A Simple Superhet
DIP — is the LM386N-1 low voltage
audio power amplifier. This IC is
designed for use in low voltage
consumer applications and can
provide gains up to 46 dB. This
amplifier provides sufficient output
power to drive a small speaker when
the receiver is tuned to local stations.
■ FIGURE 4. Prototype of
the Simple Superhet
receiver. Do not force the
IF transformer into the
board as the plastic case
may split. Instead, solder a
short wire extension onto
each pin to insert into the
corresponding hole to
complete the connection.
Now that we’ve introduced the
three ICs, let’s return to Figure 3 and
see how to combine them to form the
Simple Superhet. The primary of the
ferrite antenna loopstick, L1, and the
variable capacitor, C1, form a parallel
tank circuit that “preselects” the
desired signal, while attenuating any
image signal that might also be present.
The antenna loopstick also converts the
electromagnetic field of incoming radio
waves into a small RF voltage that is
applied through the loopstick’s secondary winding to the input of the double-balanced mixer, pins 1 and 2 of U1.
Variable capacitor C2, transformer
T1 (red can), and capacitors C3–C5,
along with the oscillator transistor
internal to the SA602 form a Colpitts
oscillator that serves as a tuneable LO
for the Simple Superhet. The Colpitts
oscillator creates oscillations by
feeding back the output signal from
the emitter of the oscillator transistor
(pin 7) to the base of the oscillator
transistor (pin 6) through the capacitive
voltage divider formed by C3 and C4.
Note that the primary of the red
can (side with three pins) is connected
to C2, while the secondary (side with
two pins) is connected to C3 and C4.
R1 and C6 form a decoupling network
that keeps RF and LO signals off of the
power supply line, while also limiting
the supply voltage for the SA602 to less
than eight volts as required for this IC.
The output of the SA602 is the desired
IF signal which appears as a balanced
output signal across pins 4 and 5.
Transformer T2 (yellow can) and
the MK484 IC form the heart of the IF
amplifier. It is important to observe that
in this application the IF transformer is
turned around “backwards” in order to
convert the balanced output of the
mixer into a high impedance, singled-ended output
to drive the MK484. Note
that in this case the primary
of the yellow can (side with
three pins) is connected to pin 2 of the
MK484, while the secondary (side with
two pins) is connected between pins 4
and 5 of the SA602. The single IF
filter is extremely selective because
the equivalent load resistance on the
primary of T2 is very large.
Continuing with the operation of the
MK484, resistors R3 and R4 form a voltage divider that reduces the 9V supply
voltage to approximately 1.6V necessary
to power the IC. Resistor R2 and capacitor C7 provide decoupled bias to the IC’s
input, while capacitor C8 shorts any RF
present at the IC’s output to ground.
The operation of the LM386
audio amplifier is very straightforward.
Potentiometer R5 attenuates the
audio signal to provide the user with a
volume control. Capacitor C10
maximizes the amplifier’s audio gain,
capacitor C11 decouples the supply
voltage, and capacitor C12 blocks DC
current from the speaker coil.
A good strategy for receiver
construction is to start at the speaker
and build towards the antenna. This
allows the builder to aurally verify
circuit operation as work progresses.
Additionally, a very fast way to
construct this circuit is by plugging the
components into a proto board as
shown in Figure 4. Once circuit operation is verified, it is straightforward to
construct the circuit on something
more permanent, such as a PC board
construction shown in Figure 5.
Begin constructing the audio
amplifier by wiring in the LM386 IC,
speaker, R5, and C9–C12. Apply
power to the IC along with an audio
tone from a signal generator to the
free end of C9. You should be able to
hear a healthy tone at the speaker
with volume adjustment provided by
R5. Next add the MK484, R2–R4,
C7–C8, and the yellow IF can.
Connect a low level 455 kHz tone
modulated signal to the secondary of
the IF can, apply power to the circuit,
and the demodulated tone should be
plainly audible on the speaker.
Adjust the screw on the IF transformer to obtain maximum volume
with the modulated 455 kHz signal.
If you don’t have a 455 kHz signal
■ FIGURE 5. An example of a PCB
implementation of the superhet
receiver with a 3-1/2” speaker used as
the base. The PCB is supported above
the speaker by 3” long wooden
dowels. The loopstick antenna (not
visible) is mounted underneath the
PCB using a small metal bracket.
March 2008 45