Category : Science and Education
Archive   : SWRADIO4.ZIP
Filename : ITWORKS.HOW

HOW THE SHORTWAVE CONVERTER WORKS: A SHORT EXPLANATION
======================================================

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 give the reader ideas for further experimentation.

Let us begin with the antenna. A radio antenna in its simplest form is just
a few feet of wire, almost any kind of wire. The wire receives 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 radio's
audio section.

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 hang on.

The SW converter connects between the antenna wire and your AM radio. The
short wave signals are frequency-shifted downward into the AM band 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 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).

________________
short wave frequency ------------> | | frequency
| NE602 chip | =====> within AM band
(local) oscillator signal -------> | (mixer) | (550 - 1600 Khz)
----------------

The 10.7 Mhz mini transformer, the crystal, and maybe 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 short wave signals shifted into the AM band, which the radio very nicely
amplifies and outputs through its speaker.

For a more detailed discussion 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.