Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Combining of plural signals
Reexamination Certificate
1999-11-15
2001-06-05
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Combining of plural signals
C455S326000, C455S333000
Reexamination Certificate
active
06242964
ABSTRACT:
BACKGROUND OF THE INVENTION
Mixers are used in communications circuits for the purpose of generating a modulated carrier for transmission, demodulating a modulated carrier in reception, or converting a signal at some input intermediate frequency (IF) to another output radio frequency (RF) by multiplying two input signals and thereby generating a third. A number of mixer realizations, both passive and active, are known in the art, and double-balanced mixers are known particularly well due to their advantages in the suppression of unwanted spurious signals and the isolation of any one of three ports to the other two, there generally being two inputs and one output. The Gilbert Cell has been the most widely used active mixer circuit for performing the above tasks, usually incorporated within an integrated circuit. It does, however, possess certain limitations in terms of intermodulation (IM) distortion and noise figure (NF) that precludes it's use in communications systems having demanding performance specifications. The series-shunt feedback double-balanced active mixer delivers a much improved IM performance, but the lossy nature of the feedback topology does not improve the NF performance. The lossless feedback double-balanced active mixer overcomes the noise limitations of the series-shunt feedback active mixer, but still retains a significant source of IM distortion. The purpose of the present invention is to address the source of IM distortion in the lossless feedback double-balanced active mixer and significantly reduce it's impact on the mixer linearity.
Referring to
FIG. 1
, a schematic diagram of a lossless feedback double-balanced active mixer is shown in functional form. Here, the mixer is comprised of switching transistors
101
,
102
,
104
, and
105
, which are turned on (saturation) and off (cutoff) alternately by a differentially applied local oscillator (LO) signal. By this switching action, a pair of currents generated by driver transistors
103
and
106
are divided into four paths, there being two paths for each of two currents. The currents generated by driver transistors
103
and
106
are the result of an input intermediate frequency (IF) signal applied differentially to the input windings of a pair of feedback transformers
107
and
108
. The hybrid transformers
111
and
112
combine the four currents from switching transistors
101
,
102
,
104
, and
105
, creating a differential pair of feedback currents
119
and
120
, as well as an output RF signal
121
. The feedback currents
119
and
120
are coupled to the output windings of feedback transformers
107
and
108
, respectively, thereby forming a pair of lossless feedback amplifiers which serve to establish the conversion gain and improve the IM performance of the mixer.
Those familiar with the art will readily understand that the improved NF performance of the lossless feedback double-balanced active mixer is a result of the lack of additional noise sources in the embedding topology. This active mixer offers considerable advantages over the more traditional Gilbert Cell active mixer, especially in terms of signal-handling and performance variations over temperature due to the temperature dependency of the emitter resistance r
e
of the driver transistors, and the tradeoffs that are encountered in receiver and transmitter system design. It further provides substantial NF improvement over the Gilbert Cell mixer and the series-shunt feedback mixer.
Those familiar with the art will also readily understand that the IM performance of the lossless feedback double-balanced active mixer is impaired by the nonlinear emitter resistance r
e
of the driver transistors
103
and
106
. Although this mixer offers substantial advantages in IM performance over the more traditional Gilbert cell active mixer, the presence of the nonlinear driver transistor emitter resistance causes the IM performance of the lossless feedback double-balanced active mixer to be less than ideal. This resistance is also the principal cause of conversion gain variation with temperature. It has long been desirable that a mixer, either passive or active, be available that has improved IM and temperature performance, and at the same time has an improved NF performance without the expense of added power.
It is the purpose of this invention to further advance the art of feedback mixers by addressing the primary source of IM distortion present in the lossless feedback double-balanced active mixer, and to therefore provide an active mixer of markedly improved IM performance, while at the same time conserving power consumption and retaining the NF performance and overall sense of simplicity and cost effectiveness of the lossless feedback double-balanced active mixer.
SUMMARY OF THE INVENTION
A lossless feedback double-balanced active mixer circuit with improved intermodulation (IM) and noise figure (NF) performance is described which includes a pair of lossless feedback balanced active mixer circuits, each of which includes a differential pair of switching transistors which divide a controlled current into two paths at a rate determined by an input local oscillator (LO). A hybrid transformer in each lossless feedback balanced mixer, consisting of a centre-tapped primary winding and a secondary winding, combines the two currents to provide a recombined amplified IF signal and an output radio frequency (RF) signal. A third driver transistor in each lossless feedback active mixer circuit provides the controlled current, which is determined by an input intermediate frequency (IF) signal. Each lossless feedback active mixer circuit further includes a feedback transformer, comprised of an input winding and a tapped output winding, which compares the input IF signal with the recombined amplified IF signal from the hybrid transformers and applies the difference as a correction to the amplifying transistors, thereby completing a lossless feedback amplifier circuit and in turn improving the IM performance of the mixer circuit. An augmentation circuit is included which improves the IM performance still further. Since the feedback transformer introduces no significant sources of noise to the active mixer circuit, the NF of the of the lossless feedback active mixer circuit remains unimpaired beyond the NF of the transistors themselves. The connection of the secondary windings of the hybrid transformers of the lossless feedback active mixer circuits effectively cancels the output LO and IF signals and provides an output RF signal.
REFERENCES:
patent: 2337965 (1943-12-01), Bode
patent: 3891934 (1975-06-01), Norton et al.
patent: 5551074 (1996-08-01), Vice
Norton, David E., “High Dynamic Range Transistor Amplifiers Using Lossless Feedback,” Microwave Journal, May 1976, pp. 53-57.
Norton, David E., “High Dynamic Range Transistor Amplifiers Using Lossless Feedback,” Proceedings of the 1975 IEEE Int'l Symposium on Circuits and Systems, pp. 438-440.
Sartori, Eugene F., “Hybrid Transformers,” IEEE Transactions on Parts, Materials, and Packaging, vol. 4 No. 3, Sep. 1968, pp. 59-66.
Trask, C., “A Linearized Active Mixer,” Proceedings RF Design 98, San Jose, CA, Oct. 1998, pp. 14-23.
Trask, C., “Disturtion Improvement of Lossless Feedback Amplifiers Using Augmentation,” Proceedings of the 1999 Midwest Symposium on Circuits and Systems, Las Cruces, NM, Aug. 1999.
Trask, C., “Feedback Technique Improves Active Mixer Performance,” RF Design, Sep. 1997, pp. 46-52.
Cunningham Terry D.
Tra Quan
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