Ham Radio News: Adding a Beat Frequency Oscillator

If you're going to use single sideband mode with an older ham radio rig, adding a beat frequency oscillator will better enable you to tune in voice communications.

| September/October 1979

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    If you're interested in soldering one together yourself, a beat frequency oscillator looks something like this.
  • copthorne-macdonald-1980.jpg
    Copthorne Macdonald is a ham radio enthusiast, the inventor of slow scan television, and the founder of New Directions Radio.

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  • copthorne-macdonald-1980.jpg

It used to be that any old shortwave receiver could be used to eavesdrop on ham radio activities as long as the unit covered reasonable chunks of the 3 to 30 mHz frequency range. Then—in the 1950's—amateur radio operators switched their voice transmission from AM to the more efficient single sideband (SSB) mode.

You see, in AM voice transmission a constant frequency carrier signal is transmitted along with the sidebands that "contain" the actual voice information. When the signal reaches a receiver, this carrier helps to demodulate (or make sense of) the voice sidebands.

In the SSB mode, on the other hand, no carrier at all is transmitted. This allows the radio builder to use less transmitter power, but also requires that an artificial carrier be generated within the receiver. The circuit that produces the receiver carrier signal is called a Beat Frequency Oscillator, or BFO (such units are also used to allow Morse code signals to be heard as on-off tones rather than a starting and stopping rush of noise).

Some modern shortwave receivers—such as the Sony ICF-5900W—come equipped with a BFO, but many broadcast-type receivers don't have this feature. Fortunately, it's easy to add BFO to such a set. Since most post-1930 receivers use 455kHz IF amplifier stages, you can simply generate a 455-kHz signal and couple it into the receiver's IF amplifier.

Furthermore, if you use a crystal-controlled oscillator to produce the signal, you can be sure the frequency will be very close to the necessary 455 kHz and you won't have to tune or calibrate the oscillator. Let me describe how just such a BFO circuit can be built by anyone with a little electronic experience.

First of all, you'll have to locate a 1 1/2" x 1 1/2" one-stage circuit board kit. I bought mine (a Model OF-1 LO Oscillator, catalog number 035108) from International Crystal Manufacturing Company for $4.48, postpaid. In addition to the kit, you'll need the following:

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