Research Of A Balun For Ultra-Wideband Receiver

Balun.which has been widely used in various passive and active microwave devices,is also an essential component in some transceiver systems,i.e.,providing AC coupled input signals to chips,e.g.,ADCs.Marchand balun.which is one of the most widely utilized balun structures,has been deemed as a hot research topic due to the simple structure,easy integration and large bandwidth.Bandwidth and imbalance of the balanced port in passband determine the working effect of a balun,which are also the difficulties for balun design.Besides,considering the integrated applications,the volume of balun and the ease of integrated application also cause additional difficulty for UWB balun with high amplitude and phase balance.In this paper,we propose to design an UWB balun with a relative bandwidth over 9:1 and two all ports matching baluns.One of the all ports matching balun is applied to a direct RF sampling receiver circuit working within 5GHz-15GHz.As for the design of balun,we firstly analyze the working characteristic of balun using common-and differential-mode analytic methods based on the equivalent circuit of balun and mixed-model S parameters.Then we further study the relationship between the relative bandwidth and structural parameters on the basis of the lumped-parameter equivalent circuit.Simulation and experiment results demonstrate that the designed UWB balun works within the range 2GHz-18GHz and performs satisfactory amplitude-and phase-balance within the passband.Since the reflection coefficient and isolation of the balanced ports of the typical Marchand balun are both-6dB,we add an isolation network between the output ports to improve the VSWR and isolation by analyzing the equivalent circuit and balun’s scattering matrix.As for the circuit design of direct RF sampling receiver,SiP package and BGA technology are adopted to integrate the front-end amplifier circuit into one substrate,which improves the integration level and greatly reduces the size of the receiver.Amplifiers are embedded in LTCC substrate and connected with other chips through bond wire.The amplified RF signal is converted to a pair of differential signals by balun printed on the BCB before being sampled by ADC.Simulation results illustrate a good matching and signal integrity of the proposed receiver circuit.