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This paper presents the design and experimental characterisation of a coaxial comb-line band-pass filter for a switched-mode power amplifier (class-S), for an operating frequency of 450 MHz. The class-S amplifier architecture is laid out for very efficient amplification and high-power operation. Both requirements present challenges for the design of the reconstruction filter terminating the final stage. According to measurement results, an initially developed filter provided a bandwidth of 26 MHz, an insertion loss of 0.55 dB, and a second pass-band only at 2.8 GHz. The maximum input power handled by the filter amounted to 54 W, staying below the requirement. This power level was limited by electrical breakdown. Therefore, a highpower adaptation was devised and realised in an improved design subsequently. The resulting filter not only provided an even lower insertion loss of 0.37 dB but also handled more than 300 W input power. I. INTRODUCTIONRecently, the interest in increasing the power-added efficiency (PAE) of RF-and microwave power amplifiers in the final stage of a transmitter chain for mobile communications has grown significantly. The most efficient microwave power amplifiers operate in a switching mode, e.g., in class-S operation for input signals of arbitrary waveform. A switched-mode class-S power amplifier can provide high efficiency (> 70%) and, at the same time, high linearity, which makes it very attractive for, e.g., applications in mobile communications. Such an amplifier system consists of the following three basic building blocks, as indicated by Fig. 1 [1]: 1. A one-bit modulator, e.g., a pulse-length modulator (PLM) or a band-pass delta-sigma modulator (BDSM), maps the information contained in the high-frequency carrier signal into a train of pulses of variable length but constant amplitude. 2. A switching amplifier, possibly consisting of a driver and a push-pull final stage, provides the desired amplification by switching a bias source. 3. A band-pass filter is required for the reconstruction of the amplified signal. The output signal of the final stage of a class-S power amplifier includes noise-type signals from previous stages over a very broad frequency range. For the case of a BDSM, the output signal has a noise-like behaviour, whereas a PLM creates parasitic harmonics with significant power levels. Therefore, the band-pass filter has to provide not only extremely low insertion loss -an insertion loss of 1 dB reduces the overall efficiency by as much as 20 % -

Abstract. This paper presents a coaxial combline filter for a class-S power amplifier for an operating frequency of 450 MHz. According to the measurement results, the filter provides a bandwidth of 26 MHz, an insertion loss of 0.55 dB, and a second pass-band only at 2.8 GHz. As the filter dimensions are crucial for the envisaged application, different ways to miniaturise the filter are proposed and their features and disadvantages discussed.