The Design Of A Local Oscillator Source Based On Digital Radio

Today trunking mobile communication also steps into digital times, digital trunking communication has drawn the special attention with its many business types, high spectrum efficiency and good secrecy characteristics and so on, at the same time digital radios that based on DMR (Digital Trunking System) treaty has started rapid development. For wireless digital radio, frequency synthesizer is a very important part, it can supply high accuracy and stability LO (Local Oscillator) signal for RF transceiver.The performance of frequency synthesizer will directly affect the whole transceiver system, so we also take frequency synthesizer as the "heart" of the wireless and it is a very important part of RF circuit. In this design we mainly supply LO signal for Super heterodyne Receiver used in the first down-conversion.In this paper we choose PLL frequency synthesizer to produce LO signal, the major structure of paper is as follows:First, introduce three kinds of frequency synthesizers:Direct synthesizer (DS), Direct Digital Synthesizer (DDS) and Phase-Locked Loop (PLL), make the right choice by comparing the advantages and disadvantages between the above methods, here we adopt PLL frequency synthesizer because its good phase tracking characteristics and board range of output frequency.Then,give a detailed introduction for every component of PLL and expound the working principle of PD (phase detector), loop filter and VCO (voltage control oscillator) and say how to design a three order low-pass filter,compare two different types of PLL chips that are integer and decimal frequency synthesizers by simulation, here we choose integer PLL chip ADF4111to complete our design because it has good performance on phase margins and spur levels.VCO can realize with field-effect transistor and variable capacitance diode and give the design principle of VCO circuit.Here we use clapp circuit and frequency selective circuit consists by variable capacitance diode and inductance to form an oscillating circuit of VCO, and then add a buffer circuit and an amplifying circuit can easily achieve our design goal.Last, we make the frequency synthesizer that applied in digital trunking communication at the frequency of305.15MHz to345.15MHz and give the test and analysis results,repeating debug the board,the test indicates that the performance of LO satisfy the requirement of the whole system.