■ FIGURE 3.
Schematic diagram of
the Simple Superhet AM
broadcast band receiver.
in the TRF are required to separate
signals as close as 10 kHz across the
entire frequency range of the receiver.
5 100 μF
C9 10 μF R5 10K
Contained inside T2
12 pF trimmer
10 – 60 pF
15 – 160 pF
12 pF trimmer
Now that we understand the
basic operation of Armstrong’s superhet receiver, we are ready to build a
simple radio that incorporates all of
these concepts. The schematic of a
receiver I call the Simple Superhet is
shown in Figure 3. I chose this name
because I believe the circuit is just
about the simplest, fully functional
superheterodyne receiver one can
construct with just a handful of parts.
Many electronics experimenters
will be familiar with the three ICs used
in this design as they are commonly
found in many homebrew receiver
projects. Let’s take a look at them one
at a time before we see how they
work together in the Simple Superhet.
The SA602AN (which is a pin-for-pin equivalent to the NE602N
originally produced by Signetics) is an
eight-lead dual in-line package (DIP)
intended for low-power, high performance communications systems. It
contains an onboard oscillator transistor that requires only a few passive
components to implement the LO
function. The IC also contains a double-balanced mixer that produces the IF
output by combining the internally
generated LO with the input RF signal.
Amplification at the IF and detection of the audio signal is handled by
the MK484 (originally produced as the
ZN414Z by GEC Plessey). This IC contains a 10 transistor TRF receiver circuit
packaged in a three-pin TO- 92 package.
The MK484 implements a TRF receiver
by cascading three high-gain RF amplifiers followed by a transistor detector.
This IC provides a very high
power gain of 72 dB using a supply
voltage of only about 1.5V! Although
the IC functions from 150 kHz to
3,000 kHz, manufacturer’s performance curves show that maximum gain
for small input signals occurs very
near the 455 kHz IF — a perfect match
for the Simple Superhet!
The final IC — also an eight-lead