Signal Frequency Beat Frequency Oscillator

 

The days when you could listen to amateurs on a simple shortwave AM receiver are with us again, thanks to the development of this one-transistor, frequency-agile signal frequency beat frequency oscillator. When teamed up with a low-cost AM set covering 3.5 and 7 MHz, this device provides effective reception of local eighty and forty metre SSB transmissions.

It is an ideal project for the aspiring amateur, as it allows them to monitor amateur activity. Its usefulness, low cost, and ease of construction would make it a good group project for schools, radio clubs or amateur theory classes.

The device is a miniature transmitter. It provides a steady carrier signal to the receiver to replace that suppressed within the transmitter (refer to any radio theory book for a more detailed explanation). It is the ultimate in simplicity, employing but eight components. The unit costs approximately ten dollars to build from all-new parts, and requires no alignment or connections to the receiver. Anyone with basic soldering skills can construct this project, and have it working first time.

Though receivers covering the short wave bands are no longer in every home, suitable sets can be picked up cheaply at garage sales and swap meets. Tuning the medium wave and one or two short wave bands, their performance is lacking in many respects. Nevertheless, they work better than might be expected when used with this circuit. The reasons for this are given later.

This unit is a one transistor 3.5 MHz RF oscillator whose frequency can be varied. As mentioned before, it replaces the carrier in the receiver that was suppressed during the transmitter's SSB generation process.

A 3.58 MHz ceramic resonator sets the oscillator frequency. This two-dollar component is similar to a crystal. Its main advantage is that it can be shifted over a 100 kHz frequency range by connecting a variable capacitor in series with it. While the frequency stability is somewhat inferior to that of a crystal, it is still acceptable for stable SSB reception.

Because the BFO operates directly on the received frequency, many of the limitations of low cost AM receivers (such as frequency drift, coarse frequency readout, hand-capacity and difficulty of tuning) are either eliminated or made less apparent. This is because the tuning in of SSB transmissions is effectively performed by a stable, easy to tune BFO, rather than the unstable free-running coarse-tuning local oscillator within the receiver. The latter would have been the case had a conventional 455kHz fixed-frequency BFO been employed.

The circuit shown (see below) covers the popular 3.525 - 3.625 MHz frequency range. This permits reception of CW and SSB activity, WIA Divisional Broadcasts and Morse practice transmissions. The second harmonic of this range covers the 7.050 to 7.250 MHz segment of forty metres, while the fourth might be useable for twenty metre reception.

Construction

Virtually any construction method may be used to assemble the BFO. However, large stray capacitances must be avoided if the full tuning coverage is to be obtained. Several prototypes were built. Almost any construction technique can be used.

Full frequency coverage will only be obtained if leads are kept short. Those to the ceramic resonator and variable capacitor are particularly critical. Whereas most RF projects are built in metal cases to provide shielding, the BFO's operation depends on there being a lack of shielding between it and the receiver. Thus either a plastic or wooden box is recommended.

Testing/Operation

To verify BFO operation, your AM short wave set is required. Position the receiver near the BFO, and tune it across the 3.5 - 4 or 7 - 8 MHz frequency range. At a certain point on the dial, the receiver will go quiet; all normal background noise will be silenced. Switching off the BFO will restore the normal band noise, while adjusting the BFO's 'Tune' Control will move the 'silence' to a different frequency. If the BFO passes these two checks, you know that it works.

Now switch off the BFO, attach a piece of wire (preferably outdoors) to the receiver's telescopic antenna, and tune in a strong SSB signal for maximum volume. Assuming the received signal is within the BFO's tuning range, it will be possible to resolve the signal by correctly adjusting the BFO. Place the BFO near the receiver, and adjust the BFO's tune control until the receiver quietens. Move the BFO away from the set, and adjust it carefully until the SSB signal is intelligible. Note that this setting is critical; the BFO's frequency must be equal to that of the transmitter's suppressed carrier. While at first this process is somewhat fiddly, it becomes easier with practice. For optimum results, experiment with the physical distance between the BFO and the receiver; weaker signals require less signal from the BFO (ie a greater separation). However, it should be possible to find a compromise position for the BFO where reception from all stations is satisfactory.

A novel device to allow the reception of amateur signals on domestic AM-only short wave receivers has been described. It is cheap, very simple to build, and can be expected to work first time. It fills a definite need amongst potential amateurs, and has the advantage of being expandable to a direct conversion receiver or CW/DSB transmitter or transceiver as interest develops.