It is easy to add CW and SSB capability to a low cost shortwave radio.When you do, you’ll be able to hear not only CW transmissions in morse code, but also single-sideband (SSB) voicecommunications from hams, aircraft pilots, and the military.
That is accomplished by adding a beat-frequency oscillator (BFO) to the radio circuit; the output of the BFO mixes with the incoming signal.The circuit, which we will henceforth refer to as the CW/SSB Adaptor, replaces the missing carrier in an SSB signal, and it converts a CW signal into an audible tone.
The CW/SSB Adaptor can be used with a portable transistor receiver or the car radio that you use with our High Performance Shortwave Converter (Popular Electronics, October 1989).
All that is required is that the receiver have a 455-kHz intermediate frequency (IF).Practically all do, but it’s a good ideal to check the IF before modifying the receiver.
Parts List:R1 = 1Meg R2 = 2K7C1 = 39pFQ1 = 2N2222T1 = 455KHz IFS1 = on/off switch A complete KIT will be available soon.
How it Works:
A schematic diagram of the CW/SSB Adaptor is shown in Fig. 1.
The circuit–consisting of a 455-kHz IF transformer, a 2N2222 general-purpose transistor, and three support components, is little more than a simple Hartley-type, beat-frequency oscillator (BFO) designed to put out a clean 455-kHz sinewave.
For the technically minded, here’s how it works.Suppose power has just been turned on. Resistor R1 feeds a small amount of current into the base of Q1, which makes Q1 conduct.With Q1 conducting, a surge of current is sent from the emitter of Q1 through T1.The primary of T1 has a capacitor across it, forming a tuned circuit, which immediately converts the surge of current into an oscillation, rather like the way a bell rings when hit sharply.Left alone, the oscillation would die out, but Q1 amplifies it and keeps it going.
This circuit is designed to be built from salvaged parts, so check your junkbox before shopping for parts.Transformer T1, a 455-kHz IF transformer, can be taken from any discarded AM radio. Given a choice, use the first or second IF transformer, usually color-coded yellow or white.if the radio is AM/FM, be sure you don’t get a 10.7Mhz transformer from the FM section (usually orange, blue, or green).
Transistor Q1 is likewise a junkbox item. Almost any general-purpose NPN silicon transistor will do; the 2N2222 (used in Fig. 1) and 2N3904 are good examples.A junked radio will probably contain at least one C945 (a general-purpose amplifier transistor), which will also do just fine.
The author’s prototype of the CW/SSB Adaptor was built on a small piece of perfboard, the layout of the CW/SSB Adaptor should go into a shielded cable, as shown in Fig. 1.You can get pieces of small-diameter shielded cable from junked tape-recorders, record players, or microphones.
After you have assembled the CW/SSB Adaptor, but before installing it in the radio, take a moment to adjust it.
Remove the shield (but not the insulation) fro the final inch or two of the CW/SSB Adaptor output cable and place the unshielded part near the radio’s detector diode.If you don’t know where the detector is located, just put it close to the radio’s IF transformers.
Next, turn the radio and tune in a shortwave broadcast station that has a fairly strong signal.Then apply power to the CW/SSB Adaptor.Adjust the transformer’s tuning slug until you get a nice loud whine in the receiver.If the transformer was taken fro a correctly aligned radio, you’ll probably have to turn the slug counterclockwise two or threeturns.There may be more than one position that gives a whine; select the loudest one.Don’t be disappointed if the whine is not especially loud.
How to Tune CW and SSB:
Should BFO’s be Tunable?: