FFT-based Digital Receiver Architecture for Fast-scanning Application

Receivers covering the VHF/UHF communications bands are the important parts of the EW/ESM systems. Currently used frequency agile signals using frequency hopping and burst transmission technique rise eminent requirements for scanning and search receivers. The fast scanning speed can be achieved with parallel signal processing based the spectrum estimation of a wide bandwidth of incoming signal. One of the technology, which is available for that purpose is a traditional FFT, but the key element of the FFT based receivers is still remain the analog to digital converter (ADC) which is used. Currently lot of high-speed, high-dynamic range ADC chips are on the market developed for the digitized mobile radio market based on Software Defined Radio (SDR) technology. In the paper we would like to introduce our currently developed digital receiver platform for fast-scanning application which is able to process up to 10MHz of instantaneous input bandwidth with down to 10 KHz resolution within 1ms processing time. Connecting it to our analog receiver front-end with highspeed synthesizers able to tune to any frequency up to 3 GHz the result is a high-speed fast scanning receiver with up to 10GHz/sec scanning speed at 10 KHz resolution. I. RADIO PLATFORM The radio front-end elements of a traditional analog radio is used for digital applications too. The SR-2000 radio platform form Sagax Communications could be divided to an analog and a digital part. The analog part contains the down-converter, the analog IF processor and a platform controller which enables to use these parts of a platform as a conventional analog receiver (see figure 1.). Fig. 1. Digital and analog partition of the device. The front-end contains the optional block down-converter for frequency extension up to 3 GHz and the basic receiver covers the 20-520 MHz range with an upper IF converted structure. It has got a wide-band IF output for further digital processing too. The analog IF processor contains the IF filter bank and the basic WFM/NFM/AM/CW/SSB demodulators. The microprocessor based platform controller provides a multi-drop RS-232 based serial or an TP Ethernet based TCP remote control. See figure 2. and figure 3. Fig. 2. Signal path from RF to IF. Fig. 3. Analog IF processor and control elements. Its fast switching frequency synthesizers allow 1000 ch/s scanning in the frequency band or channel memory to fulfil traditional ESM requirements in the VHF/UHF communication bands. The digital parts of the platform based on the high-speed, wide-band converters based on 14 bit ADC and deep FIFO memory realized as a standard desktop PCI board with onboard FPGA resource for glue-logic and signal processing, and a master-mode PCI controller. Fig. 4. Block diagram of the digital part. Fig. 5. The PCI controller scheme. The captured data could be transported to the PC based host computer memory trough a PCI bus or in real-time application external dedicated, high throughput data ports are available to connect the converter to the DSP processors directly. The platform uses a four processor DSP engine with PCI interface and external data ports. For a demodulation applications a codec chip is provided to generate the demodulated audio signal. The introduced elements are composing a cost effective and functional radio platform for EW/ESM missions. As the analog processing elements are well described in the literature in the following we deal with the signal processing attributes of the digital elements focusing on the fast scanning functionality. II. DFT BASED SPECTRUM ANALYSIS The discrete Fourier transformation (DFT) is a basic tool of spectrum analysis. The most algorithms use this method because of its good properties. The theoretical advantages are such as direct mathematical connection with the coefficients of Fourier series, orthogonal property as a transformation, noise whitening. In practice after the AD converter the spectrum can be computed only at a finite frequency grid. But if assumptions of the Shannon sampling theorem are true, i.e. the maximal frequency in the signal is less than half of the sampling frequency fs then the original analog periodic signal can be reconstructed. Studying the DFT transform of a sampled signal can be divided up into two essentially different situations: coherent and non-coherent sampling. These cases are studied hereafter. The N point DFT is defined as