Comments on U.S. Patent No. 6,091,940 – May 12, 2005Related Terms:
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.
Over six pages of references to prior art were supplied within the ‘940 patent. While references typically focus on scientific journals establishing the existing state-of-the-art, an entire page of references in the ‘940 patent was devoted to ParkerVision press releases. The references included topics such as “The available time statistics of rain attenuation on earth-space path”. It is not clear to this reviewer how rain attenuation statistics and company press releases pertain to the prior art of frequency upconversion. Looking over the ‘940 patent does make this reviewer pity the poor patent examiner who had to diligently review all of the references and material in this patent in a limited amount of time.
The independent claims of a patent form the foundation of the patent, as all the other claims add restrictions and conditions to the independent claims. For example, the ‘940 patent is 119 pages long with 374 claims in all, and 8 independent claims. The second independent claim, Claim No. 4, recites what appears to be the simplest embodiment of the invention:
4. An apparatus for frequency up-conversion, comprising:
a switch module that receives an oscillating signal and a bias signal, wherein said oscillating signal causes said switch module to gate said bias signal and thereby generate a periodic signal having a plurality of harmonics, said periodic signal having an amplitude that is a function of said bias signal; and
a filter coupled to said switch module to isolate at least one of said plurality of harmonics.
Comparing the above claim to a conventional frequency converter such as that shown in Fig 5.26 from Carlson’s Communication Systems showing striking similarities . Fig 5.26 in Carlson contains a mixer, often implemented as a switch [2,3], and oscillating signal, and a filter used to select part of the switched signal as an output.
(click image to enlarge)
Drawing of Fig 5.26, labeled “Frequency Converter” in Carlson 
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 . 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.
 A.B. Carlson, Communication Systems, McGraw-Hill, New York, NY, 1968.
 S.A. Maas, Microwave Mixers, Artech House, Dedham, MA, 1986.
 A.A. Abidi, “Direct Conversion Radio Transceivers for Digital Communications,” IEEE JSSC, Vol. 30, No. 12, pp. 1399-1410, Dec 1995.
 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.