Variable bi-phase modulator circuits and variable resistors

Abstract: A variable bi-phase modulator circuit for microwave signals comprises a quadrature power divider (1) having signal input and output ports (2 and 3) and two control ports (4 and 5), and comprises two variable resistors each having an input port (11). In accordance with the invention each of the two variable resistors comprises first and second microwave field-effect transistors (F1 and F2) the drains of which are coupled together via an intermediate resistor (R). These resistors can be formed using microwave monolithic integrated circuit technology and can have very good impedance characteristics. The input port (11) of the variable resistor comprises a connection to the intermediate resistor (R) and to the drain of the first transistor (F2). Each transistor is connected with zero dc bias between its source and drain and has a channel resistance which changes with change in gate voltage (VG1, VG2). A shunt stub (L1, L2) is connected to the drain of each transistor (F1, F2) to at least partially compensate at the frequency of operation of the transistor for the source to drain capacitance and for shifts in reference plane due to changes in the gate voltage of each transistor.

Description: This invention relates to variable bi-phase modulator circuits for microwave signals, particularly but not exclusively to such circuits suitable for use in, for example, reflected power cancellers for frequency modulated continuous-wave (FMCW) radar, or for use in transmit/ receive modules for phased-array radar. The invention also relates to variable resistors, particularly but not exclusively for use in such circuits. European Patent Application 89203056.0 published as EP-A-0 372 641 on 13 June 1990 describes, in a reflected power canceller, a bi-phase modulator circuit (Figure 5) comprising a quadrature power divider (62) which has signal input and output ports (72 and 74) and two control ports (68 and 70), and comprises two variable resistors (64 and 66) each having an input port connected to the respective control port (68 and 70). EP-A-0 372 641 forms part of the state of the art under Article 54(3) EPC. Thus (although of earlier priority date than the present application) EP-A-0 372 641 is not a prior publication. In the form disclosed in EP-A-0 372 641 each of the two variable resistors is PIN diode which is partially forward-biased. These PIN diodes form good variable resistors but such diodes cannot be readily fabricated in presently used microwave monolithic integrated circuit technologies. A different circuit construction of a bi-phase modulator is known from the paper "Gigabit-Rate GaAs FET RF Phase Modulator" by Dobratz et al, published in the 1978 IEEE International Solid-State Circuits Conference Digest of Technical Papers, February 1978, pages 76 and 77. In this modulator circuit each of the two variable resistors comprises a respective microwave field-effect transistor, and the transistor is connected with zero dc bias between the source and drain and has a channel resistance which changes with change in gate voltage. These two transistors connected to the control ports are switched together to give bi-phase power reflection from the input port to the output (isolation) port. In the circuits shown in this IISSCC Digest paper the transistor power reflection coefficients in the ON and OFF stages are made equal by placing a thin-film resistor across the transistor source and drain. According to one aspect of the present invention, there is provided a variable bi-phase modulator circuit for microwave signals comprising a quadrature power divider having signal input and output ports and two control ports, and comprising two variable resistors each having an input port connected to the respective control port, wherein each of the two variable resistors comprises first and second microwave field-effect transistors the drains of which are coupled together via an intermediate resistor, the input port of the variable resistor comprising a connection to the intermediate resistor and to the drain of the first transistor, each transistor being connected with zero dc bias between its source and drain and having a channel resistance which changes with change in gate voltage, and wherein a shunt stub is connected to the drain of each transistor to at least partially compensate at the frequency of operation of the transistor for the source to drain capacitance and for shifts in reference plane due to changes in the gate voltage of each transistor. Each variable resistor constructed in this manner as two zero-biased tuned field-effect transistors coupled by an intermediate resistor can be formed using microwave monolithic integrated circuit technologies so that the whole bi-phase modulator circuit may be fabricated in this manner on a single substrate. The resistance value can be varied by varying the gate voltages applied to the transistors. Furthermore, as will be described in more detail hereinafter, the variable resistors formed in this manner with suitably-tuned transistors can have very good characteristics and can permit performance enhancement of variable bi-phase modulators in relation to their phase noise performance, bandwidth, power handling ability, dynamic range and phase accuracy. In general a vector modulator which uses these variable bi-phase modulators will have lower amplitude and phase errors, and this has important systems consequences. When used as a reflected power canceller for FMCW radar, the receiver sensitivity will be increased. In phased array radar, sidelobe levels are reduced and the direction finding accuracy is improved. The quadrature power divider may be formed conveniently as, a Lange coupler, for example, and propagation losses may occur in the power divider which affect the phase characteristics of the control signal from the control ports. In accordance with the present invention, the respective control port is preferably connected to the intermediate resistor of the transistor pair via a length of transmission line which compensates at least partly for the effect of these propagation losses in the phase characteristics. According to another aspect of the present invention, there is provided a variable resistor in a microwave circuit, the resistor comprising a microwave field-effect transistor connected with zero dc bias between its source and drain and having a channel resistance which changes with change in gate voltage, characterised in that the variable resistor comprises first and second of said transistors the drains of which are coupled together via an intermediate resistor, in that an input port of the variable resistor comprises a connection to the intermediate resistor and to the drain of the first transistor, and in that a shunt stub is connected to the drain of each transistor to at least partially compensate at the frequency of operation of the transistor for the source to drain capacitance and for shifts in reference plane due to changes in the gate voltage of each transistor. Such a variable resistor in accordance with the present invention may be designed for use as a reflection attenuator in a variable bi-phase modulator circuit in accordance with the invention, but such resistors may also be designed for use in other microwave circuits, for example as transmission attenuators, phase shifters and power switches. It may be noted that it is already known from the paper entitled "GaAs Monolithic Wideband (2-18 GHz) Variable Attenuators by Y Tajima et al on page 479 to 481 of IEEE MTT-S Digest of 1982 to use a zero-biased field-effect transistor to form a variable transmission attenuator. With zero drain to source bias, the depletion region of the field-effect transistor is symmetrical and its channel resistance then varies with applied gate voltage from a very low value to a very high value. Three such transistors are connected in a T or a π shape network to form a transmission attenuator having a good transmission match at input and output ports. However, as will be described hereinafter with reference to a Smith chart (Figure 5), the variable impedance obtained with just one tuned zero-biased field-effect transistor can have a large imaginary component which degrades the impedance match, thus decreasing the dynamic range of attenuation and introducing phase errors. This is especially significant for (one-port) reflection attenuators. By replacing such a single transistor with an arrangement of paired transistors and an intermediate resistor in accordance with the present invention, the transistor attenuator arrangement can be tuned closer to the ideal, as will be described hereinafter with reference to an example in another Smith chart (Figure 7). These and other features in accordance with the invention are illustrated, by way of example, in specific embodiments of the invention now to be described with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit representation of one form of variable bi-phase modulator; Figure 2 is a more detailed schematic representation of this form of modulator circuit having variable resistors in accordance with the present invention; Figure 3 is a pictorial representation of the effect of changes in gate voltages VG1 and VG2 on the impedance of the variable resistors in accordance with the invention; Figure 4 is a schematic circuit representation of a single tuned zero-based FET as a variable transistor; Figure 5 is a Smith chart representation of the impedance of this variable resistor in Figure 4; Figure 6 is a schematic circuit representation of a variable resistor in accordance with the present invention; Figure 7 is a Smith chart representation of the impedance of this variable resistor of Figure 6; and Figure 8 is a top view of one embodiment of the variable bi-phase modulator of Figure 2 formed as a microwave monolithic integrated circuit. Ideally a variable bi-phase modulator with an RF input (RFi) of e j should have an output (RFo) of Ae j , where A is continuously variable between +1 and -1 and where  is a constant. Figure 1 shows a circuit realising this variable bi-phase modulator function and comprising a quadrature power divider 1 having signal input and output ports 2 and 3 respectively and two control ports 4 and 5. The modulator circuit also comprises two variable resistors 10 each having an input port 11 connected as control elements to the respective control ports 4 and 5 of the power divider 1. The quadrature power divider 1 is conveniently realised as a Lange coupler. However, a most important factor in obtaining a good quality modulator is for each load 10 to be a near pure variable resistance. The use of PIN diodes to form the loads 10 is not readily achievable in microwave monolithic integrated circuit technology, and so tuned unpowered microwave field-effect transistors such as MESFETs (metal Schottky-gate field-effect transistors) F1 and F2 are used instead, together with a resistor R connected in a configuration in accordance with the present invention. Thus, as illustrated in the examples of Figures 2 and 6, each variable resistor 10 comprises first and second microwave field-effect transistors F1 and F2 the drains of which are coupled together via an intermediate resistor R. The input port 11 of the variable resistor comprises a connection to the intermediate resistor R and to the drain of the first transistor F2. Each transistor F1 and F2 is connected with zero dc bias between its source and drain and has a channel resistance which changes with change in gate voltage VG1 and VG2. A shunt stub L1 and L2 is connected to the drain of each respective transistor F1 and F2 to at least partially compensate at the frequency of operation of the transistor for the source to drain capacitance and for shifts in reference plane due to changes in the gate voltage VG1 and VG2 of each transistor F1 and F2. With zero drain to source bias, the depletion region below the gate of the FET is symmetrical and the resistance then varies with applied gate voltage VG from a very low value for VG of 0 volts to a very high value for VG greater than the pinch-off voltage (for example, about -4 volts). One main difficulty with using a FET as a switch is the presence of a large drain to source capacitance which varies with gate voltage VG, even though this range of variation is much less than that for the resistance. Since this capacitance is essentially a low resistance path at high frequencies, it is tuned out in order to create a variable resistive load, and this can be achieved by using a short-circuit shunt stub L. The transmission line stub L forms a shunt inductance to ground as illustrated in Figure 4. The length of the stub L is less than a quarter wavelength and is optimised to give a 180° phase shift in ρ (the reflection coefficient for voltage), between the on and off states of the FET. The value of ρ is given by: ρ = V1/V0 which also corresponds to: ρ = (ZL - Z0)/(ZL + Z0) where V0 is the incident voltage and V1 is the voltage reflected from the load formed by the FET, both voltages being measured at a reference plane along a transmission line terminated by the FET load, and where Z0 is the characteristic impedance of the line and ZL is the load impedance. It will be assumed that Z0 = 50 ohms, as is the situation for most circuits of interest. In Figures 5 and 7, reflection coefficients ρ are plotted on a Smith chart with coordinates of normalized resistance (r = R/Z0, for Z0 = 50 ohms) corresponding to the circles, and of normalized reactance (x = X/Z0, for Z0 = 50 ohms) corresponding to the arcs orthogonal to the circles. In this situation, the value of ρ becomes: ρ = (z - 1)/(z + 1) where z = (r + jx) A pure resistance has a locus of ρ which, when plotted on a Smith chart, is a horizontal straight line passing the through the centre point (Z = 50 ohms, r = 1, x = 0) of Figures 5 and 7. Any straight line (e.g. line A) across the Smith chart passing through the centre point represents a reflection coefficient (ρ) which has a 180° phase difference between the opposite halves of the chart and has a magnitude which varies between 0 and 1 in each half. This satisfies the requirements of a biphase reflection attenuator. With a single tuned zero-biased FET as in Figure 4 the locus of ρ with applied control voltage VG deviates considerably from a straight line through the centre, as is shown by the curve B in Figure 5. In this case, the impedance has a large imaginary component in the centre region of the chart between curve B and line A; this degrades the best match case, thus decreasing the dynamic range of attenuation and introducing phase errors. Phase errors can be thought of as unwanted shifts in reference plane. By replacing the single tuned FET with two zero-biased tuned FETs F1 and F2 of appropriate gate width and a resistor R as shown in Figure 6, the length of the short-circuit tuning stubs L1 and L2 can be selected so as to force the locus of ρ to be closer to the ideal. A particular example is illustrated by curve C in Figure 7, for which it can be seen that the locus of ρ with applied control voltages VG1 and VG2 passes from a near short circuit (approaching r = 0,  = 180°, x = 0) to a near open circuit (approaching r = ∞,  = 0°, x = 0) through a point close to the centre (r = 1, x = 0) of the chart in a near straight line. This is a much improved variable resistive load, compared with the single tuned FET in Figure 5. Except for allowing the desired 180° reversal in phase from short circuit to open circuit, the tuning stubs L1 and L2 connected to the drain of each transistor F1 and F2 compensate for most of the variations in phase characteristics due to the changes in source to drain capacitance with gate voltage VG1 and VG2. Thus, when the load terminations 10 are a low resistance, signals are strongly reflected with a phase inversion; when they are high in resistance, the signals are strongly reflected with no phase inversion; and when the resistances approach Z0 (50 ohms, i.e. r = 1) the reflections tend to zero. The two gate voltages VG1 and VG2 control the variable load as follows: With VG1 = VG2 = 0 volts, both FETs are in the low resistance state ("on"). When VG2 is varied from 0 volts to above the pinch-off voltage (e.g. -4 volts), the load changes from a near short circuit (r = 0) to a good match. If VG1 is then varied from 0 volts to above the pinch-off voltage (eg - 4 volts), the load changes further from a good match to a near open circuit (r = ∞). Figure 3 gives a pictorial representation of this adjustment. The FET F2 preferably has a larger gate width (wider channel) than FET F1, since it must handle nearly all of the short circuit current. A smaller gate width (narrower channel) for F1 also provides a better open circuit, with minimal degradation of the short circuit. The superior performance of this Figure 6 circuit (as compared with that of Figure 4) greatly out-weights a slight disadvantage of requiring two control voltages. However with appropriate control circuits these could be derived from one master control voltage. As described with reference to Figures 1 and 2, the initial quadrature power split is conveniently accomplished using a Lange coupler. Real Lange couplers have propagation losses associated with both the conductors and the dielectric. If the coupled and through ports 4 and 5 respectively were terminated in open circuits, total reflection would take place and the signal would appear at the isolated port 3, slightly attenuated due to the coupler losses. If the coupled and through ports 4 and 5 were terminated in short circuits, total reflection would again take place but with an accompanying 180° phase shift. The signal now appearing at the isolated port 3 would not be, however, of the same magnitude as with open circuit terminations but would be attenuated by a further 1dB. Also the phase difference would not be 180° but approximately 170°. The reason for this is that in the short circuit case current flow in the coupler is a maximum and conductor losses predominate. In the open circuit case, voltage is a maximum and dielectric losses predominate. Conductor losses have a more pronounced effect on overall propagation losses than dielectric losses. Including a suitable length of transmission line L3 between the ports 4 and 11 and between the ports 5 and 11 of the variable loads 10 and the coupler 1 compensates at least partially for the effect of the coupler losses on phase. Whilst the length of the line L3, before the variable resistive load 10, corrects phase errors due to coupler losses, it does not correct the slight difference that would be observed between the amplitudes of ρ if the open and short circuits were perfect. However, in the real variable load, the FETs present a better quality short circuit than open circuit, and this almost completely eliminates the magnitude error. A near 180° phase difference is obtained over the whole dynamic range. The whole bi-phase modulator including these variable resistors 10 can be formed using microwave monolithic integrated circuit technology. Figure 8 illustrates one possible example of the layout of such a circuit on a gallium-arsenide substrate, in which the various components are labelled as in Figure 2. With the Figure 6 circuit configuration for the variable resistors 10, a very compact layout of the variable bi-phase modulator can be obtained, as compared with a variable bi-phase modulator formed of a Lange coupler (the through and coupled ports of which are terminated in switches to provide the optional 180° phase shift) followed by a transmissive attenuator to adjust the magnitude. The squares containing diagonal crosses (X) indicate vias through the substrate to a bottom ground-plane so grounding these parts of the circuit, e.g. the short-circuit shunt stubs L1 and L2 and the sources of the FETs F1 and F2. The control voltages VG1 and VG2 are applied to contact-pads which are connected to the gates of F1 and F2 by bias lines 25 and which are connected to ground through capacitors 26. Transistors F1 and F2 of conventional form with interdigital source and drain are illustrated. A pair of the variable bi-phase modulators (each with a Lange coupler 1 and a pair of variable loads 10) of Figures 2 and 8 may, for example, be connected in parallel arms between an input quadrature power divider (e.g. a Lange coupler) and an output power combiner (e.g. a Wilkinson combiner) to form an I-Q vector modulator. In this I-Q vector modulator, the input signal is split into two orthogonal components by the input Lange coupler, these two components (I and Q vectors) are adjusted in amplitude by the pair of variable bi-phase modulators, and these I and Q vectors from the pair of modulators are then added in the Wilkinson combiner to give the final output signal. The whole vector modulator may be formed as one monolithic integrated circuit. Such an I-Q vector modulator may, for example, be used to provide the modulator unit 28 illustrated in Figure 4 of GB 8828561.4 in a reflected power canceller of a CW radar. As well as ease of manufacture in microwave monolithic integrated circuit technology, the variable resistors 10 formed in this manner with suitably-tuned transistors F1 and F2 and resistor R can have very good characteristics and can permit performance enhancement of variable bi-phase modulators in relation to their phase noise performance, bandwidth, power handling ability, dynamic range and phase accuracy. In general a vector modulator which uses these variable bi-phase modulators will have lower amplitude and phase errors. This has important systems consequences. When used as a reflected power canceller for FMCW radar, the reduction in phase errors improves the suppression of unwanted reflections and so the receiver sensitivity will be increased. In phased array radar, sidelobe levels are reduced and the direction finding accuracy is improved. Instead of being designed as a one-port reflection attenuator for use in a variable bi-phase modulator circuit, such resistors 10 may also be designed for use in other microwave circuits in accordance with the invention and with two ports, for example transmission attenuators, phase shifters and power switches. Thus, for example a transmission attenuator may be designed with a variable resistor 10 having an input port 11 and an output port at the connected sources of F1 and F2 of Figure 6. Three such resistors 10 may be connected in a T or a π network similar to that described in said Tajima paper, except that each of the three resistors comprises F1, F2, R, L1 and L2, i.e the T or π network comprises a total of six FETs and three resistors. From reading the present disclosure, other variations will be apparent to persons skilled in the art. Such variations may involve other features which are already known in the design, manufacture and use of systems, circuits and component parts of variable bi-phase modulators and other microwave circuits comprising variable resistors, and which may be used instead of or in addition to features already described herein. It has been stated above that the drawings illustrate examples of embodiments of the invention and, in order to avoid any misunderstanding, it is hereby further stated that, in the following claims, where technical features mentioned in any claim are followed by reference signs relating to features in the drawings and placed between parentheses, these reference signs have been included in accordance with Rule 29(7) EPC for the sole purpose of facilitating comprehension of the claim, by reference to an example.

Claims: A variable resistor in a microwave circuit, the resistor comprising a microwave field-effect transistor (F2) connected with zero dc bias between its source and drain and having a channel resistance which changes with change in gate voltage (VG2), characterised in that the variable resistor comprises first and second of said transistors (F1,F2) the drains of which are coupled together via an intermediate resistor (R), in that an input port (11) of the variable resistor comprises a connection to the intermediate resistor (R) and to the drain of the first transistor (F2), and in that a shunt stub (L1,L2) is connected to the drain of each transistor (F1,F2) to at least partially compensate at the frequency of operation of the transistor (F1,F2) for the source to drain capacitance and for shifts in reference plane due to changes in the gate voltage (VG1,VG2) of each transistor (F1,F2). A variable bi-phase modulator circuit for microwave signals comprising a quadrature power divider (1) having signal input and output ports (2,3) and two control ports (4,5), and comprising two variable resistors each having an input port (11) connected to the respective control port (4,5), wherein each of the two variable resistors comprises first and second microwave field-effect transistors (F1,F2) the drains of which are coupled together via an intermediate resistor (R), the input port (11) of the variable resistor comprising a connection to the intermediate resistor (R) and to the drain of the first transistor (F2), each transistor (F1,F2) being connected with zero dc bias between its source and drain and having a channel resistance which changes with change in gate voltage (VG1,VG2), and wherein a shunt stub (L1,L2) is connected to the drain of each transistor (F1,F2) to at least partially compensate at the frequency of operation of the transistor (F1,F2) for the source to drain capacitance and for shifts in reference plane due to changes in the gate voltage (VG1,VG2) of each transistor (F1,F2). A bi-phase modulator circuit as claimed in claim 2, wherein propagation losses occur in the power divider (1), and the respective control port (4,5) is connected to the intermediate resistor (R) via a length of transmission line (L3) which compensates at least partially for the effect of these propagation losses on the phase characteristics of the control signal from the control port (4,5). A bi-phase modulator circuit as claimed in claim 2 or claim 3, and formed as a microwave monolithic integrated circuit. A variable resistor as claimed in claim 1 or a bi-phase modulator as claimed in any one of claims 2 to 4, wherein the first transistor (F2) has a wider channel than the second transistor (F1).

Inventors: DEVLIN LIAM MICHAEL, MINNIS BRIAN JOHN, DEVLIN, LIAM MICHAEL, MINNIS, BRIAN JOHN
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS UK LTD, PHILIPS ELECTRONICS N.V., PHILIPS ELECTRONICS UK LIMITED
Application year: 1990
Main classification: H01P1
IPC codes: H03H11, H03C3, H01P1, G01S13, H03C7, G01S7