The operation of a microwave-microwave double resonance spectrometer with modulated pump power strongly depends on complete suppression of leakage signals from high pump power. Possible mechanisms of leakage signals have been investigated carefully. Adequate countermeasures have been designed to efficiently suppress leakage signals for both amplitude and frequency modulation of the pump power. A versatile microwave-microwave double resonance spectrometer is described, which offers convenient operation over large frequency ranges. In an appendix typical line shapes are given and compared to theoretical expectations.