The Characteristic Of Reflection Phase Of Microwave Filters And Its Application To Microwave Circuits

Microwave filters are key components in various microwave systems,which are used to allow signals in the desired frequency band to pass but suppress and filter out other signals.The main performance specifications of microwave filters include the insertion loss in the passband,the suppression in the stopband,the band edge transition speed,and the phase linearity of in-band transmission,etc.Microwave filters are indispensable to such microwave devices as detectors,mixers,multipliers,and so forth.As the trend of miniaturization and integration,filters are often integrated inside these devices,in this case,it is important to notice another specification i.e.the reflection phase of filters,which is closely related to the input impedance of the filter and implies its level of loading effects,and hence has a great influence on the total performance of the device.This dissertation will present a comprehensive research on the reflection phase characteristic of microwave filters,including the way to reduce the loading effect in order to improve the total performance.Both simulations and experiments are introduced to demonstrate the theory and analysis of this dissertation.The main contents of this dissertation are as follows:The theory analysis of the reflection phase of filters is conducted.Based on their features in the aspects of reflection phase and input impedance,filters are divided into two categories,namely the high stopband-impedance filter and the low stopband-impedance filter.Through the detailed synthesis procedure of three kinds of classical filter functions,the ultimate reason of why there are two phase categories is explained,and the amplitude-frequency response and phase-frequency response of the two categories are illustrated.Through the comparison of the reflection phase characteristics of filters with different response types,orders or in-band ripples,it is shown that the in-band ripple has the biggest influence on the changing speed of the reflection phase.The bigger the in-band ripple is,the faster the reflection phase approaches the desired value,and the smaller the load effects are.The parasitic responses of the reflection phase of microwave lowpass filters are studied theoretically and experimentally.The equivalent capacitance or inductance of the fundamental element of microwave filters,i.e.the short transmission line,is analyzed based on its equivalent circuit,and then the relationship between each section and corresponding parasitic response of the stepped impedance lowpass filters is concluded.The ways to eliminate or reduce the parasitic response are provided,including using suspended substrate microstrip line to increase the characteristic impedance of high-Z lines,employing shunted stubs that feature transmission zeros to increase out-band suppression and smooth the phase response,and choosing the substrate with appropriate thickness and dielectric constant.After utilizing these techniques in two microwave lowpass filters,the results of very wide stopband,high suppression degree,and wide band with smooth reflection phase and consistent phase property are obtained,demonstrating the correctness of the theory analysis and the validity of the ways.The fractional bandwidths of the stopband with 20 dB suppression are 157% and 162%,respectively,which are larger than that of most lowpass filters in the literatures.Several typical microwave bandpass filter examples are listed to illustrate the characteristics of bandpass filters in the two phase categories,and a fast way to judge the category is given.Besides,an ultra wideband bandpass filter is designed.Through loading stepped impedance stubs with different length ratios,multiple transmission zeros near the band edges are introduced,increasing the out-band suppression and band edge roll-off speed,which satisfies the strict requirements of in-door UWB systems.The measured results show that the reflection phase agrees with the aforementioned characteristics and validate the method for fast category judgment.The development process of two kinds of wideband microwave detectors capable of high data rate demodulation are detailed,which are respectively in shunt and series configuration.A design criteria about which kind of reflection phase to select for a specific circuit configuration is provided,i.e.the high stopband-impedance filter should be chosen for the shunt configuration,and the low stopband-impedance filter for the series one.The two detectors both operate at 6~26 GHz,and the designed demodulation data rate of the ASK signal reaches 2.5 Gbps.The measured detection voltages under open condition are respectively 26~94mV and 68~146mV,and the ripples of adjacent frequencies are respectively 1.59 dB and 1.51 dB.Moreover,another wideband microwave detector in shunt configuration with flat detection output is also designed,which utilizing the previously designed lowpass filter with a wide stopband and a high suppression,in order to further improve the operating bandwidth and make the detection output flat.For this detector,the measured detection voltage under open condition is larger than 200 mV in the frequency band of 1.8~13.5 GHz,and the total ripple is less than 0.77 dB.All the three detectors show good agreements between simulations and measurements,proving the theories and methods.The design,fabrication and measurement of a W-band sub-harmonic mixer are presented.Among the passive circuits in the mixer,two of them are designed with the help of the reflection phase theory: one is the LO lowpass filter which make the mixer easier to match at RF band;the other is the LO-IF diplexer,which shows good isolation after combine two separately designed filters.In addition,a simplified 3D model and two pairs of quasi-linear parameters extracted from a measurement based approach are used to model the commercially available diode,which not only increases the efficiency of simulation but also guarantees the accuracy of the parameters.Using the diode model in the full-wave EM simulator,the reflection of RF and LO port of the mixer can be optimized directly,thus good port reflection performances can be obtained,which make the mixer directly integratable with RF low noise amplifier and local oscillator source.Finally,the measured results of the mixer show that the RF S11 is better than-10 dB in 88~100 GHz,the LO S11 is better than-10 dB in 42~48 GHz,and the conversion loss is lower than 14 dB under all LO frequencies,which meet the requirements of the project.At last,the conclusions are drawn for the entire dissertation and the prospects about topics that deserve further researches are made.