A Simple Superhet
loop of wire. Very slowly slide the ferrite
bar in and out of the loopstick antenna
until the loudest audio signal is
produced. Then secure the ferrite bar in
place with a small wedge of paper.
Retune the radio to a station (or the
signal generator, if used) near the upper
end of the AM band. This time, very carefully adjust trimmer capacitor C1a until
the loudest audio is produced. Repeat
this procedure, adjusting the ferrite bar at
the low frequency end and the trimmer
capacitor at the high frequency end, to
maximize volume for stations located at
both ends of the AM band. This
completes construction, testing, and
alignment of the Simple Superhet receiver.
Here are some other things you
can try on your own:
• Try increasing or decreasing the number of turns on the secondary of the
ferrite antenna bar to improve receiver
sensitivity. If your loopstick antenna did
not come with a secondary coil, you
can wind your own secondary coil
using about a dozen turns of fine magnet wire over the existing primary turns.
• Substitute the “white can” or “black
can” for the yellow can IF transformer
to see how sensitivity and/or selectivity change.
• Increase or decrease R3 one or two
standard resistor values to see how it
affects the gain and stability of the
MK484 IF amplifier. (Note: Keep the
voltage at pin 3 of the MK484 below
1.8V to avoid destroying the IC!)
• Advanced experimenters may modify
the basic design to build a superhet
receiver for other interesting frequency
bands such as Citizen’s Band or WWV.
Proceed by constructing a crystal oscilla-
tor or Phase Locked Loop (PLL) frequency synthesizer for the LO, as well as modifying the preselector to properly receive
signals in the desired frequency range.
I hope you have as much fun
building, using, and modifying your
Simple Superhet receiver as I have had
with mine! You may contact me with
any questions or comments at john.
ICs, loopstick antenna, and tuning
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March 2008 47