A Miniaturized Harmonic Reject Waveguide Filter for Ku-Band VSAT Transmitter

Abstract

In this paper, a novel miniaturized low-pass waveguide filter with wide stop-band is proposed for the harmonic rejection of Ku-band Very Small Aperture Terminal (VSAT) transmitters. The proposed filter consists of a corrugated waveguide low-pass filter and a metal septum in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> -plane at the center of the narrow side wall. Because it does not require additional impedance transformers, the proposed filter achieves a compact size. To estimate the amplitude level of the higher-order modes at the input port of the harmonic reject filter in a RF transmitter, higher-order modes generated by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -plane probe-type microstrip-to-waveguide (M2W) transition are analyzed and calculated using commercial numerical tools. To analyze the response of the proposed filter in higher-order modes, the mode conversions in the structure are also analyzed and calculated. The analysis shows that a wide stop-band free of spurious response in all higher-order modes can be obtained by co-designing M2W transition and filter. To validate the design concept, a Ku-band harmonic reject filter is designed using WR-75 waveguide. Simulation results show that the reflection is less than −23dB between 13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 15 GHz, while the suppression stop-band performance in TE 10 mode is better than 35dB in 17.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 29.6 GHz. The effective length of the filter is 29 mm. Simulation results also indicate that the spurious pass-band performance in TE 20, TE 21 and TM 21 modes is improved by the M2W transition. Finally, to validate the proposed idea, a Ku-band GaN power amplifier is fabricated and measured. The measurements show that the amplifier has a minimum output power of 72.5 W with better than 20 dBc <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2^{nd}$ </tex-math></inline-formula> harmonic rejection. By using the proposed filter, the measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2^{nd}$ </tex-math></inline-formula> harmonic rejection of the power amplifier is greater than 52 dBc.