ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ HOW THE SHORTWAVE CONVERTER WORKS: A SHORT EXPLANATION ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ How can as simple a circuit as the SW converter described in the enclosed files make a common AM radio into a shortwave receiver? It seems almost magical that a bare handful of parts, maybe $5 worth, can accomplish this. The following brief discussion will clear up some of the mystery and hopefully inspire the reader to further experimentation. Let us begin with the antenna. A radio antenna in its simplest form is just a few feet of wire, almost any type of wire. The wire intercepts radio signals of ALL frequencies, from the very long waves generated by natural phenomena, such as lightning, to the far microwave spectrum and beyond. Of course the signals picked up by the antenna wire are very weak, and it is the radio's amplifier sections that strengthen them enough to drive the audio section and finally the speaker, and the tuning sections that select them. You might well ask why the AM radio does not receive all the signals being picked up by the antenna. If international short wave signals are arriving at the antenna, why can't you TUNE them in? Tune is the magic word. The internal circuits of the AM radio select only the signals in the AM band and reject all the others. So, is there any hope at all of trying to receive short wave stations on an ordinary AM radio? It would seem not, but stay tuned... The SW converter connects between the antenna wire and your AM radio. The short wave signals are frequency-shifted downward into the AM band by the converter before being fed into the radio. The radio, of course, tunes and amplifies both "true" AM signals and down-shifted shortwave ones. But wait, would not the local stations on AM interfere with the short wave stations? Fortunately, the SW converter has also amplified the short wave signals enough to drown out all except the strongest local stations. So you will tune short wave stations on your AM dial, mixed in with only a very few local AM stations. The SW converter functions as a "down-converter", an idea long familiar to Ham Radio operators and some electronics hobbyists. The Signetics NE602 chip is what makes it possible to cheaply and easily build the SW converter. It is a "double-balanced mixer", in other words, it mixes two signals and outputs ONLY the sum and difference frequencies of the input signals. In the case of the SW converter, the one input signal is a short wave frequency and the other an oscillator signal, that when mixed together within the innards of the NE602 produce a signal in the AM band. The NE602 also amplifies the output signal power by a factor of 8 (18 dB). As an example, suppose you wish to listen to an international short wave station in the 49 meter band, at about 6 Mhz. You would then plug a 5 Mhz crystal into the converter. The "local" oscillator input to the NE602 is set to 5 Mhz by the crystal. The desired station(s) at about 6 Mhz come from the antenna (remember, the antenna picks up all frequencies). Both frequencies go to the NE602, which "mixes" them together. The "mixing" consists of both adding and subtracting the two frequencies. The resulting frequencies output from the NE602 are 6 Mhz - 5 Mhz = 1 Mhz, or 1000 Khz, tunable on the AM dial at about 1000, and 6 Mhz + 5 Mhz = 11 Mhz, which is unused in this case. The station you will tune at 1000 on your dial is therefore a 6 Mhz short wave station. ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ short wave frequency ÄÄÄÄÄÄÄÄÄÄÄÄ> ³ NE602 chip ³ frequency ³ ³ ÄÄÄÄÄ> within AM band (local) oscillator signal ÄÄÄÄÄÄÄ> ³ (mixer) ³ (550 - 1600 Khz) ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ The 10.7 Mhz mini transformer, the crystal, and sometimes a capacitor make up the oscillator (called the local oscillator) that produces the signal to be mixed with the short wave signal. The only signals coming out of the NE602 are shortwave signals shifted into the AM band, which the radio very nicely amplifies and outputs through its speaker. For more detailed discussions of the theory of operation of the NE602, of oscillators, of radio and electronics theory in general, see the enclosed file BIBLIO.TXT for further reading.