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This invention relates to an image rejection mixer circuit arrangement
comprising first and second signal mixers each having first and second
inputs and an output, a signal input coupled to the first input of each
mixer, a local oscillator, a coupling from an output of said local
oscillator to the second input of the first mixer, a coupling from an
output of said local oscillator to the second input of the second mixer,
means for producing a relative phase difference of substantially
90.degree. between signals applied in operation to the respective said
first inputs or to the respective said second inputs, and signal combining
means having inputs coupled to the mixer outputs for combining the output
signals of the mixers in such manner as to produce at an output thereof,
when oscillator output signals of a given frequency are applied to said
second inputs and first and second input signals having respective
frequencies which are symmetrically spaced with respect to, and have a
specific spacing from, said given frequency are applied to said signal
input, the said first input signal translated to a lower frequency while
suppressing at said output said second input signal also translated to
said lower frequency.
One known arrangement of this kind is disclosed in German
Offenlegungsschrift No. 2730153. In this known arrangement input signals
applied to the signal input are fed, either in phase with each other or in
antiphase, to the first inputs of the mixers and oscillation signals are
applied from the oscillator in quadrature to the second inputs of the
mixers. The signal combining means comprises a summing or differencing
amplifier (depending on whether the signals applied to the first inputs of
the mixers are in phase or in antiphase), a first surface acoustic wave
device bandpass filter coupling the output of the first mixer to one input
of the amplifier and a second surface acoustic wave device bandpass filter
coupling the output of the second mixer to the other input of the
amplifier. Said filters have the same pass frequency characteristic and
share a common piezo-electric substrate. The spacing between the input and
output electro-acoustic transducers of one saw filter device is different
from the spacing between the input and output electro-acoustic transducers
of the other, the difference being chosen so that a relative phase shift
of 90.degree. is produced between signals traversing respective ones of
the two devices when these signals have a frequency equal to the
centre-frequency of the pass characteristic of the two devices. The result
is that, when input signals having respective frequencies which are
symmetrically spaced with respect to the frequency of the oscillation
signals and differ therefrom by an amount equal to the centre-frequency of
the bandpass filters are applied to the signal input, only one such input
signal translated to a lower frequency (the difference between the
frequency of the input signal and the frequency of the oscillation signal)
appears at the amplifier output, in spite of both being present in the
output signals of the mixers. This is because the components of the output
signal of one mixer arising from the translation to a lower frequency of
the two input signals will be in phase with each other, whereas the
components of the output signal of the other mixer arising from the
translation to a lower frequency of the two input signals will be in
antiphase with each other and in mutually opposite quadrature to the
corresponding components of the output signal of the first-mentioned
mixer. Thus, introducing a 90.degree. phase shift in one of these output
signals and adding the result to or subtracting the result from the other
output signal will result, in the ideal case, in a signal which contains
the components arising from one input signal but not those arising from
the other.
The known arrangement has the disadvantage that a rather large area of
piezo-electric substrate is required for the two surface acoustic wave
devices. Moreover, the two devices tend to suffer from a rather large
insertion loss due to the fact that half the acoustic surface wave energy
produced by each input transducer travels away from, rather than towards,
the corresponding output transducer. It is an object of the invention to
mitigate these disadvantages.
The invention provides an image rejection mixer circuit arrangement
comprising first and second signal mixers, each having first and second
inputs and an output, a signal input coupled to the first input of each
mixer, a local oscillator, a coupling from an output of said local
oscillator to the second input of the first mixer, a coupling from an
output of said local oscillator to the second input of the second mixer,
means for producing a relative phase difference of substantially
90.degree. between signals applied in operation to the respective said
first inputs or to the respective said second inputs, and signal combining
means having inputs coupled to the mixer outputs for combining the output
signals of the mixers in such manner as to produce at an output thereof,
when oscillation signals of a given frequency are applied to said second
inputs and first and second input signals having respective frequencies
which are symmetrically spaced with respect to, and have a specific
spacing from, said given frequency are applied to said signal input, the
said first input signal translated to a lower frequency while suppressing
at said output said second input signal also translated to said lower
frequency, characterized in that said signal combining means comprises an
acoustic surface wave device comprising a piezoelectric substrate for
propagating acoustic surface waves at a surface thereof, a first
electro-acoustic transducer structure formed on said surface for launching
acoustic surface waves along a propagation path at said surface and a
second electro-acoustic surface wave transducer structure formed on said
surface for receiving acoustic surface waves propagating along said path,
said first transducer structure being of a four-phase or pseudo-four-phase
unidirectional type having first and second inputs for mutually
quadrature-related signals, the outputs of said first and second mixers
being coupled to said first and second inputs, respectively, of said first
transducer structure.
Such unidirectional electro-acoustic transducer structures (see, for
example, the paper by R. C. Peach and C. Dix entitled "A low loss medium
bandwidth filter on lithium niobate" in Proc. IEEE Ultrasonic Symposium,
1978, pages 509-512, published British patent specification Ser. No.
2018081, and the paper by D. C. Malocha and S. Wilkus entitled "Low loss
capacitively weighted TN IF filter" in Proc. IEEE Ultrasonics Symposium,
1978, pages 500-503, particularly FIG. 5 on page 502 thereof) require, if
they are to operate in a unidirectional manner, i.e. in such a manner that
acoustic surface waves are propagated thereby in substantially only one
direction, that identical signals lying within a specific narrow frequency
band are applied in mutual quadrature one to each of their first and
second inputs. This quadrature relationship is normally obtained by
feeding both inputs from a common signal source and including a suitable
phase shifter in one or both feed paths. It has now been recognized that
in a mixer circuit arrangement of the kind defined in the first paragraph
the output signals of the first and second mixers are suitable for
application directly to these first and second inputs, i.e., without the
interposition of a phase-shifting arrangement, because of the inherent
phase relationships between the aforesaid components of the mixer output
signals. Provided that their frequencies lie within the aforesaid specific
narrow frequency band the components of the mixer output signals arising
from the translation to a lower frequency of one of said first and second
input signals applied to said signal input will give rise to corresponding
acoustic surface waves propagating in a given direction (only) and the
components of the mixer output signals arising from the translation to a
lower frequency of the other of said first and second input signals will
give rise to corresponding acoustic surface waves propagating in the
opposite direction (only). Thus, if the second transducer is positioned to
receive the acoustic surface waves propagated by the first transducer in
the given direction, to the exclusion of those propagated in the opposite
direction, its output signal will correspond to only one of the
frequency-translated first and second input signals as required. The
propagation of the waves corresponding to the two frequency-translted
input signals in opposite directions arises because of the fact that,
although both input signals give rise to components of the mixer signals
which are in quadrature with each other, the quadrature relationship of
the components arising from the translation to a lower frequency of one
input signal is opposite to the quadrature relationship of the components
arising from the translation to a lower frequency of the other input
signal. Thus the arrangement can achieve a similar result to that obtained
by means of the arrangement disclosed in German Offenlegungsschrift No.
2730153without the acoustic surface wave device required two input
transducers together with the area of piezo-electric substrate which would
then be required to accommodate the second transducer and a propagation
path therefrom to an output transducer, and without a substantial
proportion of the wanted mixer output signal components giving rise to
acoustic surface waves which are unused.
Embodiments of the invention will now be described, by way of example, with
reference to the accompanying diagrammatic drawings in which:
FIG. 1 is a block diagram of an embodiment,
FIG. 2 shows a possible construction for a part of the embodiment of FIG.
1,
FIG. 3 shows a first possible configuration for an acoustic surface wave
device which forms part of the embodiment of FIG. 1, and
FIG. 4 shows a second possible configuration for the said acoustic surface
wave device.
In FIG. 1 an image rejection mixer circuit arrangement comprises first and
second mixers 1 and 2 respectively. Mixer 1 has a first input 3, a second
input 4 and an output 5. Similarly mixer 2 has a first input 6, a second
input 7 and an output 8. A signal input 9 is coupled to the first inputs 3
and 6. An output 10 of an oscillator 11 is coupled to the second inputs 4
and 7 via outputs 12 and 13 respectively of a frequency divider-by-two
arrangement collectively denoted by reference numeral 14. Divider-by-two
arrangement 14 is constructed so that oscillation signals of half the
output frequency of oscillator 11 appear in mutual quadrature at its
outputs 12 and 13 respectively, and hence at the mixer inputs 4 and 7
respectively. The outputs 5 and 8 of the mixers 1 and 2 respectively are
coupled via low-pass filters 15 and 16 respectively to first and second
inputs 17 and 18 respectively of signal combining means 19. Combining
means 19 combines the output signals of the mixers 1 and 2, filtered by
filters 15 and 16 to remove the components of these output signals having
frequencies equal to merely the frequencies of input signals applied to
signal input 9 and the oscillation signals applied to second inputs 4 and
7 and also frequencies equal to the sum of these frequencies, so as to
leave only components having frequencies equal to the difference between
these frequencies, in such manner as to produce across output terminals 20
and 21 thereof, when oscillation signals of a given frequency are applied
to the mixer second inputs 4 and 7 from divider arrangment 14 and first
and second input signals having respective frequencies which are
symmetrically spaced with respect to, and have a specific spacing from,
said given frequency, the said first signal translated to a lower
frequency (the difference between the frequency of the first signal and
that of the oscillation signals) while suppressing at the output terminals
20 and 21 the said second signal (which signal is at the so-called image
frequency) translated to the said lower frequency. To this end combining
means 19, possible specific constructions for which will be described with
reference to FIGS. 3 and 4, comprises an acoustic surface wave device
comprising a piezo-electric substrate for propagating acoustic surface
wave at a surface thereof, a first electro-acoustic transducer structure
formed on said surface for launching acoustic surface waves along a
propagation path at said surface, and a second electro-acoustic surface
wave transducer structure formed on said surface for receiving acoustic
surface waves propagating along said path. The first transducer structure
is of a pseudo-four-phase unidirectional type and has first and second
inputs for mutually quadrature-related signals, these inputs being
connected to the inputs 17 and 18 respectively. A common terminal of the
first transducer structure is connected to point 22, which is connected to
ground. The second transducer structure is connected to the output
terminals 20 and 21 which may be connected in turn to, for example, the
input of an intermediate frequency amplifier in a broadcast receiver, the
signal input 9 then being fed from an aerial (not shown).
FIG. 2 shows a possible construction for the frequency divider-by-two
arrangement 14 of FIG. 1. In FIG. 2 the output 10 of the oscillator 11 of
FIG. 1 is coupled to the base of a transistor 49 the emitter of which is
connected to the output of a current source 50 and to the emitter of a
transistor 51 the base of which is held at a reference potential
V.sub.ref. The collector of transistor 51 is connected to the commoned
emitters of a pair of transistors 37 and 39 and the collector of
transistor 49 is connected to the commoned emitters of a pair of
transistors 38 and 40. The collectors of transistors 37, 38, 39 and 40 are
connected to a positive power supply terminal 41 via equal-value load
resistors 45, 46, 47 and 48 respectively. The collector of transistor 37
is connected to the collector of transistor 38 via the series combination
of resistors 52 and 53, the collector of transistor 38 is connected to the
collector of transistor 39 via the series combination of resistors 54 and
55, the collector of transistor 39 is connected to the collector of
transistor 40 via the series combination of resistors 56 and 57, and the
collector of transistor 40 is connected to the collector of transistor 37
via resistors 58 and 59. Resistors 52-59 have equal values. Output
terminals 12, 13, 12' and 13' are connected to the common point of
resistors 52 and 53, the common point of resistors 54 and 55, the common
point of resistors 56 and 57, and the common point of resistors 58 and 59
respectively. A negative power supply terminal 60 is connected to the
current source 50.
In operation transistors 49 and 51 are turned on alternately in response to
successive half-cycles of the input signal applied to terminal 10 and this
results in a voltage wave travelling around the loop formed by the
resistors 52-59 once every two cycles of the input signal. Thus a first
output signal at half the frequency of the input signal appears in a
balanced manner at terminals 12 and 12' and a second output signal also at
half the frequency of the input signal and in quadrature with the first
output signal appears in a balanced manner at terminals 13 and 13'. The
circuit of FIG. 2 is substantially as described and claimed in British
Pat. No. 1,507,877 (PHN 7833) and as shown in FIG. 9 on page 60 of the
article "Frequency-dividers for ultra-high frequencies" by W. O.
Kasperkovitz published in Philips Tech, Rev. 38, 1978/79 no. 2, pages
54-68.
FIG. 3 shows, diagrammatically, a plan view of a first possible
configuration for the combining means 19 of FIG. 1. As shown in FIG. 3 it
is in the form of an acoustic surface wave device comprising a
piezo-electric substrate 23, for example of lithium niobate cut and
orientated in known manner. For example it may be Y-cut Z-propagating, or
41.degree. or 124.degree. to 131.degree. rotated Y-cut X-propagating. On
the surface of substrate 23 are provided, for example in conventional
manner by a process of photolithography, a first or input transducer 24 of
a pseudo-four-phase unidirectional type and a second or output transducer
25. Transducer 24 comprises an interdigital array of first, second and
third sets of electrods 26, 27 and 28 respectively with the electrodes of
these sets connected to bus-bars 29, 30 and 31 respectively. Bus-bars 29,
30 and 31 are provided with contact pads which constitute the terminals
22, 17 and 18 respectively of FIG. 1. In operation transducer 24 launches
an acoustic surface wave along a propagation path towards transducer 25,
this path being indicated by an arrow 32. Transducer 25 comprises an
interdigital array of first and second sets of electrodes 33 and 34
respectively with the electrodes of these sets connected to bus-bars 35
and 36 respectively. Bus-bars 35 and 36 are connected to contact pads
which constitute the terminals 20 and 21 respectively of FIG. 1. The
centre-to-centre distances or pitch of the electrodes in each array is
equal to .lambda./4, where .lambda. is the wavelength of acoustic surface
waves which would be launched along the path 32 should suitably
quadrature-related electric input signals having a frequency f.sub.i be
applied to pads 17 and 18 respectively, relative to pad 22, where f.sub.i
is the difference frequency between signals applied to terminal 9 and the
oscillation signals applied to mixer inputs 4 and 7 to which the
arrangement of FIG. 1 is required to respond, i.e. to the centre-frequency
of the desired IF signal at the output terminals 20 and 21. The widths of
the electrodes in each array may be equal to .lambda./8. Transducer 24 is
as described and claimed in British patent specification No. 2018081.
Transducer 24 launches waves along path 32 and not in the opposite
direction, i.e. along a path denoted by arrow 44, only if the signal of
frequency f.sub.i applied to pad 18 leads the signal applied to pad 17 by
90.degree.. If the signal of frequency f.sub.i applied to pad 18 lags the
signal applied to pad 17 by 90.degree. transducer 24 launches waves along
path 44 and not along path 32. Any waves launched along path 44 are
absorbed by an area 43 of acoustic surface wave absorbent material, for
example silicone rubber, provided where shown on the top surface of the
substrate 23. Thus, when oscillation signals of a given frequency f.sub.o
are applied to the second inputs 4 and 7 of the mixers 1 and 2 of the
arrangement of FIG. 1, and input signals having frequencies equal to
(f.sub.o +f.sub.i) and (f.sub.o -f.sub.i) respectively are applied to
signal input 9, the transducer 25 will receive surface acoustic waves
corresponding to only one of these input signals translated to the
difference frequency f.sub.i, because of the phase relationships between
the various resulting components of the output signals of mixers 1 and 2
as set forth hereinbefore.
In practice the transducers 24 and 25 will contain more than the eight
electrodes shown in each. They may each be extended in an exactly similar
manner to that shown to contain for example, eighty electrodes. The
transducer 25 may be apodized in known manner, if desired, so as to obtain
a particular required IF frequency response characteristic. Moreover
transducer 25 may be of the single-finger type (with successive electrodes
connected to the bus-bars 35 and 36 alternately and having
centre-to-centre distances of .lambda./2) instead of the double-finger
type shown. It should be noted that substantially complete separtion of
the difference signal corresponding to a wanted input signal from the
difference signal corresponding to an input signal at the image frequency
is only obtained by means of transducer 24 when the difference signals
have a frequency exactly equal to f.sub.i ; increasing deviations from
this frequency give rise to increasing proportions of acoustic surface
waves which should be launched in only one of the paths 32 and 44 in fact
being launched in the other. Thus the arrangement described gives
satisfactory image rejection only when the IF fractional bandwidth is
sufficiently small. The actual IF fractional bandwidth with which the
arrangement can operate satisfactorily will depend on the degree of image
rejection required in the particular context in which it is employed. If
desired the IF fractional bandwidth with which the arrangement can cope
satisfactorily may be increased by applying an appropriate phase
correction to the mixer output signals.
FIG. 4 shows, diagrammatically, a plan view of a second possible
configuration for the combining means 19 of FIG. 1. The configuration of
FIG. 4 is very similar to that of FIG. 3, and corresponding components in
each have been given the same reference numerals. The difference between
the configuration of FIG. 4 and that of FIG. 3 is that the unidirectional
transducer 24 is now of the "group" type, as shown in FIG. 5 of the paper
by Malocha and Wilkus previously referred to, thus avoiding the crossover
in the connections of the electrodes to the bus-bars 17 and 18 in FIG. 3.
The centre-to-centre distances of the electrodes in each transducer is
again .lambda./4, and again the transducers will in practice contain more
electrodes than the number actually shown.
Although in both FIG. 3 and FIG. 4 the unidirectional transducer 24 is of
the pseudo four-phase type it will be appreciated that, as an alternative,
it may be actually four-phase, with successive electrodes having
centre-to-centre distances of 80 /4 being supplied fro the mixers 1 and 2
alternately, the signals applied to alternate electrodes always being
inverted with respect to each other. However, such a four-phase transducer
requires even more crossovers than those shown in the transducer 24 of
FIG. 3 and is in consequence rather difficult to manufacture. Pseudo
four-phase transducers (of which there are other configurations than those
shown) are therefore preferred.
Although in FIG. 1 the oscillation signals applied to the second inputs 4
and 7 of the mixers are in mutual quadrature, and the signals applied to
the first inputs 3 and 6 are in phase (or in antiphase if an inverter, not
shown, is included in the signal path to one first input) it will be
appreciated that, as an alternative, it may be arranged that the
oscillation signals applied to the second inputs are in phase with each
other and the signals applied to the first inputs are in mutual
quadrature. To this end the divider circuit 14 of FIG. 1 may be replaced
by a direct connection and, for example, a 90.degree. phase shifter may be
included in the signal path from signal input 9 to one of the first inputs
3 and 6, or phase shifters giving phase shifts of +45.degree. and
-45.degree. respectively may be included in respective one of the signal
paths from signal input 9 to the first inputs 3 and 6. Normally, however,
it will be more convenient to apply oscillation signals to the second
inputs 4 and 7 that are in mutual quadrature.
There are, of course, alternative ways of producing the mutually
quadrature-related oscillation signals for the mixer inputs 4 and 7. For
example, oscillator 11 may be arranged to operate at the frequency which
it is required be applied to the inputs 4 and 7 and divider-by-two
arrangement 14 may be replaced by a 90.degree. hybrid.
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