Design of a compact low pass filter for high power RF generators

Abstract

The low-pass filter (LPF) is a key component for high power RF generators generating 40.68MHz sine wave, since the LPF should reject harmonic noises and handle high input power simultaneously. We propose a novel LPF, operating at 30kW with good skirt performance. The proposed filter is a 7th order Butter-worth type and consists of four capacitors and three inductors. The capacitors and inductors are implemented by copper plates, teflon insulators and copper plates with screws, respectively. The measured values of S-11, S-21 and phase of the proposed LPF are -26.4dB, -0.57 dB, and -163.5 degrees at 40.68MHz, respectively. The S-21 values at 2nd (81.36 MHz) and 3rd (122.04 MHz) harmonics are -19.5 dB and -29.3dB, respectively. The measured spectrum data on the surface of RF cable that is connected to the LPF output were -41.0dBm at 40.68MHz and -78.8dBm at 81.36MHz. The difference value of 37.8 dB is sufficient for harmonic suppression. In addition, arcing and thermal problems do not occur.