3.10. Miniature Receiver with ZN414 (ZN414Z) IC
The receiver from Pic.3.25 can be made portable (without an external
antenna) if ferrite antenna is used instead of the carton-body coil, right?
Well, not exactly. The ferrite antenna is a directional - type antenna,
meaning that it does matter where are the waves made by transmitter coming
from. The voltage being induced in it under the effect of EM field is the
biggest if the ferrite rod is in horizontal position, aimed in such a manner
that its axis is perpendicular to the direction of spread of the waves (i.e.
perpendicular to the direction towards the transmitter). If the rod is being
turned (in horizontal plane), the voltage decreases and gets to minimum when
the rod is turned for 90 degrees, i.e. When the rod axis is parallel to the
direction pointing towards the transmitter. That means that when the
receiver from Pic.3.25 (with the ferrite antenna) is tuned to some station,
the reception can be additionally improved by simply rotating it. The
directional feature of the antenna is useful here. This, however, can be bad
if the receiver is used in motion, since the volume would be constantly
changing, and the receiver should be constantly turned towards the
transmitter. It would clearly prove itself useful to have an electronic
circuit in the receiver that would be automatically changing the
amplification of the receiver, in such a manner that it would be increased
when the signal in the antenna weakens and vice versa. This would even be
convenient in the stationary - type receivers, since it would provide the
constant sound strength in the loudspeaker during the reception of signals
of various power. Such circuit does exist in modern receivers, and is called
the Automatic Amplification Regulation circuit (AAR).
AAR circuit could be added to some of the receivers described in previous
projects. The mishap is that electrical diagrams would then become
significantly more complex, which would reflect itself both on the price and
the realizability. The solution, however, does exist, and is rather simple:
it’s the ZN414 IC, being promoted as an “AM Receiver”. Its block diagram is
given on the Pic.3.30. The signal from the input


circuit (with ferrite antenna) is led on pin 2 of the IC, which is the
input of the amplifier (Z) that has very big input impedance (about 4
MOhms). This is very significant, since such amplifier does not load the
oscillatory circuit and doesn’t reduce its Q- factor. The signal is then
being amplified through 3-stage HF amplifier (HFA) and demodulated in the
detector, thereby exiting the IC as an LF signal (music, speech...). In the
right part of the Pic.3.30 the pin description of the ZN414 is given. As you
can see, it is placed in a housing usually used for small-power transistors,
either plastic (on top, like BC547) or metal (bottom, like BC107).
One end of the oscillatory circuit is connected to the ground over the C1
capacitor (for AC voltages), since input into the amplifier Z is between pin
2 and the ground, where the input signal is to be brought.
The automatic amplification regulation (control) is achieved by returning
the DC component of the detected signal from the output to the input of the
IC, over the R1 resistor. This DC voltage is being created on R2 resistor.
It is substantial for the correct operation of ZN414, its resistance must be
such that DC voltage on the pin 1 (to the ground) when no station is being
received, is equal to 0.9 V. While calculating the R2 one must have in mind
that the idle current of the IC goes through it, its typical value being 0.3
mA, and maximum 0.5 mA (more data about ZN414 can be found in table on
Pic.3.36).
The electrical diagram of a small portable MW radio receiver, reproduction
being done over the headphones, is given on Pic.3.31. The LF signal is led
from the ZN414 output over the coupling capacitor C3 to simple amplifier
made with BC547 transistor (or similar), which we discussed about before.
This can even be done without the LF amplifier. If you have high-resistance
headphones that are sensitive enough connect them between the right end of
C3 and the ground, and omit the transistor, R3 R4 and C4.
The 1.5V battery is being used in this device, therefore the necessary 0.6V
voltage drop is done with R2=1.5 kOhms. You should, just in case, connect
first a 5 kOhms trimmer instead of R2, put its slider in the mid position,
turn on the receiver and set it on an empty place on the scale, where no
signal can be heard. Connect the voltmeter between the pin1 and ground, and
carefully move the slider until the instrument shows 0.9V. If you have no
instrument, tune the receiver to some station and move the slider carefully
until you reach an optimum receipt. Then turn the variable capacitor’s knob
across the entire scale, to ensure that receiver is working well throughout
the entire reception range. If everything is OK turn off the receiver,
disconnect the trimmer, measure its resistance and solder an appropriate
resistor on the PCB. While experimenting with R2 please have in mind that
its resistance should be in any case no less than 600 Ohms.
On Pic.3.32 the PCB, ferrite antenna and look of entire device are shown.
If you plan to make a different PCB, since the device works on high
frequencies, you have to obey certain rules in order to have a reliable and
stable operation:
a. The splitting capacitor C2 has to be mounted as close as possible to the
pin 1 of the ZN414. Its capacitance affects both the amplification (which
increases with increase of C2) and the limit frequency of the LF signal
(which decreases with increase of C2), so the compromise has to be found.
You may put for start C2=82 nF (or even 100 nF), and if the reproduction
quality pleases you - everything is in order. You could try with smaller
capacitance, the amplification will decrease but the reproduction will be
better, etc.
b. All the connections, especially those near the ZN414, must be kept as
short as possible.
c. The ferrite antenna and variable capacitor should be placed as far away
from the battery, loudspeaker (if existing) and the cables connecting them
to the PCB.
d. The rotor (G- leg) of the variable capacitor must be connected with the
junction of R1 and C1.
Regarding the ferrite antenna, the best thing would be using some that is
retrieved from some disused conventional receiver, more on this was told in
the project No.3.8. If you can’t find one, or it is unsuitable for some
reason, you can make it according to Pic.3.32-c. The length of the ferrite
rod is 42 mm. If you have a longer rod, cut it down to size. This cannot be
done with the saw, but a groove must be made with the rasp all around, after
which the rod can be simply broken in two. The coil body is, again, made of
paper tape that is spooled and glued onto the rod. Before you start with
spooling, several pieces of 0.5 mm wire (3 on the picture) should be
inserted between the rod and the paper. The coil has got 80 quirks of
lacquer -isolated copper wire, its diameter being app. 0.2 mm. The beginning
and the end of the coil are fixed with the scotch tape (the starting quirks
are pressed on the coil body and fixed with several reels of 3 mm tape. The
same is done with the ending ones). When the body is finished, the wires are
removed. It is thus achieved that the coil body doesn’t lay firmly on the
rod, which can now be moved side-to-side, changing thereby the inductance of
the coil, so that its optimum value can be established. If you still decide not to use the ferrite antenna, you can use our coil
from Pic.3.6. In that case, the leg should be kept “in air”, i.e. it is not
used.

* The battery can be connected to the PCB with two pieces of wire that are
soldered to it. This solution is fine if you are skilled in soldering and
can easily un-solder the old battery and attach the new one. But if you
intend to give the receiver to someone, and he/she is not a soldering-lover,
you’ll have to find another solution. The simplest thing to do is take the
battery housing from an old receiver, do the necessary adjustments and
connect it with the PCB with two pieces of flexible (litz) wire. If you
cannot do the former, make two battery platforms of brass, as shown on
Pic.3.33 and solder them on two copper areas on PCB that are big enough to
support them. If you accept this solution, your PCB must be bigger (The
additional part is shown in dashed line, on Pic.3.32-a). The board now also
contains the holes for the screws, which are fixing it onto the device box.
On the platform that supports negative (-) battery pole, a small spring can
be attached, to provide a good contact. If you don’t have such a spring,
bend the platforms inwards a little, to keep the battery firmly in place.
If you are using a power source whose voltage is greater than 1.5 V, the R2
resistance should be increased. The exact value for it is best to find as
previously described, by using the 50 kOhms trimmer. Even better solution is
using one of the circuits from the Pic.3.34. Which one should it be? The one
on the Pic.3.34-d gives the best operating performance. The setting is done
with the TP trimmer. The slider is put in the lowest position, and then is
slowly moved upwards until the voltage on the pin 1 doesn’t reach the level
required. However, this circuit applies a big load onto the battery, surging
from it the current I=(9V-3V)/680Ohm=8.8 mA.


3.11. Pocket Receiver with ZN414 & LM386 IC’s
The author was taking notes in his lab while testing the receivers being
described herein, and used them later to write this book. One remark about
the receiver whose electrical diagram is given on Pic.3.35 was: “Works
EXCELLENT”. The reader will probably ask himself: Isn’t this the
aforementioned “The Best Receiver”? No, it isn’t, just remember: DE
GUSTIBUS... If, however, you consider it to be “The One”, please send your
vote on E-mail: tesla@drenik.net;
This device is very similar to the one being described in the previous
project. The most important difference is that the LF signal exiting the
ZN414 does not go to the transistor amplifier but to the power amplifier
built around the LM386, which was used in some previous projects. Any other
audio amplifier can be used instead, e.g. those on Pics. 3.15, 3.21 and
3.22.
With this receiver, special care should be taken regarding the voltage on
pin No.1. As you did in previous project, put the potentiometer’s slider in
mid-position, turn the receiver on and tune it to some station. Move the
slider carefully, until you reach the optimum reception. Start changing the
capacitance of the variable capacitor, covering its entire scope, to make
sure that receiver works well in its entire operating range. If a problem
occurs, re-position the slider again. When everything gets OK, turn the
receiver off, disconnect the potentiometer, measure its resistance, and
solder the resistor of such resistance on the board. The R2 resistance must
be no less than 600 Ohms.

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