The direct sampling approach has long been sought after.
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These challenges are significant with the modern quest toward low size, weight, and power (SWaP), combined with the desire for wide bandwidth, and lead to designers considering other architecture options when possible. In addition, frequency planning can be quite challenging at large fractional bandwidths. It typically requires the most power and the largest physical footprint relative to the available bandwidth. Unfortunately, this architecture is the most complicated. Through proper frequency planning, the heterodyne receiver can be made with very good spurious energy and noise performance. Additionally, the receiver gain is distributed at different frequencies, which minimizes the risk of oscillation in high gain receivers. It is also common to add an additional mixing stage to lower the frequency where very high dynamic range ADCs are available. The IF is chosen at a high enough frequency to allow practical filters in the operating band to provide good image rejection and LO isolation. The implementation is to mix to an intermediate frequency (IF). The heterodyne approach is well proven and provides exceptional performance.
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The basic topology is shown along with some of the benefits and challenges of each architecture. Table 1 compares the heterodyne, direct sampling, and direct-conversion architectures. The intention is not to promote one option over others-but rather describe the pros and cons of the options, and encourage the designer to select, through engineering discipline, the architecture most appropriate for the application. Additional consideration on spurious system noise and dynamic range is also discussed. This article compares the benefits and challenges of three common receiver architectures: a heterodyne receiver, a direct sampling receiver, and a direct-conversion receiver. In recent years, the rapid advance of analog-to-digital converter (ADC) sampling rates, the inclusion of embedded digital processing, and the integration of matched channels now offers options for the receiver architect that were not practical only a few years ago. The heterodyne receiver has been the standard receiver option of choice for decades.
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A Review of Wideband RF Receiver Architecture Options