Note: Descriptions are shown in the official language in which they were submitted.
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Field of the Invention
This invention relates to mixers, and is particularly
concerned with a tree mixer, which can provide linear and low-
noise operation. Such mixers are specially useful in radio
communications systems.
Background of the Invention
Tree mixers, also known as analog multipliers or
Gilbert multipliers, have been widely used in integrated
circuits for communications systems for many years. As is
known for example from B. Gilbert, "A Precise Four-Quadrant
Multiplier with Subnanosecond Response", IEEE Journal of Solid-
State Circuits, Vol. SC-3. Pages 365-373, December 1968, such a
mixer or multiplier typically comprises a first or lower
differential pair of common emitter transistors to the bases of
which a first differential analog input signal is supplied, and
two second or upper differential pairs of transistors whose
bases are supplied with a second differential analog input
signal and whose collector-emitter paths conduct the currents
of the lower pair of transistor~ to produce in their collector
circuits an analog output signal which represents the product
of the input signals. A single current source in the emitter
circuit of the lower pair of transi~tors provides bias current
to all six transistors. For use as a mixer in a radio
communications receiver or tran~mitter, for example an input
signal is applied to the lower pair of transistors and a local
oscillator signal is applied to the two upper pairs, or upper
quad, of transistors.
Degeneration resistor~ connected to the emitters of
the lower differential pair of transistors serve to linearize
the input stage to accommodate larger input signals without
distorting.
Such a circuit provides advantages of good rejection
of the input signals at the output, good power supply
rejection, and the possibility of conversion gain.
There are three main disadvantages, however, of this
conventional tree mixer.
Firstly, there is a trade-off between noise and
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distortion, which limits the achievable dynamic range. Noise
can be reduced, but distortion is increased, by decreasing the
bias current and resistance in the emitter circuit of the lower
pair of transistors. Conversely distortion can be reduced
(linearity increased) by increa~ing these parameters, but this
increases noise, especially shot noise from the upper quad of
transistors, this being proportional to the bias current.
Secondly, energy and noise from the input image
frequency may appear at the mixer input and mix to the output,
at which point it will mask the desired signal. Image noise in
the degeneration resistors and bias circuit will also mix to
the output. The result is an increase in output noise and a
loss in dynamic range.
Thirdly, local oscillator (LO) energy (fundamental
and harmonic) may couple to the mixer input through parasitic
capacitances and circuit imbalances. LO feed-through to the
input is also enhanced by the wide-band coupling between lower
pair and mixing transistors. This may limit the dynamic range
of the mixer by overloading the mixer input. Spurious
frequencies (e.g. LO harmonics, associated mixing terms) or DC
offsets (e.g. leakage to the mixer input mixing with LO) may
also be observed at the mixer outputs due to LO coupling to the
input.
Attempts to improve the dynamic range of tree mixers
have been made. For example, two identical tree mixers have
been combined with quadrature phase shifting networks to
suppress the energy at the image frequency. Good image
rejection (e.g. greater than 50dB) can be achieved with this
topology, at a cost of additional circuit complexity and power
consumption (e.g. typically a factor of 2). Note that
intrinsic image noise in the mixer is not suppressed.
As another example, class AB biasing schemes have
been employed. These schemes reduce the DC bias in the mixer,
thereby reducing shot noise in the mixing quad transistors,
without degrading linearity. However, these circuits offer no
image rejection capability. Two biasing schemes falling within
this category are described by J. Durec et al in "Motorola's
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Mosaic V Silicon Bipolar RF Building Blocks Fill Gaps in High
Performance Low Power Wireless Chip Sets", Proceedings of the
4th Wireless Symposium, Santa C}ara, U.S.A., pages 218-223,
1996 and by B. Gilbert in "Design Considerations for BJT Active
Mixers. (course notes)" 1995.
A further example is described in U.S. Patent
5,532,637 which issued on July 2, 1996 to Khoury et al. This
scheme also improves noise performance by reducing the DC
current flowing in the upper quad thereby reducing the shot
noise in the upper quad transistors. While this circuit
maintains good linearity for small signal inputs, it ultimately
reduces the maximum undistorted signal that can be obtained at
the output. It also offers no image rejection capability.
None of the circuits that have been described above
provide any suppression of local oscillator energy at the mixer
input. Insertion of a cascode stage between the lower pair and
upper quad may improve LO suppression but will cost in terms of
voltage headroom, which could be a problem in low voltage
circuits.
A common practice for improving overall noise
performance and linearity in radio receivers is the use of an
image-reject filter between the amplification and mixer stages.
However, in the case where the mixer stage is a tree mixer, the
image filter is usually inserted at the mixer (i.e. lower pair)
input and hence does not suppress image noise contribution in
the lower pair, nor does it prevent LO energy from leaking to
the input.
An object of the invention is to provide a tree mixer
in which one or more of the above described disadvantages is
obviated or mitigated.
Summary of t,he Invention
The invention involves the insertion of a filter
between the mixer lower pair and upper quad of a tree mixer.
Ideally, the filter will pass the desired input signal to the
upper quad transistors while blocking all out-of-band signal
energy and noise. The theory of operation of the tree mixer
with interstage filter is best understood if one considers the
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tree mixer to be composed of two portions; a linear
transconductance gain stage defined by the lower pair and a
transimpedance mixing stage defined by the mixing quad and
loads. The interstage filter will prevent/suppress undesirable
signal energy and noise from entering the mixing quad.
Broadly, the invention may be summarized as a tree
mixer having a gain stage, a mixing stage and an interstage
filter connected between the gain stage and the mixing stage
and tuned to pass an input signal at a desired frequency from
the gain stage to the mixing stage and reject one or more
unwanted frequencies.
The invention may result in one or more of the
following advantages:
1) Improved image rejection and lower circuit noise are
achievable, resulting in an improvement in dynamic range.
2) Local oscillator energy (fundamental and harmonic) may be
effectively suppressed from the tree mixer input. This
enhancement may improve overall dynamic range and may minimize
spurious energy and offsets from appearing at the mixer output.
3) The tree mixer with interstage filter can be fully realized
on a single chip using commercially-available integrated
circuit process technologies.
4) The tree mixer with interstage filter as a building block
can be incorporated in more complex mixer topologies.
5) The performance gains that can be achieved by the invention
may result in a lower overall product cost through a relaxation
in the specifications of other components in the system.
Brief Descri~tio~ of the Drawinqs
Preferred embodiments of this invention will now be
described with reference to the attached drawings in which:
Figure 1 is a schematic diagraph illustrating a
conventional tree mixer;
Figure 2a is a schematic diagram illustrating a tree
mixer according to one embodiment of the present invention;
35Figure 2b is a schematic diagram of an interstage
filter used in the mixer of Figure 2a;
Figures 3a, b and c are schematic diagrams
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illustrating three variants of the circuit illustrated in
Figure 2;
Figure 4 is a block diagram illustrating how two
mixers of the present invention may be combined;
Figures 5 through 8b are schematic diagrams
illustrating further variants of the circuit illustrated in
Figure 2; and
Figure 9 illustrates a single-ended input version of
the mixer according to the invention.
Detailed Descri~tion of ~he Preferred Embodiments
Referring to Figure 1, a known tree mixer includes a
differential pair of transistors 11 and 12 whose emitters are
connected to a constant current source 10 via respective
emitter resistors 13 and 14. The constant current source 10 is
connected to a negative power supply VEE- and may be
constituted simply by a resistor having a suitable value for
deriving a desired current or it may be constituted by a
specific semiconductor circuit. A first differential analog
input signal is supplied to the bases of the transistors 11 and
12 via input terminals Vin+ and Vin- respectively. Two
differential pairs of transistors 15, 16 and 17, 18 are
connected in the collector circuits of the transistors 11 and
12 respectively. A second differential analog input signal is
supplied to the bases of the transi~tors 15 and 16 via input
terminals VLO+ and VLO- respectively. The transistors 15 and
16 have their emitters connected together and to the collector
of the transistor 11, and have their collectors connected to
differential output terminals Vout+ and Vout- respectively and
via respective resistors 19 and 20 to a positive supply voltage
VCC+. The second differential analog input signal is also
supplied via the input terminal~ VLO+ and VLO- to the bases of
the transistors 17 and 18 respectively, whose emitters are
connected together and to the collector of the transistor 12,
and whose collectors are cross-connected to the differential
output terminals Vout- and Vout+ respectively.
In operation of the tree mixer of Figure 1, the
current I passed by the current source 10 is divided between
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the transistor~ 11 and 12 according to the first input signal,
the degeneration resistors 13 and 14 increasing a maximum
useful input voltage for the mixer and setting an effective
transconductance for this input stage of about 1/(2 Re) where
Re is the resistance of each of the resistors 13 and 14.
Transistors 15 to 18 multiply the currents of the transistors
11 and 12 alternately by +1 and -1 at the frequency of the
second signal supplied differentially to the inputs VLO+ and
VLO-. In a radio communication~ mixer, the second signal is
typically a local oscillator (LO) signal. The collector
currents of the transistors 15 to 18 are converted to a
differential output voltage by the collector resistors 19 and
20, with a single sideband conversion gain of (2/~)(Rc/Re)
where Rc is the resistance of each of the resistors 19 and 20.
As explained above, in a conventional tree mixer of
the type illustrated in Figure 1 signal energy and noise at the
image frequency can appear at the outputs of the mixer and mask
the desired signal. To counteract this, the tree mixer is,
according to the invention, provided with an interstage filter
24 as shown in Figure 2a. The circuit of Figure 2a is
identical to that of Figure 1 except that the filter 24 is
inserted between the lower pair of transistors 11 and 12 and
the upper quad 15 to 18. More ~pecifically the filter 24 is a
notch filter tuned to pass a wide band of frequencies except
the image frequency.
As shown in Figure 2b, the notch filter 24 comprises
a first resonant circuit 25 comprising inductors 26 and 27
connected in series with a capacitor 28. This resonant circuit
25 is connected across the collectors of the lower transistor
pair. Two second resonant circuits 30 and 31 are also
provided. One second resonant circuit 30 is connected between
the collector of transistor 11 of the lower pair and the
emitters of transistors 15 and 16 of the upper quad and the
other second resonant circuit 31 is connected between the
collector of transistor 12 and the emitters of transistors 17
and 18 of the upper quad. Each resonant circuit 30, 31
comprises an inductor 32, 32' cannected in parallel with a
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capacitor 33, 33'.
The first resonant circuit 25 is tuned to present low
impedance to the image frequency and the resonant circuits 30
and 31 are tuned to present a high impedance to the image
frequency. In this way image rejection is achieved.
Alternatively, the tuning may be designed with respect to the
Lo frequency to suppress fl-n~A~ental and harmonic LO
frequencies at the tree mixer inputs.
Figures 3a, b and c illustrate variants of the
circuit of Figure 2a in which a cascode stage 35 is provided
between the filter 24 and the upper quad (Figure 3a), between
the filter and the lower pair (Figure 3b) and between the upper
quad and the outputs (Figure 3c). A cascode amplifier stage
(i.e. cascaded common-base stage in the case of bipolar
transistors) offers the following improvements in a circuit: 1)
Improved isolation between circuit terminals (output to input);
2) Higher circuit bandwidth due to the absence of the Miller
Effect (related to collector-base capacitive coupling); 3)
Improved linearity (related to non-linear collector-base
capacitance).
Figures 3a and 3b offer improved LO-input isolation.
One alternative may be more suitable than the other, depending
on the filter topology. Figure 3c offers better LO-output
isolation, higher circuit bandwidth (especially important for
up-converter applications) and higher linearity.
Referring now to Figure 4, the novel tree mixer 22
can also be used in conventional image-reject downconverters
and single-sideband upconverters with an improvement in overall
noise performance and image rejection. The circuitry comprises
two mixers 22 both connected to an input 37. A local
oscillator in-phase frequency i9 mixed into one of the mixers
22 and a quadrature phase shifter 38 derives a quadrature
component of the LO frequency which is then mixed into the
other mixer 22. The output of that mixer is phase shifted with
another quadrature phase shifter 3g the output of which is
summed in a summer 40 with the output of the first mixer.
Figure 5 illustrates a variant of the circuit of
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Figure 2a in which the filter 24 is tunable. In this case the
filter components are identical to those shown in Figure 2b
except that varactors 42 are added to fixed capacitors 28, 32,
32'. This allows the filter to be tuned to different image (or
LO) frequencies thereby permitting use of a single device in
applications that must comply with different radio standards.
A "folded" tree mixer topology can also be realized
with the use of an interstage filter. This is illustrated in
Figure 6. As is known in the art, the folded topology saves
voltage headroom allowing the mixer to be used in applications
with lower supply voltages.
Modified tree mixers with common-base inputs may also
incorporate an interstage filter. Such a variant is shown in
Figure 7. As is known in the art, the common-base input stage
offers a lower input impedance than the common-emitter input
stage. The lower impedance may be desirable for improved power
matching between the input source and the input stage. A
drawback to the common-base input stage is a lack of current
gain (vs. the common-emitter stage which has current gain).
The disclosed invention may be applied to a variety
of device and process technologies (e.g., NPN BJT, PNP BJT, N-
FET, P-FET, CMOS, BiCMOS) as well. Examples of PNP and NMOS
variants are illustrated in Figures 8a and 8b.
Figure 9 illustrates a version of the mixer according
to the invention in which, instead of a differential input
signal, a single-ended input signal Vin is applied. Thus, as
is known in the art, there is a single transistor 11 in the
gain stage and a single pair of transistors 15, 16 in the
mixing stage. The interstage filter 24 illustrated comprises
an inductor 32 in parallel with a capacitor 33 serving as a
series notch filter, and inductor 26 and a capacitor 28
connected in series to ground serving as a shunt notch filter.
The type of interstage filter described in the
illustrated embodiments of the invention is a notch filter.
Such a filter was chosen as a practical, effective topology to
couple the high impedance collectors of the lower transistor
pair to the low impedance emitters of the upper quad
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transistors. The filter is bilateral (operates in both
directions) which means that if tuned properly, it can reject
LO leakage from the upper quad to the lower pair. A notch
filter has the advantage that it can be achieved while
providing direct coupling from input to output. This is
essential where DC bias to the mixing quad transistors is
derived from DC bias in the lower pair transistors as is the
case with conventional mixers. For low voltage circuits where
separate biasing is established for the upper quad and the
lower pair, direct coupling should be avoided.
A disadvantage of the notch filter is that it rejects
only one unwanted frequency and cannot reject both the image
and LO frequencies at the same time. A bandpass filter would
be better such that all or most unwanted frequencies could be
rejected but it may be difficult to design such a bandpass
filter that also provides direct coupling. However, it is
considered that any interstage filter which passes the desired
frequencies and rejects some or all of the undesired
frequencies falls within the scope of the present invention.
