Dick Smith K-6000 Assembly Manual

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Assembly Manual
K-6000
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eproduced in part by arrangement with Silicon Chip from
their March 1989 edition.
L CTRONICS
KI T
eed a spare VHF receiver for monitoring the local
repeater? How about a dedicated unit for fox ­
hunting, or a radio direction finding receiver? If the
answer to any of these question is yes, then this may
be the project for you. A few hours of construction
time is all it takes to build this little unit.
The heart of the receiver is a new
integrated circuit from Motorola,
the MC3362. Unlike many previous
" single chip receivers " (such as the
CA3089), this new integrated cir ­
cuit provides all receiver functions
from the antenna input to the audio
preamp output. It is a low power
dual conversion design with low
power drain, excellent sensitivity,
and good image rejection in narrow
band voice and data link appli ­
cations.
Dual conversion
A dual conversion receiver is an
extension of the basic super ­
heterodyne principle. A normal
superhet has one local oscillator
which is " heterodyned " or mixed
with the incoming signal to produce
an intermediate frequency which is
then amplified and demodulated.
This is referred to as single conver ­
sion and the intermediate frequen ­
cy is typically at 10.7MHz for FM
receivers or 455kHz (or 450kHz) for
most AM receivers.
By contrast, a dual conversion
receiver has two local oscillators.
The first one beats with the incom ­
ing signal to produce an in ­
termediate frequency (IF) of
10.7MHz. This is amplified in the
first IF stage and then mixed with
the second local oscillator which
operates at 10.245MHz. This pro ­
duces the second intermediate fre ­
quency of 455kHz (ie, 10.7MHz -
10.245MHz = 455kHz).
Dual conversion receivers with
this arrangement are commonly us­
ed for narrow band FM reception.
By " narrowband " we are referring
to the fact that normal broadcast
FM has a deviation of + 75kHz and
a
total channel bandwidth of
150kHz. This is wideband FM.
For the amateur bands and com ­
mercial use, narrowband FM nor ­
mally refers to a much smaller fre ­
quency deviation, such as + 5kHz.
This can vary though, for amateur
operations, depending on which
band is in use. This will be the sub ­
ject of a future article.
Using the chip
Fig.l shows a typical application
of the MC3362 in a circuit from
Motorola ' s application literature. It
shows the incoming signal fed from
The parts are all mounted off a small double-side printed circuit board. Note that where the ground plane comes right
up to the edge of a hole, the component lead must be soldered on both sides of the board.
Text and Illustrations courtesy of Silicon Chip