The Design And Implementation Of RF-IF Circuit In The Front-end Of All-digital Shortwave Receiver

Short-wave communication is a means of wave transmissions communication, which back to the ground by ionospheric reflection or reflection. It is widely applied to the field of broadcast, navigation, military affairs and so on. Because of the anti-jamming ability, simple equipment and easy for temporary networking, short-wave radio is the vital part in military operations. However, the electromagnetic environment is more and more complex, and short-wave radio communication equipments are increasing, so the short-wave signals become more and more crowded, which leads to radio disturbance among each other and even produces obstruction. Therefor, it is required to design the RF-IF circuit in the front-end of short-wave receiver which has characteristics such as anti-noise, anti-interference, anti-fading, anti-multipath ability and intelligent control.Structure, performance, the advantages and disadvantages of several shortwave receivers are analyzed firstly in the paper. Based on the main technical indicators of short-wave receiver such as frequency resolution, insensitivity, IF selectivity, image rejection, IF recjection, blocking, reciprocal mixing and frequency stability, the digital IF receiver structure based on software radio is defined. And RF-IF circuit in the front-end of the digital IF receiver is discussed importantly in the paper. In order to improve the image rejection, the output of the mixer is designed to high-intermediate-frequency. In order to improve the ability of anti adjacent channel interference and blocking interference, the short-wave band is divided into eight bands, and each band is tuned and controlled by chip microcomputer. In order to improve the IF rejection, adopt a IF notch lowpass filter before the mixer. In order to improve the sensitivity of the system, employ a dual-gate FET3SK223whose noise figure is very low to design low noise amplifier. In order to reduce the interference of combination frequency, select double-balanced mixer ADE-1to complete the frequency conversion function. In oeder to increase the gain controlled dynamic range, use multi-stage automatic gain control circuits. In order to achieve high precision and high stability local oscillator whose frequency resolution is1Hz, direct digital synthesizer is choosed, and take some measures to improve the local oscillator performance, such as select oscillator368.64MHz which is produced by OCXO18.432MHz and external high-precision phase-locked multiplier with a factor of20as the reference oscillator to degrade the effects of phase noise of on-chip multiplier, employ the tracking filter to effectively eliminate the spurious signals and harmonics of sinusoidal waveform, Add gain control circuit controlled by frequency amplitude control word and DAC output to keep amplitude basically steady in required frequency range.Each module is produced, installed and debugged, and then test the technical indicators of each module according to the given testing methods, and then test the various indicators of the system. All of the results of the short-wave receiver meet the subject requirements.