Digital Receiver Handbook: Basics of Software Radio

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Digital Receiver Handbook: Basics of Software RadioApplicationsApplications of Digital Receivers●●●●●Tracking Receiver SystemSignal Intelligence ReceiverDirection Finding SystemRadar Signal Processing SystemWireless Cellular Development SystemDigital receivers can be used in many different systems:Tracking receivers and signal intelligence receiverscan be highly automated because digital receivers allowDSPs to perform the signal identification and analysisfunctions as well as the adaptable tuning functions.Direction finding is an ideal application for digitalreceivers because of their excellent channel-to-channelphase matching and consistent delay characteristics.Radar applications benefit from the tight couplingof the A/D, digital receiver and DSP functions toprocess wideband signals. FPGAs are especially wellsuited to handle FFT and pulse compression tasksnormally required in the signal processing sections.Some of the digital receiver chips employ specialfeatures like a “chirp” sweep generator function in the localoscillator, ideal for high-performance radar applications.Cellular phone applications are one of the strongesthigh-volume applications because of the high density oftightly-packed frequency division multiplexed voicechannels.As a general capability, any system requiring atunable bandpass filter should be considered a candidatefor using digital receivers. Take a look at the followingapplication examples to give you some more details.Product overviews of all models described in theapplications section are included in the last section ofthe handbook.14Pentek, 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 RadioApplicationsTracking Receiver SystemModel 6821 215 MHz A/D Converter with FPGASystem Highlights● A/D data delivered into SDRAM that acts as a digital delay memory● A/D data also delivered into a Pentek FFT IP core in FPGA● FFT core detects the strength of signals at each analysis frequency● PowerPC controller in FPGA sorts signals according to peak strenthModel 6821 commercial (left) andconduction-cooled version● PowerPC controller also tunes DDC IP core in FPGA to the strongest signal frequencies● Delayed data from SDRAM feeds DDC IP core to compensate for FFT calculation time● DDC captures these moving signals in real time and downconverts them to basebandFigure 21A tracking receiver locates unknown signals, locksonto them and tracks them if their frequency changes.As shown in Figure 21, to implement this receiver,we use the 128 MB SDRAM of the Model 6821 tocreate a delay memory function.Samples from the A/D are sent into a circular bufferwithin the SDRAM and also to a Pentek FFT IP coreimplemented in the FPGA. The spectral peaks of the FFTindicate the frequencies of signals of interest present at theinput.The PowerPC microcontroller of the FPGA digeststhis frequency list and decides which signals to track. Itthen tunes the Pentek DDC core, also implemented inthe FPGA, accordingly. The delayed data from thecircular buffer feeds the input of this DDC core.The digital delay can be set to match the time ittakes for the FFT energy detection and the processoralgorithm for the tuning frequency decision, so thatfrequency-agile or transient signals can be recoveredfrom their onset. The dehopped baseband output isdelivered to the rest of the system through the FPDPport or, optionally, across a VXS link.See page 22 for a more detailed description of thePentek Model 6821 A/D Converter. This Model is alsoavailable in a dual-channel version as Model 6822, seepage 23. Both Models are available in commercial andconduction-cooled versions.15Pentek, 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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