High power millimeter wave sources, such as Megawatt, 170 GHz gyrotrons, require window-materials with very low dielectric loss for their normal operation. In an ideal crystalline material, theory predicts that the intrinsic lattice loss (ILL) in this frequency region arises from two-phonon processes and is expected to be quite small. In practice, a higher level of loss is observed, due to lattice disorder, defects, impurities, and free charges. The role of these various mechanisms has not been clearly established. In search for low-loss vacuum windows for high power gyrotrons, currently only the very expensive CVD diamond has shown satisfactory performance. Recently high-purity, semi-insulating silicon carbide (HPSI-SiC) has also been shown to have relatively low loss and is a promising alternative material. Some materials, such as CVD diamond, crystalline silicon carbide, and single-crystalline Au-doped Silicon have loss tangent as low as 10-5 to 10-6 in the millimeter wave range. Various methods of dielectric measurements have been reported for such low loss material, however, the open resonator method is the most convenient and sensitive technique. With this technique, the determination of the loss tangent for low loss materials is accomplished by measuring the changes in the response curve of an open resonator when a material sample is inserted into the resonator.
The evaluation of very low loss materials in the millimeter-wave region requires precision measurement of the change in the linewidth of a resonant cavity when a sample is inserted. The precision required is on the order of a few kHz change in a 200 kHz line-width. Backward wave oscillators (BWOs) are convenient sources as they can generate milliwatts of power over a wide tuning range. However, a free running BWO is too noisy for the precision required. Phase locked loops (PLL) are used for many radio applications and can translate the frequency accuracy of a high quality signal source to a tunable signal source. Synchronizing the BWO with the local oscillator (LO) has the additional benefit of eliminating the frequency fluctuations of the BWO. This locking was accomplished at NCCU by the use of a harmonic mixer and a digital PLL as described below.
To perform measurements created is a system with the block diagram shown in Fig. 1. The high voltage power supply can tune the BWO (OTK 443) from 120 to 170 GHz. The spectrum analyzer (Tek2782) is designed to operate with external, harmonic mixers for down-conversion over the frequency range from 40 GHz to above 300 GHz. The millimeter wave output from the BWO is first coupled into free space by a horn, then divided by a wire-grid polarizing beam splitter, with one polarization directed to the resonator measurement system and the orthogonal polarization utilized for the PLL system. The RF power for the PLL system is focused by a TPX lens into a horn that feeds the waveguide mixer. This harmonic mixer was selected from the series (WM782) originally supplied with the Tek2782. The LO output of the Tek2782 (at 14 to 18 GHz) is amplified and applied to the mixer through a diplexer, which also serves to separate the IF signal (at 3.525 GHz) generated by harmonic mixing. The diplexer’s IF output is followed by a chain of LNAs that have the capability of up to 60 dB of gain, to ensure the required minimum signal level of 15 dBm is delivered to the AD4007 the phase of the divided IF signal is compared to an oven-stabilized crystal oscillator. Any difference generates an error signal, via the charge pump, that is input to a third order active low pass filter (LPF) chip. This digital chip contains pre-scalers, a phase-frequency detector (PFD) and a charge pump (CP). In the AD4007 chip, the phase of the divided IF signal is compared to an oven-stabilized crystal oscillator. Any difference generates an error signal, via the charge pump, that is input to a third order active low pass filter (LPF).
Fig. 1. Schematic diagram of the system.
Measurements and simulated closed loop frequency response calculated at the CREST Center are presented in the findings section of this report.