Digital “Crystal” Breathes New Life Into Old Radio

[Bill Meara] of the Soldersmoke Podcast has a nice old Drake 2B radio, and wanted to use it for the 12 meter amateur band. These old radios normally make switching tuning bands easy — you just swap out one frequency crystal for another and you’re set.

Only [Bill] didn’t have the 21 MHz crystal that he needed. No problem, because he had a junk crystal, a hacksaw, and a modern direct-digital synthesis (DDS) chip sitting around. So he takes the donor crystal, cuts it open, and solders the two wires directly from the DDS to the crystal’s pins. Now he’s got a plug-in replacement digital oscillator that doesn’t require modifying the nice old Drake receiver at all. A sweet little trick.

The video’s a little bit long, but the money shot comes in around 5:00.

Now, one might worry about simply plugging a powered circuit (the DDS) in place of a passive element (the crystal), but it seems to work and the proof of the pudding is in the tasting. We wonder how far this digitally-controlled-analog-receiver idea could be extended.

11 thoughts on “Digital “Crystal” Breathes New Life Into Old Radio

    1. It isn’t a new idea. Still a nice implementation though. Other amateur radio operators have been building similar devices for years now. e.g. http://www.n4yg.com/downloads/DDSQST.pdf

      There are limitations with DDS so it wouldn’t be suitable for everything however. A big one is are artifacts and spurs caused by DDS which require heavy filtering. I’d be wary of using DDS for a transmitter unless it was designed very well and tested thoroughly. Read http://www.analog.com/static/imported-files/application_notes/131351807AN_927.pdf for more information about DDS artifacts and spurs.

  1. >>Now, one might worry about simply plugging a powered circuit (the DDS) in place of a passive element (the crystal)

    Hams have been doing this to convert old crystal based, chanelized comercial equipment into ham gear for decades. The oscillators commonly used to be big boxes that bolted to the side of the radio. They were PLLs with thumbwheel switches to set the frequency and PIN diode matrixes to set the IF offset. Now we have cheap, easy DDS. :-)

    1. Heck, hams have been doing this since a time when there were no semiconductors in the ham shack, building outboard vacuum tube variable oscillators and plugging their output into the crystal connector, changing frequency by turning a bigass variable capacitor.

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