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A software-defined radio, or SDR, does with a computer what traditional radios do with coils, crystals and mixers. An analog-to-digital converter samples a wide swath of spectrum, and software selects the signal, filters it and turns it into audio or data, producing the familiar waterfall display of a whole band at once. Most new amateur transceivers are SDRs inside, and an inexpensive USB dongle can turn a laptop into a receiver for listening to repeaters and decoding FT8 before getting licensed. Read more…