A new structure of the electro-optic modulator to compensate the third-order intermodulation distortion (IMD3) is introduced. The modulator includes two Mach-Zehnder modulators (MZMs) operating with frequency chirp and the two modulated outputs are combined with an adequate phase difference. We revealed by theoretical analysis and numerical calculations that the IMD3 components in the receiver output could be selectively suppressed when the two MZMs operate with chirp parameters of opposite signs to each other. Spectral power of the IMD3 components in the proposed modulator was more than 15dB lower than that in a normal Mach-Zehnder modulator at modulation index between 0.15π and 0.25π rad. The IMD3 compensation properties of the proposed modulator was experimentally confirmed by using a dual parallel Mach-Zehnder modulator (DPMZM) structure. We designed and fabricated the modulator with the single-chip structure and the single-input operation by integrating with 180° hybrid coupler on the modulator substrate. Modulation signals were applied to each modulation electrode by the 180° hybrid coupler to set the chirp parameters of two MZMs of the DPMZM. The properties of the fabricated modulator were measured by using 10GHz two-tone signals. The performance of the IMD3 compensation agreed with that in the calculation. It was confirmed that the IMD3 compensation could be realized even by the fabricated modulator structure.

An important barrier to the application of high-temperature superconducting microwave filters is their power-handling capability. To clarify the key parameters for improving the power-handling capability of rf filters based on high-temperature superconductors with microstrip structures, we synthesize bandpass filters with different layouts using several kinds of thin film high-temperature superconductors, and subject them to third-order intermodulation measurements. By improving the sensitivity of the measurement set-up through the selective reduction of the fundamental output signals, we succeed in measuring the intermodulation signals of the superconducting filters. The experimental results indicate that increasing the film thickness and utilizing MBE-grown films of NdBa2Cu3O7 films are effective in obtaining high-power handling microstrip filters.

This paper describes the distortion characteristics of an even harmonic type direct converter (EH-DC) used in earth stations for CDMA satellite communications. Direct conversion technique is known as a method to simplify circuit topologies of microwave transceivers. In satellite communications, multi carriers which have high and nearly equal level are provided to a quadrature mixer of the EH-DC. Hence, the third-order intermodulation degrades receiving characteristics. In this paper, we show the relationship between the distortion characteristics and noise figure of the EH-DC for CDMA satellite communication systems. Furthermore, we show NPR of even harmonic quadrature mixers caused by the third-order intermodulation. Experimental results in X-band indicate that the proposed EH-DC has almost the same BER characteristics compared with a heterodyne type transceiver.