GB2203307A

GB2203307A - Variable phase shifter

Info

Publication number: GB2203307A
Application number: GB08807159A
Authority: GB
Inventors: Colin Stuart Aitchison
Original assignee: ERA Patents Ltd
Current assignee: ERA Patents Ltd
Priority date: 1987-03-30
Filing date: 1988-03-25
Publication date: 1988-10-12
Also published as: GB8807159D0; GB8707510D0

Abstract

A variable wide band for use at high frequencies such as those in the microwave range comprises means (1) to receive an input signal and to produce therefrom first and second signals with a 90 DEG (or 180 DEG ) phase difference between them; first (4) and second (7) amplifiers to receive and amplify the first and second signals respectively; 180 DEG (or 90 DEG ) phase shift means (24) to receive and combine the amplified first and second signals to produce two output signals which are phase-shifted relative to each other by an amount dependent upon the relative magnitudes of the amplified signals; and means to control the gains of the amplifiers (5 and 7) so that the sum of the gains is substantially constant. The phase shifter introduces no insertion loss and may even afford a signal gain between its input and output terminals. <IMAGE>

Description

VARIABLE PHASE SHIFTER This invention relates to a variable phase shifter, and particularly to a wideband phase shifter for use at high frequencies, for example in the microwave range.

Circuits providing a variable phase shift between two signals in the microwave frequency range have previously been proposed. However, such circuits have suffered from a serious disadvantage in that they inevitably introduce an insertion loss.

It is an object of the present invention to provide a variable phase shifter which can be arranged to introduce no insertion loss and can even be arranged to afford a signal gain between its input and output terminals.

According to the invention there is provided a variable phase shifter comprising means to receive an input signal and to produce therefrom first and second signals with a 900 (or 1800) phase difference between them; first and second amplifiers to receive and amplify the first and second signals, respectively; 1800 (or 900) phase shift means to receive the amplified first and second signals, to combine them and to produce from them two output signals which are phase-shifted relative to each other by an amount dependent upon the relative magnitudes of the amplified signals; and means to control the gains of the amplifiers so that the sum of the gains is substantially constant.

Preferably the means to produce the first and second signals comprises a 900 or a 1800 hybrid and preferably the means to produce the two output signals comprises a 1800 or a 900 hybrid. Each amplifier may comprise a MESFET distributed amplifier.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Fig. 1 is a block schematic diagram of a wideband microwave variable phase shifter; and, Fig. 2 indicates how the relative phase of two output signals produced by the phase shifter of Fig. 1 is varied.

Referring to Fig. 1, a wideband phase shifter comprises a 900 hybrid 1, to an input port 2 of which an input signal is applied. An isolated port 3 of the hybrid is properly terminated. An output port 4 is coupled to an amplifier 5, and an output port 6 is coupled to an amplifier 7.

Preferably the amplifiers 5 and 7 are distributed amplifiers. Such amplifiers comprise a number of transistors, preferably GaAs MESFETs, such as the transistors 8-11. The amplifier basically comprises two microwave transmission lines; a "gate" transmission line in which the gate electrodes of the transistors 8 to 11 are connected, and a "drain" transmission line in which the drain electrodes are connected. The source electrodes are connected to a "ground" line 12, which is common to the two transmission lines and to a microwave signal input 13 and a microwave signal output 14. The gate electrodes of successive transistors are interconnected via inductors 15, and the drain electrodes are interconnected by similar inductors 16.Inductors 17 and 18 are provided in the end sections of the lines The gate transmission line is terminated at its right-hand end (as viewed in Fig. 1) by a resistor 19 equal to the characteristic impedance of the line, which may be, for example, 50Q. The resistor 19 may be referred to as the "idling gate load". The microwave signal is fed into the input 13 from the hybrid 1 which presents to the gate line an impedance equal to that characteristic impedance. The drain transmission line is terminated at its left-hand end by a resistor 20 equal to the characteristic impedance of that line, again 50n, for example. The resistor 20 may be referred to as the "idling drain load".A d.c. source 21 is connected to the line 12, and is connected to the junction of the resistor 20 and the adjacent inductor 18, via a low-pass filter 22, to provide bias for the drain electrodes. A microwave signal fed into the input 13 is amplified by the successive transistors 8 to 11. A respective travelling wave passes along each of the gate and drain transmission lines and, if the phase constants of each line are equal and corrected designed, the gain of the amplifier will be substantially independent of the signal frequency. A gain of some 6dB can be obtained over a very wide frequency band such as 50 MHz to 20 GHz.

The outputs 14 and 23 of the amplifiers 5 and 7, respectively, are connected to input ports of a 1800 hybrid 24. An isolated port 25 of the hybrid is terminated by an impedance equal to the drain line impedance of the amplifiers 5 and 7. Respective output signals are taken from the isolated port 25 and an output port 26 of the hybrid 24.

Means (not shown) is provided to apply a variable bias voltage along the gate line of each amplifier, to vary the gain of each amplifier. The biasing means is operative to maintain the sum of the power gains substantially constant.

In operation of the phase shifter, a signal is fed into the input port 2 of the hybrid 1 and the two signals relatively phase displaced by 900 are thereby fed to the amplifiers 5 and 7. The outputs of the amplifiers are phase shifted in the hybrid 24 which produces an output signal at the output port 26 which has a phase displacement relative to the output of the isolated port 25. The phase displacement varies in dependence upon the relative amplitudes of the two signals fed to the hybrid input ports, and hence in dependence upon the setting of the bias voltage supply means.

This effect is illustrated in Fig. 2. Assuming, firstly, that the amplifier 5 is operating at full gain and that the amplifier 7 is turned off, the output signal at the port 26 of the hybrid 24 will be phase-shifted by 900 relative to the signal at the port 25. It will be assumed that this phase shift is anticlockwise (i.e.

-90 ), as shown in Fig. 2(a). If, on the other hand, the amplifier 5 is turned off and the amplifier 7 operates at full gain, the signal at the port 26 will be phase-shifted by 900 in the opposite direction, as shown in Fig. 2(b). If, now, the amplifier 5 is turned on and is operated at slightly less than full gain whilst the amplifier 7 is operated at low gain (the sum of the power gains being constant for all conditions, as mentioned above), the signal at the port 26 will be phase-shifted anticlockwise, but by less than 900. Such a condition is shown in Fig. 2(c). As the gain of the amplifier 5 is decreased and that of the amplifier 7 increased, the anticlockwise phase shift will decrease, until the point is reached at which the amplifier gains are equal. The signals at the ports 25 and 26 will then be in-phase, as shown in Fig. 2(d). Reducing the gain of the amplifier 5 still further while increasing the gain of the amplifier 7 accordingly, will cause the output signal to shift clockwise in phase, for example to a position shown in Fig. 2(e).

It will be seen, therefore, that by suitable differential adjustment of the gains of the amplifiers 5 and 7, a relative phase-shift in the output signal at the port 20 having any value from -900 to +900 can be obtained.

The hybrids 1 and 24 will both cause insertion losses of 3dB giving a total insertion loss of 6dB.

However, the amplifiers 5 and 7 will have a total gain of, say, 6dB, and further gain may be obtained by cascading extra distributed amplifier stages.

Although the above embodiment is intended for high-frequency operation, say in the 20MHz to 50GHz range, other configurations may be used, particularly at lower frequencies. For example, although hybrids are shown at the input and output of the amplifiers, other types of phase shifter may be used. Furthermore, the amplifiers may be of other configuration instead of the distributed amplifiers shown.

The types of hybrid might be reversed, so that the hybrid 1 is a 1800 hybrid and the hybrid 24 is a 90 hybrid.

Claims

1. A variable phase shifter comprising means to receive an input signal and to produce therefrom first and second signals with a 90" (or 1800) phase difference between them; first and second amplifiers to receive and amplify the first and second signals, respectively; 1800 (or 900) phase shift means to receive the amplified first and second signals, to combine them and to produce from them two output signals which are phase-shifted relative to each other by an amount dependent upon the relative magnitudes of the amplified signals; and means to control the gains of the amplifiers so that the sum of the gains is substantially constant.

2. A variable phase shifter according to claim 1, in which the means to produce the first and second signals comprises a 900 or a 180 hybrid.

3. A variable phase shifter according to claim 1 or 2, in which the means to produce the two output signals comprises a 1800 or a 900 hybrid.

4. A variable phase shifter according to claim 1, in which each amplifier comprises a MESFET distributed amplifier.

5. A variable phase shifter substantially as described with reference to the accompanying drawings.