100-500MHz Digital Control Voltage-tuned Bandpass Filter Developed

Today, in a high-speed period of communication technology, it becomes a popularsubject to be research, as well as the high performance microwave filters in eachfrequency band. With the increasing requirements of the performance in the wirelesscommunication system, an alterable center frequency narrow-band filter covering theentire communication band and is urged to improve the signal-to-noise ratio of thesystem, the anti-jamming capability and electromagnetic compatibility.The frequency band segmentation used for the numerical control voltage-tunableband-pass filter to make wideband filter come true is developed in this paper, it divided100M-500MHz into four segments. The filter consists of the control circuit, switchingcircuit, DA converter circuit, amplifier circuit, filtering circuit.Filtering circuit is optimized based on the seven-order Chebyshev filter circuit, italters the value of the capacitance by changing the value of the center frequency.Varactor, which has the advantages of simple structure, good performance, continuouslyvariable performance, is selected as a variable capacitor to tune the center frequency.The voltage of the varactor is changed by Numerical control circuit to change itscapacitance value. Input nine parallel control word, processed by the microcontrollerafter latch, then strobes the different band filter components, while table look-up outputthe conversion code which corresponding to the center frequency to the DA conversioncircuit. Then the digital voltage is converted to the analog voltage which is amplified todrive the varactor diode work in the filter. To achieve the center frequency conversion,meet the functional requirement.The technology of band split is used to cover wide band a filter, which include foursets of filter work at different frequency bands in a row seamless. It solves the difficultproblem of balance the different performance demands between high frequency and low.Be optimized in groups, the high performance voltage-tunable filter with up to fiveoctave frequency coverage could be achieved. Bring Chebyshev circuit model into theoptimized voltage-tunable filter, reduce the contradiction of constant bandwidth andconstant relative bandwidth, minimize the bandwidth changes result from switching in different center frequencies. It developed an optimized circuit model with simplecontrol method to easy to achieve high-order voltage-tunable filter. The circuit modelmakes it possible that developing the high-order high-performance voltage-tunable filterwith low rectangular coefficient and out-of-band rejection.Developed electrically tunable filter samples center frequency minimum spacing of1MHz, insertion loss is less than8dB, relative bandwidth less than18percents,rectangular factor less than4, groups delay less than200ns and performance to meet thedesign requirements. In the future work, improving this sample to get more excellentperformance, to cover a wider range of work frequency, to work at a higher frequency,to get more precise alteration of the frequency, and so on. The successful developmentof this filter widens the design method of the voltage-tunable filter.