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Monster Media 1994 #1
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HAM
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SWRADIO.ZIP
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ITWORKS.HOW
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1993-12-22
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5KB
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77 lines
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HOW THE SHORTWAVE CONVERTER WORKS: A SHORT EXPLANATION
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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.
┌────────────────┐
short wave frequency ────────────> │ NE602 chip │ frequency
│ │ ─────> within AM band
(local) oscillator signal ───────> │ (mixer) │ (550 - 1600 Khz)
└────────────────┘
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.