For operation over a bandwidth of 15% or less, a mode transducer in which the microstrip line an slot line each extend a quarter wavelength beyond the point at which they cross and are respectively terminated in an open-circuit and a short-circuit may be used.

As an alternative to a split-T junction, a hybrid ring may be used. For example, with a hybrid ring having four ports spaced around a ring of 3A/2 circumference (where A is the wavelength in the centre of operating frequency band) with three intervals of A/4 and one interval of 3A/4, the two series branches of the slot-line series T are coupled to first and second ports separated by A/2 (these ports being isolated from one another), a third port between the first and second constitutes one port of the phase modulator, and the fourth port is terminated in a matched load. Such an arrangement can couple the two series branches of the slot-line series T to the port of the phase modulator without introducing any further relative phase shift. For operation over a very broad bandwidth, the hybrid ring may be of the form described in U.K.Patent Specification 1 321 978 comprising a ring of four sections of transmission line and four ports spaced around the ring at regular intervals by the four sections, the ring comprising portions of the microstrip line type and the slot line type and three sections being entirely of one type and the fourth section comprising a single portion of the other type.

As a further alternative to a split-T junction, a 3 dB directional coupler may be used; this will introduce a relative phase shift of 90 which is independent of frequency, and may therefore be used in a phase modulator for modulating between two phases differing by 90 . Two ports of the directional coupler isolated from one another are respectively coupled to the two series branches of the slotline series T; of the other two ports of the directional coupler, one is terminated in a matched load and the other constitutes one port of the phase modulator.

Although it is generally preferable for the means coupling the two series branches of the slot-line series T to a single transmission line constituting one port of the phase modulator to be symmetrical (as in Fig. 1), an asymmetrical configuration may be used. In particular, the electrical path lengths from the junction of the branches of the slot-line series T to the single transmission line may differ so as to result in a relative phase shift other than 1 80', but this will of course have the disadvantage that the phase shift will be frequencydependent.

Factors affecting the choice of the characteristic impedance of the slots of the branches of the series T are: (a) to minimise attenuation of the output signal by energy which leaks past a diode under forward bias, the ratio of the characteristic impedance of the slot line to the resistance of the diode (under forward bias) should be large; (b) to minimise reduction of the amplitude of the output signal due to reflection at a diode under reverse bias, the ratio of the capacitance of the diode (under reverse bias) to the distributed capacitance of the line should be small; this implies a low characteristic impedance. (It may be possible to compensate for the diode capacitance by locally increasing the width of the slot).

The optimum choice will of course depend on the parameters of the particular diodes used. As an alternative to diode switching means comprising two diodes (as in Fig. 2), a set of four diodes in a star configuration may be used, the centre of the star suitably being located over the centre of the junction of the branches of the slot-line series T; diametrically opposite diodes are connected to the centre of the star in the same sense, and the two pairs of diametrically opposite diodes are connected to the centre of the star respectively in opposite senses.The other terminals of the four diodes are connected as follows: the terminals of two diodes are respectively connected to a portion of the ground plane remote from the first branch of the series T at points respectively adjacent the slots forming the second and third branches and the terminals of the other two diodes are connected to portions of ground plane respectively on opposite sides of the slot forming the first branch and adjacent the slots forming the second and third branches. A connection to the centre of the star may be made with a microstrip line in an arrangement analogous to that of Fig. 2 or with a pin extending through the substrate at the centre of the junction.

Since a modulator embodying the invention may be in planar form on a single substrate and will generally not require phase adjustment for optimum performance, it may be relatively cheap to manufacture.

The phase modulators each embodying the invention and respectively producing phase shifts differing by 90 and by 1 80' may be used in series to form a 4-bit latching phase shifter for producing relative phases of 0 , 90 , 180 and 270 . It should be noted that in the 90 phase modulator, the selection of one or the other of two ports of the directional coupler respectively to form one port of the modulator and to be terminated in a matched load will determine the sense of the 90 phase shift produced by a change in a given sense of the polarity of the applied modulation signal.

The branches of the slot transmission line series T may have a geometrical disposition other than that of a letter "T", for example that of a letter "Y".

Claims (14)

1. A phase modulator comprising two ports for use one as an input and the other as an output, a slot transmission line series T having three branches meeting substantially at a common point, a first of the branches constituting a first of said ports and the second and third branches being in series with one another, diode switch means located substantially at the junction of the three brances for selectably substantially short-circuiting either one of the second and third branches while leaving the other of those two branches substantially unaffected, and coupling means formed at least partly in strip transmission line for coupling both the second branch and the third branch to a common transmission line constituting the second of said ports.

2. A phase modulator as claimed in Claim 1 which presents a substantially matched load at each of its two ports over its operating frequency range when either one of the second and third branches of the series T is selectably substantially short-circuited by the diode switch means.

3. A phase modulator as claimed in Claim 1 or 2 wherein the coupling means comprise a junction device having at least three arms of which two are isolated from one another and are respectively coupled to the second and third branches of the series T and of which a third arm constitutes the second port of the phase modulator.

4. A phase modulator as claimed in Claim 3 wherein the junction device is a split-T junction having a resistive load coupled to its two arms which are coupled to the second and third branches of the series T, the resistive laod being located at a distance from the third arm that is substantially a quarter of a wavelength at the centre of the operating frequency range of the phase modulator.

5. A phase modulator as claimed in Claim 3 wherein the junction device is a hybrid ring having a fourth arm terminated in a matched load.

6. A phase modulator as claimed in Claim 3 wherein the junction device is a 90 directional coupler having a fourth arm terminated in a matched load.

7. A phase modulator as claimed in Claim 1 or 2 wherein the coupling means comprise a junction having three arms meeting substantially at a common point, two of the arms being respectively coupled to the second and third branches of the series T and the third arm constituting the second port of the phase modulator, wherein the electrical path-length between the common point of said junction and the common point of the series T via the second and third branches thereof respectively are each substantially an odd integral number of quarter wavelengths at the centre of the operating frequency range of the phase modulator.

8. A phase modulator as claimed in any preceding claim wherein the coupling means are substantially symmetrical.

9. A phase modulator as claimed in any preceding claim wherein the diode switch means comprise two adjacent diodes respectively extending across the slots forming the second and third branches of the series T.

10. A phase modulator as claimed in Claim 9 wherein the two diodes are connected in opposite senses to a common terminal which is isolated at low frequencies from, and coupled at R.F. to, a portion of ground plane bounding the slots forming the second and third branches of the series T and remote from the slot forming the first branch of the series T.

11. A phase modulator as claimed in Claim 10 wherein the common terminal is a strip conductor overlying said portion of ground plane, being separated therefrom by a layer of dielectric.

12. A phase modulator as claimed in any of Claims 1 to 8 wherein the diode switch means comprise a star configuration of four diodes connected to a common terminal substantially overlying the common point of the series T, two of the diodes being connected in opposite senses to a portion of ground plane remote from the first branch at points respectively adjacent the slots forming the second and third branches, and the other two diodes being connected in opposite senses respectively to portions of ground plane on opposite sides of the slot forming the first branch and respectively adjacent the slots forming the second and third branches.

13. A combination in series of a phase modulator as claimed in Claim 6 or any claim appendant thereto for producing phase shifts differing by 90 degrees and a phase shifter as claimed in any preceding claim other than Claim 6 or a claim appendant thereto for producing phase shifts differing by 180 degrees, whereby to produce any desired one of four phase shifts spaced at intervals of 90 degrees.

14. A phase modulator substantially as herein described with reference to the drawings.