Input A7 is used for a 5 MHz self-test input. Under software control it is possible to use the multiplexer to switch between a wideband input (A0, displayed as In1 by the software), a low-pass filter for medium wave (MW) reception (A1, or In2), and a high-pass filter for shortwave (SW) reception (A2, or In3). In the design, a 74HC4051 is used as an input multiplexer, offering a total of eight inputs A0 to A7. The SDR covers practically the whole of the radio frequency range up to 30 MHz. The publication of the SDR hardware was also followed by a flurry of tuning and control software like Peter Carnegie’s ‘G8JCFSDR’. Almost three years since its publication, the Elektor Software Defined Radio (SDR) is still a blockbuster project, and here at Elektor we were gratified by comments from the radio amateur community like ‘good value for money’ and ‘an excellent entry-level SDR’. A tuned loop antenna is also described that lets you use the Elektor SDR receiver without an outdoor antenna. The preselector described here allows the use of up to four filters, tuned under software control using varicap diodes. A selective input circuit improves antenna matching and immunity to interference from other strong signals. RADIO Preselector for Elektor SDR Here’s the automatic tuning upgrade! By Burkhard Kainka (Germany) The performance of a receiver depends to a large extent on its input filters.
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