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AMATEUR RADIO By GARRY CHATT, VK2YBX Build this simple adaptor to receive SSB on shortwave AM receivers This project is about as simple as they come. It's a 455kHz oscillator that will allow you to receive SSB signals on your AM-only shortwave receiver. +5V OUTPUT 1M 100pF J 100pFl Fig.1: a ceramic resonator is used in a CMOS oscillator circuit to give a 455kHz source. build and will convert your AM shortwave receiver to SSB reception. AM & SSB signals The parts for the 455kHz oscillator can be mounted on a small piece of Veroboard. To demodulate an SSB signal, you simply wrap the output lead several times around the receiver and tune for best quality audio. Most experienced amateurs know that SSB (single sideband) reception requires a receiver fitted with a "product detector" or BFO (beat frequency oscillator) to reinsert the missing carrier. They also know that such receivers command significantly higher prices than AM only models. It's also a fact that signals broadcast using SSB tend to be far more 70 SILICON CHIP interesting to amateurs and shortwave listeners than the standard HF broadcasts, such as Radio Australia, the BBC and the VOA (Voice of America). The problem is, most readers only have a standard AM shortwave receiver - one that's incapable of SSB reception. This simple circuit solves that problem. It uses only a handful of parts, will cost you less than $5 to To understand how the adaptor works, it is necessary to explain the basic differences between AM and SSB transmissions. A modulated AM signal consists of a carrier and two symmetrically spaced sidebands (see Fig.2). As can be seen from Fig.3, the amplitude of the carrier is a function of the amplitude of the modulating signal. The two sidebands on either side of the carrier have the same amplitude (Figs.2 & 4) and carry the same information. This is why AM is also sometimes referred to as double sideband, or DSB for short. In fact, the carrier itself conveys no information. In mathematical terms: AM power = carrier power + sideband power = Pre + P(fc + fmJ + P(fc - fmJ· Fig.2: the amplitude of an AM carrier is a function of the amplitude of the modulating signal. Fig.5: you can use an oversize drill to cut the tracks on the Verohoard. How it works AMPLITUDE le-Im le le+lm FREQUENCY Fig.3: a modulated AM signal consists of a carrier (fc) and two symmetrically spaced sidehands (fc - fm and fc + fm). AMPLITUDE The heart of the circuit is a Murata "Ceralock" ceramic resonator. This provides a reliable 455kHz source at a fraction of the cost of a crystal. The resonator is used in the CMOS oscillator circuit shown in Fig.1 and provides an RF output level of 5V peak-to- peak. The output of the oscillator is then fed to a length of insulated hookup wire which is wrapped several times around the receiver, thus providing a degree of inductive coupling. The amount of signal required varies from receiver to receiver and can be adjusted by varying the number of turns. Construction (fe-lmmaxJ (fe-lmmin) le (fe+lmmin) (fe+lmmaxJ FREQUENCY Fig.4: the two sidebands on either side of the carrier have the same amplitude and carry the same information. By removing one sideband and the carrier, more efficient use of the available transmitter power is made without sacrificing the transfer of information from transmitter to receiver. In fact, an SSB transmitter uses approximately one quarter the power of an equivalent AM transmitter. Another advantage of SSB operation is that the bandwidth required to transmit the signal is significantly reduced. So, by removing the carrier and one sideband, we can make more efficient use of the RF spectrum. Receiving SSB To demodulate an SSB signal (ie, to turn it back into intelligible speech), it is necessary to insert a locally generated carrier at the receiver. This carrier can be generated by using an external oscillator tuned to the receiver's intermediate frequency (IF) - in this case, 455kHz. The new carrier provides a reference frequency against which the upper or lower sideband can be demodulated. Note that the opposite sideband is also generated during this process, so that a complete AM signal is available for detection. OK, so that's how we convert an inexpensive shortwave receiver to SSB operation. It's an arrangement that can be made to work quite well but don't expect it to perform as well as a fully-fledged SSB receiver with narrowband IF stages and special audio filters. The unit can easily be made up on a small piece of Veroboard. Fig.5 shows the wiring details. You can make the cuts in the tracks using an oversize drill. Note that all unused input and output pins on the 4069 are grounded to prevent spurious oscillation. The recommended maximum supply voltage when using the CSB455E resonator is 5 volts. If portable operation is contemplated, the unit could be powered from a 9V battery via a 5V 3-terminal regulator. Because the ceramic resonator operates at a fixed frequency, no tuning of the circuit is required. To demodulate an SSB signal, first tune your AM radio to the SSB signal (it will sound very distorted). You can then wrap the oscillator output lead around the receiver, apply 5V, and carefully tune your receiver for best quality demodulated audio. Footnote: the CS455E ceramic resonator is available from Dauner Electronics, 51 Georges Crescent, Georges Hall 2198 . Phone (02) 724 6982. DECEMBER1988 71