Digital Receiver Handbook: Basics of Software Radio

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