X-band Broadband Frequency Source

As the key component of the modern communication system, frequency source is the essential circuit which decides the performance of the system. Due to the rapid development of radar, electronic countermeasure, communication, the demand on high-performance frequency synthesizer is more and more urgent. Most of the modern frequency source is realized through frequency synthetic technology so it is also called frequency synthesizer. A sort of X-band frequency synthesizer with high resolution, wide bandwidth, low phase noise and spurious free dynamic range (SFDR) has been introduced. By adopting the hybrid"DDS+PLL"frequency synthetic technology controlled by MCU and optimizing the program algorithm and considering rationally for the Electromagnetic Compatibility (EMC) the target of the system has been achieved.Firstly the basic theory of DDS and PLL, as well as their characteristics and properties has been analyzed. After introducing and comparing the advantages and disadvantages of several frequency synthetic technologies which are used to expand the output bandwidth, we adopt the scheme of DDS driving PLL. By taking advantages of the low phase noise properties and fine steps of DDS and the narrow-band filter property of PLL, the targets of the system has been meet. Avoiding a lot spurs which brought in by the mixers and multiplier and the filters which is essential to filter out the spurs the system is simple and stable.Then the scheme has been demonstrated roundly. The frequency plan and the power performance have been designed reasonably. Then the feasibility about the phase noise, SFDR, hopping step, bandwidth, has been analyzed. According to the request of the system, the DDS chip AD9858, the PLL chip ADF4106, the VCO HMC588LC4B and so on, have been chosen to establish the system circuit. In this system, MCU PIC18F6722 has been used for the implement of frequency hopping (FH).Finally the debugging process has been introduced and the result has been analyzed detailed. At last through the test of the system the paper gives the results of such system as following: output frequency range 8GHz～12.5GHz, phase noise -81dBc/Hz@10kHz, SFDR -53dBc, harmonic restrain -30dBc, step 1MHz.