Research And Development Of Multi-rate Digital Intermediate Frequency System

With the rapid development of mobile communication technology, the traditional communication devices based on proprietary hardware has limited function and hard to upgrade. This causes a compatibility issue for a variety of communication protocols and software versions. This problem has prompted the development of new technologies such as software defined radio (SDR). Due to the development level of hardware, it is difficult to directly deal with RF and high frequency band signals, so it is generally used in the digital structure of broadband IF bandpass sampling, the key technology is digital IF signal processing. Therefore, it is very important to study the digital intermediate frequency technology.Digital IF signal is in between the digital baseband signal and RF signal, the external signal through RF unit (AD9364 agile transceivers) into the digital IF unit (zedboard FPGA), after digital IF signal processing which is then transmitted to the digital baseband. Digital IF frequency technology, including digital up conversion(DUC) and digital down conversion(DDC). DDC IF signal passed through the high-speed radio unit down to the low sampling baseband signal and the low-IF signal of 240kHz move to zero-IF baseband signal. DUC Will slow baseband signal rise sampling to high speed signals into the RF unit of DAC, in order to reduce the DAC quantization error and reduce the analog filter requirements.The paper presents a multi-rate digital IF system implemented by FPGA. The system designs in a full duplex mode and supports multi-rate transceiver architecture; it can also be configured according to the applications in order to achieve a variety of data conversion rates. The system includes a mixer module, a sample rate conversion (interpolation and decimation) module, and a low-pass filter module. It realized 32, 96,480 times down/up sampling and 240KHz down conversion process. The spurious signals are effectively suppressed while the signal stopband attenuation can reach 60dB. With flexible system configuration, the module has been optimized and reused, which can save hardware costs and improve the performance.