Digital Receiver Handbook: Basics of Software Radio

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Digital Receiver Handbook: Basics of Software RadioTheory of OperationDSP Demodulation FunctionsKey ey Benefits of Digital Receivers● Frequency and Phase Shift Keying (FSK, PSK)● AM, FM, and PM● Spread Spectrum● Custom Frequency Agile Schemes● Signal Analysis (FFT’s), Signal Identification● Signal Recording and Tracking●●Dedicated digital receiver hardwarepreselects only signals of interestSaves significant DSP horsepowersince DSP requirements are directlyproportional to sampling rateFigure 19 Figure 20Virtually any form of demodulation can be implementedjust by loading the DSP or FPGA with theappropriate algorithm (Figure 19). AM can be demodulatedwith an envelope detector, FM & PM can bedemodulated using a phase or frequency discriminatoralgorithm.The ability to quickly change the local oscillatorallows frequency-agile modulation schemes to beaccommodated as well. Analysis functions includeenergy detection such as required by scanning receiversthat may be implemented with an FFT, for example.Other analysis functions include cryptography,identification of transmitters based on transmissionfrequency, modulation schemes, and other signalcharacteristics.Once the signal is successfully brought into the DSParena, automated functions such as center frequency andbandwidth tuning can be implemented to track a complexsignal which may be moving or hopping. Interestingsignals can be stored on hard disk, tape or other mediaand the time of the signal event can be logged as well.With this arrangement, when new or proprietarydemodulation, processing, or analysis schemes are required,no new hardware is necessary. Instead, a new DSP softwarealgorithm is loaded.Think of the digital receiver as a hardware preprocessorfor DSP. It preselects only the signals you are interestedin and removes all others. This provides an optimumbandwidth and minimum sampling rate into the DSP.Since the number of DSPs required in a system isdirectly proportional to the sampling rate of inputdata, by reducing the sampling rate you can dramaticallyreduce the cost and complexity of the DSP systemthat follows.Even if the digital receiver outputs do not require agreat deal of signal processing, reduction of bandwidthand sampling rate helps save time in data transfers toanother subsystem, helps minimize recording time andtape or disk space, and speeds up communication channels.12Pentek, Inc. • One Park Way, Upper Saddle River, NJ 07458 • Tel: (201) 818-5900 • Fax: (201) 818-5904 • Email: digrec@pentek.com • http://www.pentek.com
Digital Receiver Handbook: Basics of Software RadioTheory of OperationDigital Receiver Chip PererformanceNarrowbandWidebandTI / GC Intersil TI / GC Intersil TI / GC Pentek IP TI / GC Pentek IP Pentek IP4014 50214B 1011A 50016 4016 Core 430 1012A / B Core 421 Core 422No of Channels 4 1 1 1 4 256 1 1 1Sample Freq (ƒ s) 62.5 MHz 65 MHz 70 MHz 75 MHz 100 MHz 185 MHz 80 / 100 MHz 160 MHz 296 MHzInput bits 14/16 16 12 16 14/16 16 12 16 16Min Decim (N) 32 2 64 64 32 1024 2, 4, 8 2, 4, 8 2, 4, 8Max Decim (N) 65k 65k 65k 131k 16k 9984 16, 32, 64 16, 32, 64 16, 32, 64Resampler No Yes No No Yes No No No NoCustom FIR Yes Yes No No Yes Yes No Yes Yes3 dB Output BW Prog Prog 0.8ƒ s/N 0.56ƒ s/N Prog Prog 0.8ƒ s/N Prog Prog0.9ƒ s/NVME Boards 6821 68216822 6822VIM Modules 6230 6235 62506231 6235 6236 62516232 6236 62506251PMC Modules 71317140 7140PCI Boards 76317640 7640cPCI Boards 72317240 7240The above chart shows the salient characteristics forseven popular digital receiver chips and three FPGA IPcores. Note that the range of decimation settings for thenarrowband chips on the left is much higher than forthe wideband receivers on the right. The output samplingfrequencies for real and complex outputs are shown as afunction of the decimation factor N.Notice also that the 3 dB output bandwidth of theFIR filter is expressed as a percentage of the inputsampling rate divided by N. This percentage reflects thefrequency characteristics of the specific FIR filterfunction implemented in the digital receiver chip. Eachfilter characteristic has its own passband flatness, rolloffrate, and stop band attenuation characteristics suitablefor different applications.As an example, if we were using the TI/GC1011Awith a 64 MHz A/D converter and needed a usableoutput bandwidth of 10 kHz, we could solve for theappropriate decimation factor setting as follows:10 kHz = 0.80 • ƒ s/ N orN = 0.80 • 64 MHz / 10 kHz = 5120Note that the decimation factors of narrowbandreceivers are programmable in steps of 1 or 4 and thewideband receivers are programmable in binary steps asshown. Some receivers allow entry of custom FIR filtercoefficients and others have output resampling stages tosupport custom filter characteristics and output samplingrates.Model numbers of Pentek products using each typeof digital receiver are shown at the bottom.13Pentek, Inc. • One Park Way, Upper Saddle River, NJ 07458 • Tel: (201) 818-5900 • Fax: (201) 818-5904 • Email: digrec@pentek.com • http://www.pentek.com
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Digital Receiver Handbook: Basics of Software Radio

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