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Sunday, May 3, 2015
Drake Dongle Derring-Do -- 2B goes SDR (video)
Here I take my Drake 2B -- arguably the quintessential Hardware Defined Radio -- and connect its first Intermediate Frequency circuitry to an RTL-SDR Dongle, allowing me to digitally process, filter, and display (panoramically!) the signals being inhaled by the ancient receiver. Another cool tech twist: To get at the 455 kHz IF signal I use the "Q-Multiplier" jack on the back of the receiver. This connector was put there to allow for the use of a selectivity enhancing regenerative stage. So I'm using that connector for a similar purpose, but using technology that wasn't even being dreamed about when that Drake 2-B was being designed in 1961.
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Actually, the field of digital signal processing was well understood when the 2B was in its prime. The work of Nyquist, Shannon, and others was published as early as the 1930's. The only thing we lacked was the compute power. That is now available in tiny, low power DSP chips, enabling the things we're about to start taking for granted. We live in interesting times...
ReplyDeleteAnd even then a lot of what's being done existed in analog. Webb had that article in CQ in 1957 or 58 about building a synchronous detector, and that's now implemented in digital processing. It wasn't anything but a sideband slicer with the ability to synchronize with the AM signal.
ReplyDeleteBoth might have used that jack in 1961, though they'd lose out on the good selectivity further on.
Of you don't know about the analog work, you'll never know to implement them with DSP. When PLLs hit in the early seventies (well they existed, and were discussed earlier, but were bulky with tubes), there were lots of methods described for better AM reception, including some that had no problem with DSBsc.
That said, there is something quite appealing to using one of these gizmos to replace selectivity and detectors in an existing receiver. Beats matching resistors and capacitors analog phasing networks. And there the receiver is offering front end selection, which is obviously lacking in these gizmos, and which I can't believe is taken care of on such cheap gizmos.
Michael
Well done Bill.
ReplyDeleteHow about a diagram for the impedance changing device?
ReplyDelete