Michael J. Rainey: July 2011

Posted on Feb 4, 2014

Some months ago a surprise package turned up in my mailbox. Ananonymous donor had sent me threewonderful, old germanium transistors. The2N170 and a pair of 2N107`s (all date-coded December 1956) werestill in theiroriginal plastic tube packaging. Athin booklet, Type 2N107 Transistor Circuits for Experimenters [and] Amateurs, was also included. Thecir

Michael J. Rainey: July 2011
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cuits shown in the booklet appear to be aimed at the rank beginner:audio amplifiers, code practice oscillators and the like. However, the "SimpleRadioReceiver" caughtmy eye (lower right-hand corner below). This was exactly the sort of project that captivated me in my early youth. Growing up inSt. Louis, Missouri in the late 1960`s, Ienjoyed listening to baseball and ice hockey games on KMOX, and pop music on KXOK, using a wide variety ofsimplehomemade AM radio receivers. Although there`s nothing much ofinterest to me on the broadcast AM radio dial these days, Ihave a nostaligia forthe simple circuits thatmarked my first stepsin amateur radio. By the way, this same circuit also appeared in the first (1957) through the seventh (1964) editions of the General Electric Transistor Manual. While I`m not now interested in listening to broadcast AM radio, a direct-conversion receiver suitable for copying CW only requires the addition of a beat-frequency oscillator (BFO) to the above receiver circuit. With sufficient BFO drivethegermanium diodecan be made to function as acommutating (switching) mixer. On the other hand, a single diodemixerhas several notable disadvantages; perhaps the worstis therelative lack ofAM signal rejection. This type of mixer alsocreates more spuriousfrequency energy and the port-to-port isolation isquite inferior as compared to more advanced designs. Nevertheless, the more I thought about it the keener I...

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