1. A circuit comprising: a digital signal generator to produce a digital signal; and a passive filter circuit for filtering the digital signal to attenuate at a radio communications frequency, the passive filter circuit comprising a first stage to receive the digital signal, the first stage having a first inductor and a first branch coupled to the output of the first inductor, the first branch having a first capacitor and a second inductor, and a second stage coupled to the first stage, the second stage having a third inductor and a second branch coupled to the output of the third inductor, the second branch having a second capacitor.