LC filter with suspended printed inductor and compensating interdigital capacitor

A radio frequency (RF) filter having an input and an output, said filter for filtering an input signal to yield an output signal therefrom, said filter comprising:

• a substrate onto which circuit elements can be printed;
• a filter circuit comprising at least two parallel resonant circuits, each of said parallel resonant circuits comprising a suspended printed inductor in parallel with a suspended interdigital capacitor, wherein said parallel resonant circuit is devoid of electrical traces extending into an area of said substrate underlying entirely said inductor and said capacitor from an area of said substrate external to said area of said substrate underlying said inductor and said capacitor, and further wherein the resonant frequency of said parallel resonant circuit is substantially independent of variations in the width of the traces used to construct said suspended printed inductor and said suspended interdigital capacitor;
• coupling means for electrically coupling said parallel resonant circuits to one another;
• input coupling means for coupling said input signal to said filter circuit;
• and output coupling means for coupling said filter circuit to said output thus forming said output signal.

A method of fabricating a suspended printed resonant circuit, said method comprising the steps of:

• providing a substrate onto which circuit elements can be printed;
• printing onto said substrate, a first electrically conductive trace using lithographic techniques such that said first electrically conductive trace functions as an inductor at RF frequencies of interest;
• printing onto said substrate, a second electrically conductive trace using lithographic techniques such that said second electrically conductive trace functions as an interdigital capacitor at the RF frequencies of interest and such that said interdigital capacitor is electrically connected in parallel to said inductor so as to form a parallel resonant circuit;
• and
• creating an area on said substrate underlying entirely said first electrically conductive trace and said second electrically conductive trace that is devoid of electrical traces extending into said area from an area of said substrate external to said area of said substrate underlying said first electrically conductive trace and said second electrically conductive trace.