Microwave/millimeter wave receiving front-end is one of the core units in the systems like military/civilian communication system, high speed wireless data transmission system and millimeter wave detecting imaging systems. The performance of a broadband RF receiving system is largely determined by the degree of spurious suppression, the noise figure, the flatness of amplitude response as well as group delay fluctuation in the pass-band. It is of great practical significance to study the theoretical issues and key technologies in the broadband microwave/millimeter wave system for the development of broadband microwave/millimeter wave system. With the goal of achieving the high-performance microwave/millimeter wave broadband receiving front-ends, a series of key technologies and theoretical issues associated with receiving front-end have been studied. Design techniques of high selectivity band-stop filter and broadband amplitude equalizing circuit are proposed. Group delay fluctuation in broad band is analyzed and optimized. The influence of inter-stage impedance mismatching on the amplitude response and noise figure performance are formulated by network theory. Accurate measurement method of noise figure in broadband system with high power gain is studied. The established theoretical model and design techniques are applied in the development of Ka-band broadband receiver with low amplitude and low phase distortion, Ku band LNB module and W-band radiometer. The main achievements of this dissertation are as follows:
1、An antisymmetrical parallel double coupling notch structure is proposed for microstrip band-stop filter. This notch has a double-resonant characteristic in the stop-band, which extends the stopband bandwidth effectively. Compared with the traditional parallel coupled notch structures, the proposed notch shows a higher selectivity and a wider stop band. Based on this new notch structure, an L-band compact band-stop filter with high selectivity is developed. In order to cope with the strong reflected signal from the conventional reflective band-stop filter, which will degrade the performance of adjacent circuit components, an absorptive band-stop filter(ABSF) using parallel coupling structure with loaded lunmped resistor is developed. By adjusting the loading position of the lunped resistor, the flexibility is enhanced for choicing the value of loaded-resistor in the ABSF. Simulated and experimental results show that the transmission parameter and the input port reflection coefficient are less than -20dB in stopband, which indicates that the signal in stopband is absorbed effectively.
2、A novel design method, which is based on a low-order band-pass filter combined with notch structures, for BPFs with low group delay fluctuation is proposed. The effectiveness of improving the fluctuation of group delay in the pass-band of a broadband system has been simulated by Ansoft’s HFSS. Experimental study on a prototype of bandpass filter at C-band demonstrates that the group delay fluctuation has been reduced effectively , while keeping a high out of band signal suppression. This method is applied in the design of low pass filters which are used in the high phase orthogonality I-Q mixer. The measurement results illumstrate that the error of phase orthogonal between I and Q channels is reduced significantly, which is introduced by the value errors of the components used in the filters.
3、To improve the engineering practicality of the negative group delay circuit (NGDC), a novel circuit scheme is presented for the design of transmission-type NGDC with low signal attenuation. The theoretical and experimental analysis of the circuit is carried out. By loading a lumped resistor at a specific point on the parallel coupling microstrip (MS) resonator, the negative group delay characteristics can be obtained. The negative group delay value of the NGDC can be reconfigured by changing the value of loaded-resistor. An open radial patch is employed to further reduce the circuit size. A measured maximum NGD of -2ns to -12ns and a corresponding signal attenuation of 6dB to 10 dB can be obtained by loading the resistor with different value. Compared to the previously reported circuits, the proposed NGD circuit in this work has advantages in regard to circuit size, signal attenuation, reflection coefficients as well as flexible configuration of NGD, which offers a practical way to realize the negative delay circuit in engineering application.
4、The relationship between impedance mismatch in cascaded devices and amplitude- frequency response in microwave/millimeter wave broadband system is studied theoretically and experimentally. Several effective ways of optimizing the gain flatness in the passband are summarized. Theoretical analysis is carried out to the principle of amplitude equalizer, and a broadband amplitude equalization scheme which can effectively improve the gain flatness in the pass-band of the broadband receiving front-end is presented. A Ka-band amplitude equalizer based on parallel microstrip resonators and an L-band wideband amplitude equalizer based on lumped parameter devices are designed. The scheme of Ka-band broadband receiver front-end is optimized by the broadband amplitude equalization technique and the low group delay fluctuation filter design method. The experimental results demonstrate that the Ka-band broadband receiver front-end has a pass-band gain flatness less than ±0.45dB at a maximum gain of 60dB, and a group delay fluctuation less than 1.8ns.
5、The relationship between impedance mismatch in cascaded devices and noise figure in the RF system is analyzed theoretically. A system gain configuration method based on the optimal system noise figure is presented. The method decomposes the system layer by layer based on the current commercial device performance, which simplifies the process of configuration and optimization of noise figure and gain of the cascaded device. The noise figure measurement method of broadband receiver system with high gain is studied. A theoretical model is established for analyzing the measurement error of noise figure caused by gain compression. A criterion which is used to judge if there is gain compression in the noise figure measurement is proposed. Based on the above methods, the design scheme of a dual band Ku-band LNB is worked out. The key circuit parts of the LNB module are designed and fabricated, including the Ku-band broadband LNA, the frequency synthesizer with switchable output frequency, the high performance Bias-Tee and the broadband amplitude equalizer. Under the condition of satisfying the requirement of out-of-band suppression, the group delay fluctuation of the LNB module is reduced by extending the bandwidth of the band-pass filter properly. The experimental results show that the noise figure of LNB module is around 0.90dB, the pass-band gain flatness is better than ± 1.5dB with an overall down-converting gain of 64dB. The performance of the developed LNB module is comparable with that of oversea’s commercial product.
6、An evaluation model is established for estimating the effect of gain fluctuation on the effective bandwidth which determines the brightness temperature resolution of a broadband millimeter wave radiometer. Design sechemes for W band single-channel and six-channel radiometers are worked out by using heterodyne structure. By the IF broadband amplitude equalization technique, the problem of gain unbalance in the broadband millimeter system is compensated. By a circuit with digital justable gain and with justable integration time, the gain and temperature unbalance between channals can be reduced obviously. An IF circuit with broadband equalizer is designed and fabricated according to the broadband gain characteristic of the LNA and downconverter. By cascading these circuits, a gain flatness less than ±1.5dB in a RF bandwidth of 11GHz has been realized for a complete channel of the radiometer. The prototypes of W-band radiometers are constructed, and the temperature sensitivity and long-term stability of the radiometers are studied experimentally. The experimental results demonstrate that the developed radiometers have a temperature sensitivity better than 0.55K, and the long term performance is stable and reliable.