Design of a triband lumped element filter for digital microwave power amplifiers

In this paper, a compact lumped element triband reconstruction filter for digital microwave power amplifier (PA) applications in the 0.8, 1.8 and 2.6 GHz band is presented. The proposed design introduces three first order passbands with additional transmission zeros for maximum isolation between bands. Detailed equations are presented for a systematic design of the filter. In contrast to common filter designs, the input characteristic impedance is optimized broadband to values different from 50Ω as is required for proper amplifier operation. The presented design strategy can be easily extended to multiband filters of higher orders. Insertion losses of 0.8 dB, 1.4 dB and 0.9 dB are achieved at 0.8 GHz, 1.64 GHz and 2.22 GHz, respectively. The filter shows a 3 dB bandwidth of 290 MHz in the 0.8 and 1.8 GHz band and almost 1 GHz in the 2.6 GHz region. The prototype represents the first triband filter suitable for proper digital RF PA operation.