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Monster Media 1994 #1
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XTAL.TXT
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1993-12-22
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43 lines
╒══════════════╕
│ CRYSTALS │
╘══════════════╛
FOR YOUR SHORTWAVE CONVERTER
The original project instructions specify a 5 Mhz and/or 8 Mhz crystal be
installed in the kit at the "xtal" location on the schematic. You would need
to use one of the optional capacitors (150 pF with 5 Mhz, or 47 pF with 8
Mhz) at C1. I have tried crystals of other frequencies with varied results.
A 14.31818 Mhz crystal pulled in more signals than any others I tried (it
gives access to the 19 and 22 meter bands). 11.111 Mhz also works well.
Even a commonly available 3.579 Mhz color burst crystal works o.k. (use a 220
pF cap for C1 with this one). Every different crystal gives you a new band
approx. 1 Mhz wide. Experiment.
A crystal rated at a given frequency should give you reception on the bands
of that frequency plus and minus 455 Khz (the car radio's or AM radio's IF
frequency). Read the technical specs of the NE602 and the theory of oscil-
lators and superhet radios (see ITWORKS.HOW, AMRADIO.TXT, and BIBLIO.TXT)
for an explanation. Additional discussion of theory is obviously beyond the
scope of this brief introduction.
Crystals over 17 Mhz or so generally do not give good results. Sometimes a
high frequency crystal (50 or so Mhz) gives reception on the lesser overtone
frequencies, but this is chancy. If you have a couple of HF crystals around,
give them a try, but I do not recommend buying them for this project.
I strongly suggest mounting a 14 or 16-pin wire-wrap DIP socket where the
crystal would be, instead of hard-wiring in one particular crystal. This
way you can solder each different crystals to a DIP Header and plug them in
one at a time for every different band you want to tune. Perhaps you can
come up with an even more convenient method of installing and removing the
various crystals, such as crystal sockets.
There are various sources for cheap crystals. Ham shows and computer flea
markets are best (obtain them there for as cheaply as 10c ea., or at most a
dollar or so). Ads in electronics magazines feature them for a dollar or
two each. They can sometimes be found on old CPU boards, plug-in cards, or
other circuit boards. As a last resort, order them from Jameco, Digikey,
JDR Microdevices or even Radio Shack at $2 - $8 each.