United States Patent [191
`Laws
`
`[54] MIXER FOR DIRECT CONVERSION
`RECEIVER
`[75] Inventor:
`Peter G. Laws, Swindon, England
`[73] Assignee:
`Plessey Semiconductors Ltd.,
`England
`[21] Appl. No.: 740,992
`[22] Filed:
`Aug. 6, 1991
`[30]
`Foreign Application Priority Data
`_
`_
`Aug. 8, 1990 [GB] United Kingdom ............... .. 9017418
`[51] Int. Cl.5 ............................................. .. H04B 1/28
`[52] US. Cl. .................................. .. 455/314; 455/324;
`455/333; 307/529
`[58] Field of Search ..................... .. 455/84, 76, 86, 87,
`455/324, 333, 226.1, 314; 307/529; 330/151;
`331/12; 370/120
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`4,032,857 6/1977 Lum .................................. .. 330/151
`4,394,626 7/1983 Kurihara
`_______ " 331/12
`4,584,710 4/1986 Hansen .... ..
`455/2261
`4,768,187 8/1988 Marshall ............................ .. 370/ 120
`
`llllllllllllllllllllllllllllllllIllllIlllllllllllllllllllllllllllllllllllll
`5,303,417
`Apr. 12, 1994
`
`US005303417A
`[11] Patent Number:
`[45] Date of Patent:
`
`FOREIGN PATENT DOCUMENTS
`
`3734882 3/1989 Fed. Rep. of Germany .... .. 455/314
`
`Primary Examiner-Reinhard J. Eisenzopf .
`Assistant Examiner-Mark D. Wisler
`Attorney, Agent, or Firm-Kirschstein, Ottinger, Israel
`& Schiffmiller
`ABSTRACT
`[57]
`A mixer system for a direct conversion receiver, the
`receiver includin an RF in ut ath which is divided
`into I and Q pathsgfor demodglatigg from the RF signals
`I and Q signals in phase quadrature with One another,
`the mixer system including ?rst and second serially
`coupled mixer means in the I path, third and fourth
`serially coupled mixer means in the Q path, and a local
`oscillator providing a plurality of local oscillator signals
`in phase quadrature with one another to the mixer
`means, such that the ?rst mixer means receives a local
`oscillator signal in phase quadrature to the local oscilla
`tor signal applied to the second mixer means, and the
`third mixer means receives a local oscillator in Phase
`quadrature to that applied to the fourth mixer means.
`
`7 Claims, 12 Drawing Sheets
`
`HALF FREQUENCY MlXER
`
`MIX 11
`
`MIX 21
`
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`APPLE 1033
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`
`1
`
`MIXER FOR DIRECT CONVERSION RECEIVER
`
`FIELD OF THE INVENTION
`
`.
`
`.
`
`.
`
`.
`
`This invention relates to a mixer arrangement for
`maximising ef?ciency whilst minimising signal distor
`tion and noise, and is intended for use in direction con
`version of a received radio signal to quadrature I & Q
`baseband signals using a tuned local oscillator.
`
`5
`
`25
`
`20
`
`BACKGROUND ART
`The superheterodyne is the most commonly used
`radio receiver system. As shown in FIG. 1, it can be
`seen that two local oscillators are required for receiving
`and a third LO signal is needed for transmitting in a
`transceiver system. The ?rst LO will probably be gen
`erated by a VCO controlled by a synthesiser whilst the
`TX LO may be generated using a second frequency
`synthesiser or be produced by mixing the ?rst and sec
`ond LO signals. An RF ?lter is required to block un
`wanted radio signals and remove image frequencies
`which would otherwise be converted by the ?rst mixer
`to an inband IF frequency. A second high frequency
`?lter is required to remove unwanted side tones gener
`ated by the nonlinear action of the ?rst mixer.
`Direct conversion can be obtained by a homodyne
`system as shown in FIG. 2. This system is simpli?ed by
`the need for only one LO signal which may also be
`directly used for tansmitting in a transceiver. Filtering is
`also easier to perform since the RF ?lter has only to
`provide a blocking function and all post mixer ?ltering
`is provided by low pass baseband ?lters. The advantage
`of these base band ?lters is that they may be totally
`constructed within an integrated circuit. Automatic
`gain control will be required which can be provided by
`an RF AGC 1 stage or a combined action of AGC 1 and
`base band AGC 2 stages as shown.
`The disadvantage with this direct conversion system
`40
`is that the local oscillator may radiate a signal which
`when picked up at the front end will be converted down
`and generate a DC offset in the mixer outputs. Since this
`signal may be a multipath reflected signal when re
`ceived, it will probably have a varying amplitude and
`phase and hence when converted down could produce
`varying offsets i.e. unwanted base band signals. This
`problem is reduced by minimising the generated LO
`signal level, however this can result in decreased gain
`and noise performance in the mixers.
`
`5,303,417
`2
`ceiver the transmit frequency may be derived by dou
`bling the LO signal as shown.
`In implementing the system with serially coupled
`mixers it will be realised that additional noise and distor
`tion will be generated by the need for the second mixer.
`Also there is also an inherent signal power loss (about
`4dB) due to the power split into sidebands with only the
`baseband signal being of use. Most of the noise and
`distortion is generated in the tail of the mixer that con
`verts an input voltage into a current, which is then
`subsequently commutated by the LO switching devices.
`
`SUMMARY OF THE INVENTION
`It is an object of the present invention to reduce the
`problems of noise and distortion associated with serially
`coupled mixers.
`The present invention provides a mixer system for a
`direct conversion receiver, the receiver including an
`RF input path which is divided into I and Q paths for
`demodulating from the RF signals I and Q signals in
`phase quadrature with one another, the mixer system
`including ?rst and second serially coupled mixer means
`in the I path, third and fourth serially coupled mixer
`means in the Q path, and a local oscillator providing a
`plurality of local oscillator signals in phase quadrature
`with one another to the mixer means, such that the ?rst
`mixer means receives a local oscillator signal in phase
`quadrature to the local oscillator signal applied to the
`second mixer means, and the third mixer means receives
`a local oscillator in phase quadrature to that applied to
`the fourth mixer means.
`In one arrangement, four local oscillator signals may
`be provided each to a respective mixer means and hav
`ing
`relative
`phase
`shifts
`of
`for
`example
`+22.5,+67.5,—67.5,—22.5 degrees.
`In a preferred arrangement said I and Q input paths
`have a quadrature phase shifting arrangement prior to
`said mixing means so that the I and Q signals are in
`phase quadrature prior to said mixing means. The local
`oscillator provides ?rst and second signals in phase
`quadrature, and the ?rst signal is applied to the ?rst and
`third mixing means whereas the second signal is applied
`to the second and fourth mixing means. Such an ar
`rangement has the advantage of simplicity of implemen
`tation.
`In a preferred arrangement, the ?rst and second
`mixer means are combined in a double balanced mixer
`arrangement as are the third and fourth mixer means.
`Thus the ?rst mixer means comprises a ?rst transistor
`means having its main current path connected in series
`with the main current path of a second transistor means
`forming said second transistor means. Appropriate local
`oscillator signals are applied to the control electrodes of
`the ?rst and second transistor means. The RF input
`signal is injected into the main current path by further
`transistor means.
`The overall con?guration may be regarded as a
`stacked mixer arrangement in that the further transistor
`means and the ?rst transistor means constitute a ?rst
`mixer, and the further transistor means and the second
`transistor means constitute a second mixer.
`Thus the abovernentioned drawbacks can be greatly
`reduced by using stacked (cascaded) mixers such that
`the differential output currents from the ?rst mixing
`stage are fed directly into the second mixer commutat
`ing devices. Hence in practice this takes the form of a
`double balanced mixer. A consequence of directly cou
`
`Half-frequency direct conversion systems as disclosed
`in Swiss Patent CI-I-A-671856
`The basic concept of the half frequency conversion
`technique is shown in FIG. 3. In essence the received
`radio signal is mixed down to baseband by two directly
`coupled mixers each being driven by an L0 at half the
`’ wanted RF frequency. I and Q baseband signals can be
`derived by providing quadrature LO drives to the sec
`ond mixer stages. Note that no intermediate ?ltering is
`required (between the ?rst and ‘second mixers) since the
`sidebands generated by the ?rst mixer do not interfere
`when converted by the second mixing stage with the
`baseband outputs.
`FIG. 4 shows how the half frequency conversion
`technique may be implemented in a transceiver system.
`The ?ltering and AGC requirements are similar to the
`basic (single mixer) homodyne system. For a trans
`
`55
`
`65
`
`14
`
`

`
`3
`pling the mixers in this way means that separate first
`stage mixers are required for I and Q branches.
`From closer inspection of the double balanced mixer
`arrangement it can be shown that optimum commuta
`tion of the RF input will result when the LO inputs are
`in quadrature phase. Indeed in this case the combined
`commutation is the same as that of a single mixer with
`an L0 input at the RF frequency as in the basic direct
`conversion system! From this it may be reasoned that
`the overall mixing ef?ciency is not reduced and that the
`only increase in noise would be due to the second mixer
`commutating devices. However, since the half fre
`quency LO does not cause any pick up problems its
`amplitude may be increased enabling the effective mixer
`gain to increase with less noise contribution from all the
`mixer commutating devices. Note that attention should
`be paid to minimising the second harmonic of the L0 as
`this will be at the wanted RF frequency.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`Preferred embodiments of the invention will now be
`described with reference to the accompanying draw
`ings, wherein:
`FIGS. 1 to 4 are diagrams of known transceivers/
`receivers;
`FIG. 5 is a block diagram ofa ?rst embodiment ofthe
`invention;
`FIG. 6 is a block diagram ofa second embodiment of
`the invention;
`FIG. 7 is a diagram of the second embodiment of
`FIG. 6 adapted to form a transceiver;
`FIG. 8 is a diagram of the arrangement of FIG. 7
`incorporated in a direct conversion transceiver;
`FIG. 9 is a detailed circuit diagram of the second
`embodiment of FIG. 6;
`FIG. 10 shows circuit diagrams of phase shift ar
`rangements for use with FIG. 6; and
`FIGS. 11 and 12 show respective direct conversion
`receivers incorporating the arrangement of FIG. 6 with
`adaptive control of the phase shift arrangement.
`
`35
`
`25
`
`5,303,417
`4
`FIG. 8 shows a possible architecture for a half fre
`quency direct conversion transceiver system. The half
`frequency receiver mixers may be implemented using
`bipolar transistors as shown in FIG. 9. Quadrature RF
`input signals are feed into Q1 R1 and Q2 R2 which
`generate the tail currents for the two stacked mixers.
`The I channel is made up from Q5 and Q6 forming the
`?rst mixer stage with Q9-Q12 forming the second stage.
`A differential output stage is shown with R3 and R4
`converting the mixer output currents into voltages. A
`cascade stage consisting of Q3 has been included to help
`reduce any LO induced signals being feed through to
`the RF input. The Q channel is constructed in a similar
`fashion.
`The phase shift circuits may be constructed in a num
`ber of known ways, two of which are shown in FIG. 10.
`As previously mentioned the LO drives may also be
`generated by the use of a frequency divider network.
`Where low I and Q output frequencies are not re
`quired, the differential mixer outputs may be monitored
`and DC balance maintained by a system as shown in
`diagram 11. Here any common DC offset in the mixer
`outputs are corrected by an error signal being feed back
`to control the phase shift circuit. Phase correction may
`be achieved by trimming R and or C components by the
`use of FETs or varicap diodes respectively. This could
`have the advantage of enabling DC coupling of the
`outputs to be used and reduce the need for setting up of
`the LO drives.
`The advantage offered by this invention is to provide
`a direct conversion system which reduces the problems
`of local oscillator radiation being picked up by the front
`end and hence being converted down to unwanted
`baseband signals. It may also have a significant advan
`tage of reducing the pulling of the half frequency VCO
`when transmitting in transceiver systems. The tech
`nique is ideally suited for systems that I and Q demodu
`lation schemes such as in GSM DECT CT2 PCN etc.
`The present invention is intended for operation up to
`l GI-lz. This is of particular use in the cordless tele
`phone market and may be extended to higher frequency
`applications in the future.
`I claim:
`1. A mixer system for a direct conversion receiver
`comprising an RF input path for received signals modu
`lated on a radio frequency carrier, means to divide said
`received signals into I and Q paths for demodulating
`from said received signals I and Q signals in phase quad
`rature, ?rst and second serially coupled mixer means in
`said I path, third and fourth serially coupled mixer
`means in said Q path, local oscillator means to generate
`local oscillator signals at a frequency of one half of the
`frequency of said carrier, means to apply said local
`oscillator signals in phase quadrature to said first and
`second mixer means, means to apply said local oscillator
`signals in phase quadrature to said third and fourth
`mixer means, and means to derive said I and Q signals
`from outputs of said second and fourth mixer means
`respectively.
`2. A mixer system in accordance with claim 1
`wherein said means to divide said received signals into
`said I and Q paths is arranged to apply said received
`signals to said I and Q paths in phase quadrature, and
`said local oscillator signals are arranged to be applied to
`said ?rst and third mixer means in phase quadrature.
`3. A mixer system in accordance with claim 1
`wherein there are provided controllable phase shift
`means to apply said local oscillator signals in phase
`
`DISCLOSURE OF THE PREFERRED
`EMBODIMENTS
`Referring now to the FIGS. 5 to 12 of the drawings,
`using two pairs of stacked mixers I and Q baseband
`45
`signals may be derived as shown in FIG. 5. It is now
`evident that four phases of the LO signal are now re
`quired such that quadrature phases are provided be
`tween ?rst and second mixer stages and that overall I
`and Q outputs are generated from the RF input. Gener
`ating these four phases from a true half frequency LO
`may be cumbersome and inefficient due to the range of
`phase angles required. If however an L0 running at
`twice the RF frequency is used, the four half frequency
`phases may be generated using a divider network.
`An alternative approach is shown in FIG. 6. Here the
`I and Q outputs are determined by the phase shifted RF
`inputs. Since the LO drives are common to the mixer
`stages good matching between the two channels will
`automatically result. The phase difference between the
`two LO drives is not critical as this should only have a
`marginal effect on the mixing ef?ciency and create a
`constant DC shift on the outputs. This shift is due to the
`stacked mixers acting as a phase comparator between
`the two LO inputs. An additional advantage is that a
`transmit frequency may be generated using a further
`stacked mixer as shown in FIG. 7, utilising the same two
`LO inputs.
`
`55
`
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`
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`
`

`
`5,303,417
`5
`quadrature to said ?rst and second mixer means and in
`phase quadrature to said third and fourth mixer means,
`and feedback means responsive to said demodulated I
`and Q signals to control said phase shift means.
`4. A mixer system for a direct conversion receiver
`comprising an RF input path for received signals modu
`lated on a radio frequency carrier, means to divide said
`received signals into I and Q paths for demodulating
`from said received signals I and Q signals in phase quad
`rature, ?rst and second serially coupled mixer means in
`said I path, third and fourth serially coupled mixer
`means in said Q path, local oscillator means to generate
`local oscillator signals at a frequency of one half of the
`frequency of said carrier, means to apply said local
`oscillator signals in phase quadrature to said ?rst and
`second mixer means, means to apply said local oscillator
`signals in phase quadrature to said third and fourth
`mixer means, said local oscillator signals being applied
`to said ?rst, second, third and fourth mixer means with
`relative phase shifts of +67.5, —-22.5, +22.5 and —67.5
`20
`degrees respectively, and means to derive said I and Q
`signals from outputs of said second and fourth mixer
`means respectively.
`5. A mixer system for a direct conversion receiver
`comprising an RF input path for received signals modu
`lated on a radio frequency carrier, means to divide said
`received signals into I and Q paths for demodulating
`from said received signals I and Q signals in phase quad
`rature, ?rst and second serially coupled mixer means in
`said I path, third and fourth serially coupled mixer
`means in said Q path, local oscillator means to generate
`local oscillator signals at a frequency of one half of the
`frequency of said carrier, means to apply said local
`oscillator signals in phase quadrature to said ?rst and
`
`25
`
`6
`second mixer means. means to apply said local oscillator
`signals in phase quadrature to said third and fourth
`mixer means, and means to derive said I and Q signals
`from outputs of said second and fourth mixer means
`respectively, said I path including a ?rst transistor hav
`ing a main current path and a control electrode, said
`?rst mixer means including a second transistor having a
`main current path and a control electrode, and said
`second mixer including a third transistor having a main
`current path and a control electrode, and there being
`provided means connecting the main current paths of
`said ?rst, second and third transistors in series, means to
`apply said received signals to the control electrode of
`said ?rst transistor and means to apply the respective
`local oscillator signals for said ?rst and second mixer
`means to the control electrodes of said second and third
`transistors respectively.
`6. A mixer system in accordance with claim 5
`wherein said ?rst mixer comprises said second transistor
`and a fourth transistor having a main current path and a
`control electrode, said second and fourth transistors
`being connected as a differential pair with the main
`current paths of said second and fourth transistors each
`connected in series with the main current paths of said
`?rst and second transistors.
`7. A mixer system in accordance with claim 6
`wherein said second mixer comprises said third transis
`tor and a ?fth transistor having a main current path and
`a control electrode, said third and ?fth transistors being
`connected as a differential pair with the main current
`paths of said third and fifth transistors each connected
`in series with the main current paths of said ?rst and
`second transistors.
`
`*_ *
`
`*
`
`*
`
`if
`
`35
`
`45
`
`55
`
`65
`
`16