U.S. Patent No. 6,091,940 (the ‘940 patent), entitled “Method and System for Frequency Upconversion” was granted to Sorrells, et al. on July 18, 2000 and was filed on October 21, 1998. This patent includes claims to methods and systems for converting one frequency to another, which are depicted in the patent with figures such as Figs 10, 16, 32A, 37A.

The claims of a patent are the heart of the patent. The remaining material in a patent specification generally includes supporting material to define the existing state-of-the-art, called “Prior Art”, and figures and description that serve as reference material for the claims of the invention. The description of the invention and the supporting figures presented in a patent specification must, among other requirements, provide adequate written description of the invention to show that the inventor(s) were in possession of the invention as claimed at the time of filing.

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Fig. 16 in the ‘940 patent

It is widely recognized by those skilled in the art of radio frequency design that frequency converters can work by switching [2,3], and that the mixer described by Carlson is often realized as a switch. Following a mixer with a filter is also standard practice in frequency converter design, as shown in the basic textbook by Carlson. So one of the core claims of patent ‘940 reads as a conventional upconverter circa no later than 1968.

Fig. 5 from the ‘940 patent

The ‘940 patent shows an example of prior art AM upconversion in Fig. 5, which shows a frequency multiplier after the mixer. This is in contrast to Fig. 16 in the ‘940 patent, which shows their invention using a switch to act as both the mixer and harmonic generator. The combination of mixer and harmonic generator shown in Fig. 16 is known as a “harmonic mixer” and is quite old [2]. Again, this portion of the ‘940 patent does not appear to add anything to the state-of-the-art.

Many of the other figures shown describing the invention can be found in the literature prior to 1997. For example, Fig. 32A is a conventional FET gate mixer and Fig. 37A is simply a shunt resonator filter. As previously stated, the connections and configurations of these devices do not appear as anything new.

While this reviewer is not in a position to discuss the validity of the ‘940 patent, certainly some of the claims read as prior art and many of the drawings of the invention appear very similar to radio frequency converters existing in the prior art. It is not clear to this reviewer what is innovative about the ‘940 patent.


References
[1] A.B. Carlson, Communication Systems, McGraw-Hill, New York,NY, 1968.
[2] S.A. Maas, Microwave Mixers, Artech House, Dedham, MA, 1986.
[3] A.A. Abidi, “Direct Conversion Radio Transceivers for Digital Communications,” IEEE JSSC, Vol. 30, No. 12, pp. 1399-1410, Dec 1995.
[4] H. Kaiuwi, S. Nagata, K. Tateoka, K. Kanazawa, and D. Ueda, “A GaAs Single Balanced Mixer MMIC with Built-In Active Balun for Personal Communication Systems,” IEEE Microwave and Millimeter-Wave Monolithic Circuits Symposium, Orlando, FL, pp. 77-80, 1995.