GB2240890A

GB2240890A - Phase shifting circuit

Info

Publication number: GB2240890A
Application number: GB9026601A
Authority: GB
Inventors: Melvyn Mcgann
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1990-02-08
Filing date: 1990-12-06
Publication date: 1991-08-14
Also published as: GB9002786D0; GB9026601D0

Abstract

A phase shifting circuit capable of producing any phase shift between 0 DEG and 360 DEG on an r.f. signal comprises a broadband quadrature network 1 of resistors and reactances (Fig 2) producing quadrature outputs and a digital controller 4 for generating cyclic functions also in quadrature, the quadrature outputs are multiplied 2, 3 by the quadrature cyclic functions and then combined to produce a given phase shift. The values of the quadrature cyclic functions may be varied to vary the phase shift produced by the network, or continuously increased or decreased to vary the frequency of the output signal. The cyclic functions may be stored in digital form. The quadrature network is a polyphase network (Fig 2) comprising rows of resistors with skew ladder connections of capacitors or inductors. The phase shifting circuit may be used to correct phase distortion in a transmitter. <IMAGE>

Description

Phase Shifting Circuit This invention relates to phase shifting circuits.

It is known (GB-A-1584557 and GB-A-2135844) for a phase shifting circuit to comprise a phase shifter for producing outputs with quadrature phase shift relative to each other, and means for multiplying each output with respective cyclic functions, which are also in quadrature, and for combining the multiplied signals to produce a given phase shift. With such a phase shifting circuit, it is possible to produce a variable phase shift over any angle without a change in the amplitude of the input signal.

However, the known phase shifting circuits operate on a narrow band of input frequencies.

The invention provides a phase shifting circuit comprising a phase shifter for producing outputs with quadrature phase shift relative to each other, and means for multiplying each output with respective cyclic functions, which are also in quadrature, and for combining the multiplied outputs to produce a given phase shift, wherein the phase shifter is a broadband phase shifting network comprising a plurality of rows of series resistors, each row containing equal numbers of resistors, and the network including reactances connected between pairs of resistors in one row and the next successive pairs of resistors in another row.

While such polyphase networks comprising rows of series resistors coupled by reactances are known as broadband quadrature networks, they have not hitherto been proposed for use in circuits for producing a variable phase shift without a change in amplitude of the input signal. The circuit of the invention provides a particularly convenient arrangement of broadband phase shifting circuit suitable for producing any phase shift in an r.f. signal over a broad band of frequencies without a change in amplitude of the signal.

The invention is especially applicable to medium or high frequency (300KHz to 30MHz) or VHF or UHF (30MHz to 3000MHz) r.f. signals.

A phase shifting circuit constructed in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the phase shifting circuit; and Figure 2 is a circuit diagram of the broadband quadrature network in Figure 1.

A phase shifting circuit is designed to shift the phase of radio frequency signals, for example medium frequency or high frequency signals e.g. tens of MHz.

The phase shifting circuit comprises a broadband quadrature network 1 which, over a range of frequencies of the input r.f. signal, produces outputs which have a constant 900 phase difference over all frequencies. The phase from input to output of the quadrature network is variable and arbitrary. Only the outputs have accurate relative phase. The phase shifting circuit also includes multipliers 2, 3 for multiplying those outputs with respective cyclic functions, which are in quadrature, provided by digital controller 4. The digital controller 4 produces outputs of the form ksinz and kcosz. The digital controller is a ROM with sine and cosine values followed by digital-to-analogue converters. The value of z is increased by providing a signal on the input marked "up", in which case the ROM addresses are clocked through in one direction.The value of z is decreased by applying a signal to the "down" input, in which case the ROM addresses are clocked through in the opposite direction. With such a digitally generated pair of functions, it is apparent that the digital controller 4 can stop on any desired angle "z", thus providing two d.c. voltages "in quadrature".

The multiplied outputs are combined and fed to an output via an amplifier 5.

The circuit is operative to produce any phase shift and with constant amplitude. For example, when Z = 0, the phase shift circuit will produce a 900 phase shift since sin 0 = o and cos 0 = 1. When z = 450, both sin 45 and cos 45 are equal to 0.707, and the vectorial addition of equal amounts of 00 and 900 phase shift will be a nett 450 phase shift to the output signal.

Because the sine and cosine functions are cyclic, there is no discontinuity at 3600. Instead of sine and cosine functions, successively rising and falling ramp signals 900 out of phase could be used instead, but some amplitude variation would then be produced. Also, the outputs of the quadrature network could be fixed relative to the input phase e.g. the outputs could always be at 0 and 900 or at 2700 and 3600 or other values in quadrature.

Referring to Figure 2, the r.f. input is fed to transformer 6, the output winding of which is centre-tapped to earth, so that rows 1, 5 and 2, 6 are fed in anti-phase to rows 3, 7 and 4, 8. Each row contains five resistors in series, R1 to RSI R6 to R10, R11 to R15 and R16 to R20, with the first in each row having the same value as each other, similarly the values of the second in each row being the same as each other, etc. Capacitors C1 to C4 are connected between terminals 1 to 4 and the junctions of the first and second resistors in each row, in a 'cylindrical' arrangement where the capacitor C1 connects terminal 4 to the junction of R1 and R2 of the first row.

Capacitors C1 to C4 are the same value as each other, capacitors C5 to C8 are the same value as each other, etc.

The phase of terminal 6 is 900 relative to that of terminal 5, that of terminal 7 1800 relative to terminal 5 and that of terminal 8 2700 relative to terminal 5.

The number of resistors in each row determines the accuracy of the relative phases at the outputs, and also affects the frequency range. The capacitors are graded, with the largest nearest the input. The absolute values also affect the frequency range, and smaller changes between the values of each group give more accurate results.

Inductors may be used in place of capacitors.

The phase shifting circuit may be used for correcting phase distortion in a transmitter, such as described in our co-pending British patent application number 9002789.7. However, the phase shifting circuit could also be used in order to produce small but accurately controlled, variations in frequency of an input signal e.g. if a continuous signal was applied to the "up" input to the digital controller 4, so that the ROM addresses were continuously clocked through and the phase of the output was continually increased relative to the phase of the input, the output frequency would be slightly higher than the input frequency to the phase shifting circuit. Similarly, the frequency of an input signal could be decreased by clocking through the ROM addresses in a digital controller 4 in the other direction. The speed could be varied in order to vary the amount of frequency shift produced. This would enable small frequency off-sets to be applied to high frequency (even U.H.F.) signals, something very difficult to do by other means. In the case of a fixed frequency change by some low frequency, it would be possible to use a source oscillator at that frequency followed by a 900 phase shifting network to generate the sine and cosine functions.

Claims

1. A phase shifting circuit comprising a phase shifter for producing outputs with quadrature phase shift relative to each other, and means for multiplying each output with respective cyclic functions, which are also in quadrature, and for combining the multiplied outputs to produce a given phase shift, wherein the phase shifter is a broadband phase shifting network comprising a plurality of rows of series resistors, each row containing equal numbers of resistors, and the network including reactances connected between pairs of resistors in one row and the next successive pairs of resistors in another row.

2. A phase shifting circuit as claimed in claim 1, in which there are four rows of resistors, pairs of the inputs being connected to an anti-phase input device, and pairs of the outputs being in quadrature phase relation.

3. A phase shifting circuit as claimed in claim 2, in which the reactances are capacitances.

4. A phase shifting circuit as claimed in any one of claims 1 to 3, including means for storing the cyclic functions in digital form.

5. A phase shifting circuit as claimed in claim 4, in which the cyclic functions alternatively increase and decrease in time.

6. A phase shifting circuit as claimed in claim 5, in which the functions are sine and cosine functions.

7. A phase shifting circuit as claimed in any one of claims 4 to 6, in which the cyclic functions are stored in a ROM.

8. A phase shifting circuit as claimed in any one of claims 1 to 7, in which the cyclic functions are continuously increased or decreased in order to produce a frequency shift.

9. A phase shifting circuit substantially as herein described with reference to the accompanying drawings.