Crystal Radio

Posted on Feb 7, 2014

My original BFO, a small EICO 330 solid state signal generator, served well to bring in a variety of distant LW beacons. In late-2004/early-2005, a new One-Active Device (1-AD) contest to complement the Crystal Set DX contest was announced. Steve McDonald suggested that I should try applying the BFO idea to BCB reception in the 1-AD contest. Beca

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use of the 1-AD Contest limit of one active device 1 tube or 1 transistor - I planned to replace the EICO with a simple, one-JFET Hartley BFO as "proof-of-concept" and then to build from there. As it turned out, there was little need to go beyond this basic one-JFET set. The BFO is connected to the crystal set by attaching a clip lead to the OUT post and loosely wrapping the lead around the crystal set`s antenna lead-in wire. The BFO and crystal set grounds are not connected. The coil and tuning capacitor should be high Q to keep the oscillations sharp as possible. The BFO is very frequency-stable in the short term in spite of there being no supply voltage regulation provisions. I planned to address voltage regulation in Phase II, but never got that far. `Warm-up time` is 10 to 15 seconds. I can tune in a station, turn off the BFO for an hour or so, then turn the set back on to hear a high-pitched whistle descend exactly to zero beat in a matter of seconds. One thing working for frequency stability is the low current draw, only about 0. 3 mA. At left is the 1-MPF 102 JFET BFO. In center is a 1000 pF variable capacitor shunted across the BFO`s tuning capacitor to get to low frequencies. (The BFO was originally designed to cover the BCB. ) The two-dial unit to the right is a double-tuned LF crystal set. Also used but not shown are sound powered phones and matching transformer. A clip lead from the BFO output is wrapped around...

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