home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Current Shareware 1994 January
/
SHAR194.ISO
/
textfile
/
swradiom.zip
/
XTAL.TXT
< prev
Wrap
Text File
|
1993-10-20
|
2KB
|
41 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 success.
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
oscillators and superhet radios (see AMRADIO.TXT and BIBLIO.TXT) for more
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 one of your different crystals to a DIP Header and plug
them in one at a time for each different band you want to tune. Perhaps you
can come up with an even more convenient method of installing and removing
the various crystals.
There are various sources for cheap crystals. Ham shows and computer flea
markets are best (you can obtain them there for as cheaply as 10c ea., or at
most a dollar or two). 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.