Design And Implementation Of Frequency Expansion Module Using For Microwave Signal Generator

Direct Digital Frequency Synthesizer(DDS)signal source has the characteristics of large output frequency relative bandwidth,short frequency switching time,very low frequency resolution and able to generate signal modulation.It has been paid more and more attention in the practical application.But the spectral purity of the DDS signal source is not high,and the upper limit of the output frequency is too low.It can not directly fulfill the requirements of measurement or signal simulation.It is necessary to extend the frequency range by using the frequency band extension technique.This paper summarized three kinds of frequency expansion structures: frequency multiplier-filter structure,frequency mixer-filter structure,and DDS-PLL hybrid structure.After analyzing the main advantages and disadvantages of the three technologies,a two stage frequency multiplier-filter structure module is designed and implemented to extend the output band of the signal source.Firstly,the key circuit parameters of the module are analyzed,which mainly includes a signal channel division method which can make the continuous signal and the LFM signal pass normally,the parameters of two filter banks and three low pass filter.Next,we focus on the simulation of the filter needed by the module.A 5 channel filter banks working in 0.75~1.5GHz are realized by microstrip interdigital bandpass filter;a 5 channel filter banks working in the 3.0~6.0GHz are realized by cavity cross finger band pass filter;the 3.0GHz and 6.0GHz low-pass filters is microstrip hairpin structure.The test results show that the designed filter has the characteristics of good rectangular coefficient and high external restraint system,and has good practicability.Finally,the whole system is integrated,and the circuit schematic diagram and PCB layout of the whole circuit module are completed.In system integration,the circuit is divided into four parts: primary frequency multiplication link,1.5~3.0GHz microstrip filter bank,two-stage frequency multiplication link,and 3.0~6.0GHz cavity filter bank.The corresponding PCB layout and test are completed respectively.Finally,in the output frequency range of 1.5~4.0GHz,four pieces of PCB were individually tested and combined debugged.The test results show that the harmonic spurious suppression of the output signal of the module is greater than 43 dBc in the range of 1.5~3.0GHz,more than 56 dBc in the range of 3.0~4.0GHz,and the non-harmonic spurious suppression is superior to 69 dBc in the range of 1.5~4.0GHz;the maximum linear frequency modulation width of any frequency point in the range of the output band is 3%~16% of the carrier frequency.