In the field of microwave technology，microwave power, frequency and phase are the main measurement parameters. Microwave signal detection is widely used in ASK, FSK, PSK, Microwave positioning, antenna phase pattern testing, near-field diagnosis. These systems are in urgent need of practical application of lightweight, small size, low power consumption, high integration of electronic equipment. The existing microwave power, phase and frequency detectors are discrete circuits. These separate devices not only have the disadvantage of bulky but also can not measure the power, phase and frequency of signals simultaneously. Therefore, an urgent need for a system that can achieve integrated detection of three microwave parameters to meet the application requirements in the field of microwave communication. In response to these needs, based on the research of discrete single detectors, the design theory and implementation of integrated detection system are completed. The main works are described as follows:
1. In term of non-linearity under high power for capacitive MEMS microwave power sensor:
A capacitive MEMS microwave power sensor with a high-power handling of 4W is proposed. The structure of capacitive MEMS power sensor was simulated by Ansys HFSS software.According to the electric and magnetic field distributions on the surface of the MEMS beam, the effective sensing range is determined. The mechanical performance of capacitive sensor was analyzed at the 4W input power to obtain displacement distribution by the Ansys FEM software. The traditional microwave-to-electricity conversion model was first modified. Further, taking into account the signal reflection caused by the increase of capacitance under large input power, a second correction was made. Finally, the output characteristic of the capacitive MEMS power sensor is measured under high power, and the validity of the nonlinear model of the sensor is verified. It has laid a theoretical foundation for the research of high-power capacitive MEMS microwave power sensor.
2. The term of MEMS microwave phase detector performance:
A four-port MEMS microwave phase detector based on MEMS power sensor was fabricated based on GaAs MMIC process. It shows that the measurement results of the MEMS microwave phase detector agree well with the calculation results in a complete cycle. The phase-detection sensitivities of the two sensors were about 16.62μV/° and 23.94aF/°, respectively, at a frequency of 10GHz with a power input of 200mW. Furthermore, to fully reveal the application potential of this MEMS microwave phase detector, this paper also carries out phase detection test under high power, and the test results still accord with the relationship of the cosine curve. The capacitive MEMS microwave power sensor is applied in this device to make up for the lack of thermoelectric power sensor for the high-power handling and can expand the dynamic range of the phase detector up to 4W.
3. In term of the MEMS amplitude demodulator :
Three kinds of the MEMS amplitude detectors are proposed based on GaAs MMIC process (thermoelectric type, online type and cascade type). These devices utilize the square-law and low-pass characteristics of thermoelectric converters and electrostatic force actuators. Experiments show that the thermoelectric MEMS detector can achieve the carrier frequency of 0.35-10GHz amplitude modulation signal direct demodulation, covering 0-23dBm power detection range. The on-line MEMS amplitude detector has the return loss of better than 20dB and insertion loss of less than 0.5dB at the 0.01-10GHz band. The cascaded amplitude detector has the advantage of high power handling capability and can cover direct demodulation of (0.35-6GHz) RF signals in the power range from 0 to 23 dBm. The manufacture of these three devices is compatible with GaAs MMIC processes. Besides, none of these MEMS detectors has DC power.
4. Reliability issues related to charging injection of dielectric layers for MEMS cantilever switch:
A balance-bridge method is proposed to characterize the reliability problem caused by the charge injection in the dielectric layer of the MEMS cantilever switch. First of all, the equivalent circuit model of the balanced bridge is established, and the precision formula of the balanced bridge method is deduced. The vibration mode of the cantilever beam was analyzed by laser Doppler velocimetry (LDV) to ensure the mechanical symmetry of the cantilever switch structure. The electrical symmetry of the cantilever switch structure was analyzed by using a signal function generator and oscilloscope. The charging process under DC voltage is studied. The experimental results show that the accuracy of the balanced bridge method can reach 10Ω/fF. The contact and non-contact charging process can be observed by using this method. The two charging effects can be distinguished by the post-fitting relaxation time and the tensile index factor. The charge in the dielectric layer occurs mainly in the first 1200s. Based on this approach, the dielectric charge process under the RF signal of 26-33dBm at 26GHz was studied, and the corresponding mechanism was analyzed and discussed. The experimental results show that the effect of charging the lower dielectric layer under RF signals is insufficient compared to DC voltage.
5. Design theory and implementation method of the MEMS microwave signal integrated detection system:
Design Theory: Firstly, the general formula of n-port signal detection is given through passive network signal superposition and synthesis process. The theoretical method of the integrated microwave signal detection system is deduced from the general formula, which provides the basis for the device structures. Secondly, based on this, the architecture of the two microwave signal integrated detection systems are proposed, and their HFSS structures simulation are carried out to determine the final structure.
Implementation Methods: Based on the GaAs MMIC process, two MEMS microwave signal integrated detection systems were fabricated. The two kinds of microwave signal integrated detection system: first, the frequency of the microwave signal is measured by the amplitude ratio and phase comparison method, and the frequency detection has no relation with the power. Secondly, the signal power is detected by the coupling method, and the results are corrected to realize the frequency independent. Two microwave integrated signal detection system using the quadrature dual-channel method to achieve the phase detection. Further two kinds of microwave integrated detection system achieve amplitude demodulation. At last, the S-parameter model of MEMS microwave device and integrated detection system is deduced based on microwave network theory. It provides robust theoretical support for the embedded application in microwave communication system. The MEMS microwave signal integrated detection system realizes the integrated synchronous detection of microwave power, phase, frequency and amplitude modulation signals, and has the advantages of small size, lightweight and low power consumption.