The RF Cage that goes on top of the circuit in resonant RFPDs is grounded. It helps in avoiding interferance, however, if some component, most likely the tunable inductors touch this cage when they are closed, it creates parasitic inductance and/or capacitance. I observed this happening in one of our RFPDs where the resonant frequency would drop significantly as well as the peak magnitude once the cage is put on. I circumvented this problem by applying a layer of insulation tape on the inside of the RF cage cover. This helps avoiding any parasitic contact with the gnd.
Issued in Public Interest.