U.S. Patent No. 6,091,940 is directed to frequency up-conversion where harmonics due to amplitude modulation are filtered (pages 3-5).  The apparatus claim recites the capability to generate integer harmonics, and the prior art has that capability even if not done (pages 9-10).  The claim recites a switch module to receive an oscillation signal where the oscillation signal causes the switch module to gate and thereby generate (page 12).  This reflects capability rather than configuration so the prior art discloses (page 12).  The method claims required generation of integer harmonics, and there was no evidence of actual use of the prior art to generate the integer harmonics (pages 13-14).

Hindsight: “Configured to” language may to avoid capability concerns in overcoming prior art, but may risk invocation of means-plus function interpretation.