Software radio is a brand-new architecture based on broadband A/D devices, high-speed DSP chips, and software as the core. The basic idea is to put the broadband A/D as close as possible to the radio frequency antenna in order to digitize the received analog signal as early as possible, and try to realize the various functions of the radio station through software. The main features are: flexibility, adaptability, and openness.
The main difference between virtual radio and software radio is that it uses general-purpose microcomputers to replace high-speed DSP chips to complete various functions. Its flexibility, versatility and openness are more than software radios. I believe that with the continuous improvement of general-purpose microcomputer computing speed, virtual radio technology will be more widely used.
1 Design Idea of Multimedia Radio Interface Card
Based on the basic principles of virtual radio, we designed a radio interface card that can receive AM and FM signals, and a multimedia radio can be formed by using speakers or headphones and other equipment carried by the computer. Its composition block diagram is shown in Figure 1.
1.1 Data collection part
The data acquisition part is mainly composed of a numerical control amplifier and an analog/digital converter AD6640, which is responsible for completing the data acquisition work. The digital control amplifier is divided into two channels: AM power amplifier channel and FM channel. The channel is selected by the microcomputer according to the channel selection. The maximum range of its adjustable gain: 20 ~ 60dB, so that the signal is stabilized at about 0.8V (because AD6640 input peak-to-peak value V = 2V), in order to increase the effective number of bits after sampling. According to the different AGC control words sent by the computer, the gain can be adjusted and controlled continuously. AD6640 is a new-generation analog-to-digital conversion device produced by AD Company, with a resolution of 12 bits, a sampling rate of up to 65MSPS, and a power consumption of only 710mW when powered by 5V.
1.2 Digital down converter
Due to the influence of the computing speed of the microcomputer, if the sampled data is directly sent to the computer for processing, the real-time performance is difficult to guarantee. For this reason, we use the special programmable down-converter HSP50214B produced by American Harris Company to do the necessary preprocessing of the sampled data and then send it to the microcomputer through the PCI bus. Its main function is to down-convert the IF data into baseband data. HSP50214B includes the following functional modules: input unit, level detection, carrier digital voltage-controlled oscillator (NCO), cascade integral/comb filter (CIC), 5 optional half-band filters (HB), 255-order Programming FIR filter, automatic gain control (AGC), resampling filter/interpolation half-band filter, resampling NCO, coordinate transformation and output unit. Its structural block diagram is shown as in Fig. 2.
The chip's front-end processing speed is up to 65MSPS, the back-end processing speed is up to 55MSPS, the total decimation factor range is 4～16384, the output sampling speed can reach 12.94MSPS, and the widest output low-pass bandwidth is 982kHz (IF bandwidth 1.96MHz) . It supports parallel data input with a word length of up to 14 bits, and the output forms are flexible and diverse. Either parallel output or serial output can be selected to output the required information such as amplitude, phase, frequency or quadrature I and Q data. In addition, HSP50214B also has a built-in level detector, which can be I. F. Automatic gain control provides support. In short, HSP50214B is very powerful and flexible in use, and can be used to demodulate AM, FM, FSK and DPSK signals. 1.3 Introduction to S5933 and PCI bus
The PCI bus is independent of the processor and has a high data transmission rate; it adopts a highly comprehensive and optimized bus structure to ensure reliable operation between the components of the system and is fully compatible with the existing bus.
The S5933 produced by AMCC is a powerful and flexible PCI bus controller dedicated chip. This chip complies with PCI local bus specification version 2.1 and can be used as a PCI bus target device (Slave) to achieve basic transmission requirements. It can also be used as a PCI bus master device to access other PCI bus devices. The peak transmission rate of S5933 is 132MB/s (32-bit PCI data line), which fully meets the data transmission requirements between HSP50214B and the microcomputer.
1.4 Microcomputer and decoding control circuit
In addition to the initialization of the HSP50214B and broadband power amplifier control tasks, the microcomputer must also complete functions such as channel selection control and voice output. For this reason, it is necessary to prepare and equip the necessary device-specific driver to complete the background operation without affecting other work. The decoding control circuit selects EPM7128SQC-10 from Altera Corporation in the United States to complete address decoding and other control tasks.\
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