Abstract:
A phasing receiver includes a quadrature mixing arrangement for frequency converting an input or high IF information signal to a pair of quadrature related low IF signals. The low IF signals are applied to a polyphase filter which functions as a low pass and adjacent channel rejection filter. One or more elements effecting a fine adjustment of relative phase away from quadrature and/or relative amplitude away from equality of the low IF signals are incorporated in information or oscillator signal paths in or about the mixing arrangement or a superhet stage preceding the quadrature mixing arrangement. These elements effect a predistortion of relative phase and/or relative amplitude of the low IF signals in order to compensate for mismatches in an input stage of the filter, and thereby improve image rejection by the filter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to phasing receivers and particularly, but not exclusively to polyphase or sequence-asymmetric receivers which may be implemented as an integrated circuit. 
     2. Description of the Related Art 
     A popular type of architecture for use as an integrated receiver is a zero-IF architecture in which an input signal is frequency downconverted to a zero-IF using quadrature mixers, the wanted signals are selected from the products of mixing using low pass filters and the wanted signals are processed further to provide a demodulated output. 
     Most of the limitations which beset the zero-IF receiver arise either directly or indirectly from the fact that components of wanted signals translated down to IF frequencies at or around DC cannot be distinguished from components of unwanted signals which appear in the same frequency range as a result of inherent circuit deficiencies. 
     The above-mentioned limitations in a zero-IF receiver may be eliminated substantially in a low-IF superheterodyne receiver but this would suffer from an image response that could not be eliminated by realistic front-end filters. 
     Another receiver architecture termed a phasing receiver, is a low-IF receiver based on the principle of the image-reject mixer in which the image response is removed by cancellation, rather than filtering, but the level of image rejection which can be achieved, even in fully-integrated form, is severely limited by the degree of matching which can be obtained between nominally identical components. A particular example of a phasing receiver is the polyphase or sequence-asymmetric receiver in which the conventional IF filters, IF phase shifters and IF signal combiner are replaced by a single polyphase IF filter. This very substantially increases the level of image rejection which can be obtained. In spite of the increased level of image rejection there may still a need to improve further the level of image rejection. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of the present invention is to improve the image rejection capability of a phasing receiver. 
     According to one aspect of the present invention there is provided a phasing receiver having a polyphase or sequence-asymmetric gyrator filter in which lack of image rejection is improved by the fine adjustment of the phase and/or amplitude of input signals. 
     According to a second aspect of the present invention there is provided a phasing receiver comprising an input, first and second signal mixing means for providing quadrature related low IF frequencies, a polyphase filter having inputs coupled to outputs of the first and second signal mixing means and signal demodulating means coupled to outputs of the polyphase filter, and means for effecting fine adjustment of the phase and/or amplitude of signals applied to the inputs of the polyphase filter to compensate for less than optimum image rejection. 
     By means of the present invention fine adjustment of the amplitude and/or phase of the pair of IF signals feeding a polyphase IF filter can further improve the level of image rejection which can be obtained, typically by at least 10dB. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a block schematic diagram of a polyphase receiver, 
     FIG. 2 is a simplified circuit diagram of a current-fed polyphase or sequence-asymmetric gyrator filter, 
     FIGS. 3A and 3B show one arrangement by which amplitude may be adjusted, 
     FIGS. 4A and 4B show another arrangement by which amplitude may be adjusted, 
     FIG. 5 shows one arrangement by which phase may be adjusted, and 
     FIGS. 6 to  13  are graphs showing the lack of image rejection due to errors in the resistance R 3 , the capacitance C 6  and the cross coupling gyrator JC 17 , shown in FIG.  2 . 
    
    
     In the drawings the same reference numerals have been used to indicate corresponding features. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the illustrated polyphase receiver comprises a low IF receiver in which the local oscillator frequency is offset by, for example, half the channel bandwidth from the nominal carrier frequency of an input signal received by an antenna  10 . 
     Ignoring the components in the box  100 , the antenna  10  is coupled by an RF amplifier  12  to a signal splitter junction  14 . First and second mixers  16 ,  18  each have a first input coupled to the junction  14 . A local oscillator  20  is coupled to the second input of the first mixer  16  and, by way of a 90 degree phase shifter  22 , to a second input of the second mixer  18 . The in-phase products I of mixing present at an output of the first mixer  16  are applied to a first input  26  of a current-fed polyphase filter  24 . The quadrature phase products Q of mixing present at an output of the second mixer are applied to a second input  28  of the filter  24 . 
     The polyphase filter  24  functions as an image rejection filter and a channel selectivity filter. Outputs  30 ,  32  of the filter  24  are coupled to a demodulator  34  having an output terminal  36 . 
     Referring to the components in the box  100 , the signal from the RF amplifier  12  is applied to a superhet frequency down conversion stage comprising a mixer  102  to which an output from a local oscillator  104  is applied. A bandpass filter  106  selects an IF signal which is amplified in an IF amplifier  108  and supplied to the junction  14 . Thus the quadrature frequency down conversion stage operates on the IF signal instead of the RF signal which will be the case if the superhet stage is not present. For convenience of description it will be assumed that the superhet stage is not present. 
     Although the polyphase filter  24  has been identified as being current fed, it may be voltage fed depending on the application. 
     FIG. 2 illustrates an embodiment of a 5th order polyphase filter  24 . The filter comprises 2 sets of stages, corresponding stages in each set being identical. For convenience of description only one set of the stages will be described and the corresponding components in the non-described set will be shown in parenthesis. A first stage of the filter comprises a source resistor R 1  (R 3 ), a capacitor C 1  (C 6 ) and a transconductor JC 1  (JC 9 ) are coupled in parallel between signal rails  38 ,  40 . A cross-coupled gyrator consisting of transconductors JC 26  and JC 17  is coupled to the signal rails  38 ,  40 . A current source J 1  (J 2 ) representing the inputs is coupled to the signal rails  38 ,  40 . The current sources J 1 , J 2  correspond to inputs  26 ,  28 , respectively, in FIG.  1 . 
     The second, third and fourth stages are of identical layout and will be described collectively. A transconductor JC 2  (JC 10 ), JC 4  (JC 12 ), JC 6  (JC 14 ) is connected in parallel with a capacitance C 2  (C 7 ), C 3  (C 8 ), C 4  (C 9 ) and another transconductor JC 3  (JC 11 ), JC 5  (JC 13 ), JC 7  (JC 15 ) between signal rails  42 ,  44 . Cross coupled gyrators JC 19  (JC 18 ), JC 21  (JC 20 ) and JC 23  (JC 22 ) are coupled to the signal rails  42 ,  44  of the respective stages. The fifth stage of the filter comprises a transconductor JC 8  (JC 16 ), capacitance C 5  (C 10 ) and resistor R 2  (R 4 ) connected in parallel between signal lines  46 ,  48 . A cross coupled gyrator JC 25  (JC 24 ) is coupled to the signal rails  46 ,  48 . Outputs  30 ,  32  are derived from the fifth stages. 
     The response of the polyphase filter  24  is centred on the low IF frequency, for example 12.5 kHz in the case of 25 kHz channel spacing. The component values can be determined by a number of known techniques and reference may be made to “Handbook of Filter Synthesis” by Anatol I. Zverev, published by John Wiley and Sons Inc., June 1967. Once the values of the resistors R 2 , R 2 , R 3  and R 4  and the transconductances of the transconductors JC 1  to JC 16  have been set the capacitances C 1  to C 5  (C 6  to C 10 ) determine the shape of the filter response and the filter bandwidth. The centre frequency of the filter, in this example 12.5 kHz, is determined by the cross-coupling of gyrators JC 17  to JC 26  as discussed by J. O. Voorman, “The Gyrator as a Monolithic Circuit in Electronic Systems” PhD Thesis of Catholic University of Nijmegen, The Netherlands, Jun. 16, 1977, pages 91 to 103. 
     A polyphase filter is normally fabricated as an integrated circuit and if the components were truly identical, the response would be as perfect as can be designed. However due to limited matching capabilities of the components, image rejection by the polyphase filter will be less than perfect. 
     The present invention endeavours to reduce, if not remove, the mismatches in component values by predistorting the amplitude and/or phase of drive signals in order to correct for the frequency dependent errors produced by the mismatches. 
     An examination of the effects of these mismatches has shown that mismatches between components in the early stages, especially the first stage, are responsible for the degrading of the image rejection. However because these components are in the early stages, the errors which they produce do not vary rapidly across the bandwidth of the filter. It has been found that it is possible to substantially correct for these errors by means of an essentially constant phase and/or amplitude correction inserted in the RF signal path, the local oscillator signal path or the low IF path. If the superhet stage is present, the correction would be in the higher IF path, the local oscillator signal path or the low IF path. 
     Referring to FIG. 2, the mismatches are considered are those between (a) the source resistors R 1  and R 3 , (b) the first capacitors C 1  and C 6 , (c) the amplitude match of the forward and reverse paths of the first cross-coupling gyrator JC 17 , and (d) the phase match of the forward and reverse paths of the first cross-coupling gyrator JC 17 . Mismatch due to (a) above can be reduced by fine adjustment of the phase of the IF input signals. The mismatch due to (b) above can be reduced by fine adjustment of the amplitude of the IF signals. The mismatch due to (c) above can be reduced by fine adjustment of the amplitude of the IF input signals and lastly the mismatch due to (d) above can be reduced by fine adjustment of the phase of the IF input signals. 
     Referring to FIG. 1, the adjustment of the amplitude and/or phase of the IF input signals can be effected in the RF signal path to the first and second mixers  16 ,  18 , for example at the junction  14 . Also phase adjustment can be effected in the output path of the local oscillator  20  but it is not considered viable to make amplitude adjustments because the mixers  16 ,  18  are normally intentionally overdriven by the local oscillator  20 . Amplitude adjustments may also be effected in the low IF signal paths between the outputs of the mixers  16 ,  18  and the filter inputs  26 ,  28 , respectively. Although phase adjustments are theoretically possible in these latter signal paths, in reality they are impractical. 
     FIGS. 3A and 3B illustrate a method of effecting an amplitude adjustment in an analogue manner. FIG. 3A shows a fixed potentiometer comprising resistors R 1 , R 2  connected in series across an input of one of the signal paths and a junction  50  of these resistors being connected to an output. FIG. 3B shows a variable potentiometer comprising a resistive potentiometer VR 1  connected in series with a fixed resistor R 3 . An output is taken from the wiper of the potentiometer VR 1 . The values of the potentiometer VR 1  and the resistor R 3  can be determined statistically to introduce a sufficient adjustment range. The values of the resistors R 1 , R 2  are then chosen to achieve the same attenuation as VR 1  and R 3 , when the potentiometer VR 1  is in its mid position. The potentiometer VR 1  permits adjustments of the amplitude error to be made in each individual receiver to maximise image rejection. 
     The amplitude adjustment arrangement shown in FIGS. 4A and 4B differs from that shown in FIGS. 3A and 3B by a digitally controlled potentiometer VR 2  being used in place of the combination of the potentiometer VR 1  and the fixed resistor R 3 . The digital value is determined by measuring image rejection and adjusting the attenuation to maximise the image rejection. 
     FIG. 5 illustrates an arrangement for adjusting phase. In one of the signal paths an adjustable nominal 45 degree phase lead network PA 1  is provided and in the other of the signal paths an adjustable nominal 45 degree phase lag circuit PA 2  is provided. The circuits PA 1  and PA 2  each comprise a varactor diode VAD and a fixed resistor R 4 . Phase adjustment is effected by altering the capacitance value of the respective varactor diodes VAD. This can be effected digitally in which a stored digital value is applied to a digital to analogue converter DAC which supplies an analogue signal to a controller  52 . The controller  52  produces control voltages VC 1  and VC Q  to adjust the varicap values electronically in the same direction. Although FIG. 5 indicates a nominal relative phase difference of 90 degrees, this can be varied to give a smaller or greater phase difference depending on the respective phase mismatch in the receiver. 
     In practice it has been found that mismatches between capacitances C 1  and C 6  are corrected by adjustments in amplitude whereas mismatches between source resistors are corrected by phase adjustments. In other words the amplitude and phase adjustments act in an essentially orthogonal manner which enables each adjustment to be carried out substantially independently of the other. 
     FIGS. 6 to  13  are graphs which illustrate the effects of mismatches and the improvements obtainable by making the adjustments discussed. All the graphs are plots of frequency in Hz against the output across the resistor R 2  in dBA. They all show a frequency shift of 300 kHz and a −3 dB bandwidth of 80 kHz. 
     In FIGS. 6 to  13 , the graphs have been folded about zero frequency and comprise a positive frequency part P and a negative frequency part N. Any imperfection is shown on the negative frequency part of the characteristic. 
     Taking FIG. 6 as an example the effects of the mismatch in the resistance R 3  are shown as a peak of increasing amplitude as the value of the resistance R 3  is increased in 5% increments up to a maximum of 25%. 
     In FIG. 7, the full line shows the peak due to a 5% error in the resistance R 3  and in broken lines the effect of improvement obtained by the fine adjustment of the input phase. The dip in the broken line is due to the fact the errors are frequency dependent. 
     FIG. 8 is a graph illustrating a lack of image rejection caused by increasing the value of the capacitance C 6  shown in FIG. 2 by 25% in steps of 5%. 
     FIG. 9 is a graph illustrating by a full line a lack of image rejection caused by 5% error in capacitance C 6  shown in FIG. 2 improved by fine adjustment of the input amplitude (broken lines). 
     FIG. 10 is a graph illustrating the lack of image rejection caused by increasing the value of amplitude of a cross-coupling gyrator JC 17  shown in FIG. 2 by 25% in steps of 5%. 
     FIG. 11 is a graph illustrating by a full line the lack of image rejection caused by 5% error in amplitude of the cross-coupling gyrator JC 17  shown in FIG. 2 improved by fine adjustment of input amplitude (broken lines). 
     FIG. 12 is a graph illustrating the lack of image rejection caused by a phase lead/lag of 5 degrees of the cross-coupling gyrator JC 17  shown in FIG.  2 . 
     Finally, FIG. 13 is a graph illustrating by a full line the lack of image rejection caused by a 5 degree error in phase of the cross-coupling gyrator JC 17  shown in FIG. 2 improved by fine adjustment of input phase (broken lines). 
     These mismatches are, in general, the most significant ones in terms of the resultant lack of image rejection although the actual degree of mismatch used in these examples is much greater than would normally be expected in practice. They have been used solely to make the resultant lack of image rejection clear. 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of phasing receivers and component parts thereof and which may be used instead of or in addition to features already described herein.