At microwave and millimeterwave bands, demand for wide system bandwidth is increasing along with the remarkable progress in high-rate data-transfer and high-resolution imaging techniques. Therefore, development and utilization of spectrum resources at W/D band are attracting extensive attention. Theory and critical techniques of broadband components and RF frontend have become challenging and frontier research projects. The objective of this work is to develop broadband high-performance RF-frontend system at short-millimetre wave bands. Relevant theory and key techniques in the development are analyzed and studied in depth. Progress has been made in many aspects, including theory and precise measurement of receiver noise figure, system scheme design of low-spurious W-band RF frontend, development of W-band broadband downconverter and receiving module, development of W-band RF-frontend prototype, and D-band broadband sub-harmonic mixer (SHM). The main progresses of this dissertation are as follows:
1、Double-side-band noise figure (DSB NF) and single-side-band noise figure (SSB NF) of a broadband receiver with unbalanced signal/image-frequency channels are theoretically modelled. The conventional NF formulas of ideal heterodyne receivers are extended to those of the non-ideal receivers. The influences of the unbalances between the signal/image-frequency channels on the NF are studied in depth. The design guidelines for non-ideal receivers are deduced. Accurate NF measurement methods are proposed for receivers with unbalanced signal/image-frequency channels based on the traditional Y-factor method and direct-measurement method. Experiments have been carried out by establishing three types of W-band heterodyne receivers. The measured results of NF by Y-factor method and direct-measurement method are in agreement with those predicted by the theoretical NF model. The theoretical NF model of receivers with unbalanced signal/image-frequency channels is applied to assign appropriate specifications. Prototype of broadband W-band downconverter and receiver module with high-performance have been developed.
2、A novel circuit design scheme is proposed for W-band broadband downconvertoer, in which a high-gain low-noise amplifier (LNA) MMIC die is integrated with a sub-harmonic mixer (SHM) based on anti-parrallel diode pair (APDP) as a whole circuit module. The interstage matching techniques and the 3D electro-magnetic (EM) simulation method are applied to improve the gain flatness of ultra-wideband LNAs and mixers. The measured gain fluctuation of the W-band downconverter is within ±1.5dB over 89~101GHz, and the DSB NF is 3.0~5.0dB. A broadband W-band receiver module with coupled-detector self-protection circuit is proposed. The coupled-detector self-protection circuit at W band with low loss and wide bandwidth is accurately designed based on the established broadband equivalent circuit model of the detector diode. Measured gain of the developed W-band receiver module is higher than 11dB with fluctuation within ±1.6dB. The image-frequency rejection is higher than 40dB. The technical indexes meet the requirements of the radar application system.
3、A system design scheme of W-band RF frontend is proposed based on dual local-oscialltor (LO) architecture in combination with two-stage down-conversion receiver design, according to the demands of broadband FMCW detection radar application systems. The theoretical model of the system is established based on a dual LO source with two different output frequencies for receiver and transceiver, respectively. The beat signal property and its phase noise are analyzed in depth. The system overall structure based on dual-LO architecture in combination with two-stage down-conversion receiver design is proposed. Feasibility of phase noise cancellation based on this overall structure is theoretically validated. On this basis, key technical indexes of the RF frontend are determined. Harmonic and spurious spectrums are analyzed in the cascaded active components, mixers and multipliers, while the suppression level and filtering method are provided to improve the spur suppression performance.
4、A novel compact microstrip lowpass filter (LPF) with ultra-wide stopband up to 40 GHz is proposed, which effectively improves the spectrum purity of the IF signal. The central part of the LPF consists of three basic circular sectors, two smaller quadrants and a number of high impedance lines that are cross-coupled to form a high cut-off rate and a moderate stop band from 2.3-10 GHz. Then, four types of resonator cells (RCs), three of which are high impedance lines loaded with semi-circulars and one of which comprises folded step-impedance lines, are added to the central part to generate additional transmission zeroes up into Ka band. As a result, an ultra-wide stopband with 18th-harmonic suppression has been attained. A demonstration filter with 3 dB cutoff frequency of 1.96 GHz has been designed and fabricated. The measured stopband bandwidth of the LPF is from 2.2GHz to 40GHz with suppression level greater than 25 dB, and the roll-off rate is 104 dB/GHz. Its roll-off rate is better than the reported similar LPFs.
5、In development of the broadband W-band RF-frontend system, the combined circuit- field simulation method is applied to the optimization of the W-band two-way power-combined frequency tripler, and the system design scheme for high spur suppression is proposed which employs bandpass filter (BPF)/LPF with high stopband suppression in the LO chain and IF port of the receiver. In this way, the key technical proplems are solved, including the optimization of gain/amplitude flatness and the spur rejection over wide bandwidth. To improve the output power at higher frequencies, the matching networks of the diodes are modelled together with the input power divider and the output power combiner in simulated and optimized. The fluctuation of the W-band output power is better than ±0.8dB over the 10GHz bandwidth. The broadband loss performances of bonding wire are tested by experiment in actual circumstance. The technical problem that the LO power decreases at higher frequencies in band is solved and the conversion-gain flatness of receiver is improved. In addition, harmonics and spurs are greatly suppressed by adding high-performance filters in LO and IF circuits. The broadband W-band RF-frontend system is developed by assembly and test of the transceiver and LO chains. Measured W-band output power is 9.5dBm±1.0dB over 10GHz bandwidth, and fluctuation of the receiver gain is within ±1.8dB. The DSB NF is 3.7~5.1dB. The spur suppression is greater than 50dBc of the system. Phase noise cancellation is validated in the RF frontend closed-loop experiments, constructed by the transmitter and the receiver together. The RF frontend is practical and available for W-band short-range detection radar systems.
6、A circuit scheme for implementation of D-band SHM is proposed based on the single-junction operation of a dual-junction Schottky diode chip. Broadband single-junction equivalent circuit model of the diode chip is developed, considering the parasitic effects. The load-pull method is employed in combination with the established diode model. In this way, impedance characteristics at RF and LO ports corresponding to optimum conversion loss can be obtained effectively, and the circuit size are optimized according to the impedances. The measured SSB conversion loss (CL) is 14±3dB over 135~165GHz，and 12±1dB over 140~160GHz. The IF frequency band covers 0~20GHz. The broadband single-junction equivalent circuit model of the dual-junction diode chip is validated and the effectiveness of the design method are verified. A symmetrical high-Q H-plane notch cavity structure is proposed and its resonance mechanism is analyzed. It is successfully applied to the development of D-band fullband waveguide BPF with enhanced stopband performance. The measured insertion loss is typically 1.2dB over 106~174GHz, and the suppression is greater than 30dB at 180GHz. The main technical indexes are comparable to similar foreign commercial products. Development of the low-cost and high-performance SHM and BPF provide an essential foundation for research on broadband D band receivers.