Switch board capacitance

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Switch board capacitance

Post by swetterlin » Wed May 05, 2010 5:22 pm

On the subject of filter matching, keep in mind that the switch board will present some shunt capacitance (roughly 50 pF) to the filters, due to internal capacitance of the switch ICs. This may affect the performance of some filters, though at 10.7 MHz this capacitance is a fairly high shunt impedance.

You can fine tune handmade filters while they are attached to the filter switch in order to take this capacitance into account. Or, it may be simpler just to include a 3.9 uH inductor from the switch input (and output) to ground. This inductor will neutralize the capacitance in a broad frequency range in the area of 10-11 MHz. The inductor does not have to be fancy. Even a relatively low Q (like 10) is acceptable. So you can use most any SMT inductor that has an SRF over 20 MHz.

It is easy to add the inductor to the board, even though there are no pads for it. Just attach it after the connectors are in place, spanning between the input (and output) connectors and the ground plane around the edge of the board. They are only needed on the main input and output, not on each filter connector. That is, you need 2.

This occurred to me after the schematic was finished, so the inductors don't show on the schematic. And they may turn out to be unnecessary.

Sam W.

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