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FIGURE 141 A software-de ned radio transmitter
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FIGURE 142
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A software-de ned radio receiver
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conversion stage, and digital IF and baseband functions incorporating signal processing hardware algorithms (Fig 142) There are two different kinds of SDR radios The first type is called heterodyne SDR, as shown in Fig 142: The antenna receives the RF signal, which is then filtered, amplified, and converted to IF by these analog stages A very fast ADC then changes the analog IF into a digital signal, where the receive signal processor (RSP) filters and tunes the signal to the selected channel to provide baseband I and Q outputs into the DSP This allows various channels, frequencies, and standards to be placed within a single radio (In fact, the RSP is where the SDR radio has its tuning and selectivity, data rate, channel bandwidth, and even channel shaping abilities, and actually supplants the local oscillator (LO), channel select filter, quadrature mixer, and data decimation filter) The RSP is essentially a special purpose DSP, and must be capable of the same speed as the ADC itself The output of the RSP is then sent on to the DSP, which demodulates the baseband signal The DSP, depending on its programming, can demodulate both digital and analog signals, and is able to receive FM, AM, QPSK, CDMA, and the like To really make quality software radios possible, a fast, low-noise ADC with wide dynamic range is a requirement Indeed, this ADC must have an excellent signal-tonoise ratio (SNR) rating, which represents both quantization and thermal noise, as well as the analog-to-digital converter (A/D, also known as ADC) sample clock s wideband phase noise The high dynamic range is also a must in order to decrease spurious responses, since within any multicarrier radio the responses of one channel can interfere with the weaker signals in another channel The transmitter of a heterodyne SDR system, as was shown in Fig 141, functions so: The DSP places digital data to be modulated into the transmit signal processor (TSP) The TSP then modulates the carrier with this information The digital modulated signal
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is transformed to analog by the DAC, sent into the analog stages and frequency upconverted to RF by the mixer/LO, amplified by the SSPA, filtered, and sent out of the antenna The second type of SDR architecture is called direct conversion software defined radio (DC SDR) DC SDR radios reduce much of the heterodyne system s disadvantages, such as the need for a front-end filter (used to set the radio s frequency, its maximum bandwidth, and to remove the image band), and converts the incoming RF directly into a zero IF This removal of the front-end filter requirement is an important feature of a receiver that must be completely configurable over multiple bands, as RF filters that are frequency and bandwidth tunable over a wide range are extremely difficult and expensive to design and mass produce As a result, for each wireless standard the heterodyne type SDR radio would need an expensive and large switchable filter bank, along with a fine-tunable LO, while the DC SDR completely removes this problem altogether, significantly lowering system costs
Direct Conversion Receivers
1421 Introduction
Direct conversion receivers (DCRs) (also called zero-IF receivers; Fig 143), have seen limited use in the past due to implementation complexities A DCR architecture comprises a receiver without an IF, since the input RF is directly mixed down baseband DCRs have a much lower parts count, and are thus cheaper to build, than the competing superheterodyne designs DCRs do not require an image filter, since no image frequency is seen by the DC receiver, nor do they need high frequency IF filters and amplifiers, since there is no IF However, direct conversion receivers have multiple problems that make a discrete DCR design almost impossible, while even RFIC implementations are rife with difficulties Nonetheless, many of the DCR s limitations can, and have been, addressed relatively successfully within the domain of some of the newer RF-integrated circuits
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