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5.5.2. Electronic Tuning Instead of the capacitor CR, that was used for fine tuning in the
previous project, a capacitive (varicap) diode can be used. It’s a special
HF diode which is polarized by exposing it to DC voltage in order to be
non-permeable (+ to the anode, - to cathode). By changing the voltage
diode’s capacitance also changes, which allows for it to be utilized as
variable capacitor. If, acc. to pic.5.13-a, the DC voltage between the
cathode and anode (UAK) varies from U1 to U2, diode’s capacitance goes from
Cmax till Cmin.
components. Their mishap is they are hard to purchase, they are quite robust
(compared to other device components), and their mounting isn’t simple
because the shaft for the knob must go through the front plate of the device
box. That is why varicap diodes are also replacing them. With the diode that
has Cmax/Cmin ratio that is big enough, say, Cmax/Cmin>15, the circuit form
pic.5.13 can be used as the variable capacitor (C is simply omitted). In
that case, some bigger knob with an arrow is mounted on the P1 handle, and
numbers from 1 to 10 are written on the panel, as shown on pic.5.13. This
scale allows the listeners to see what station is the receiver tuned at. Of
course, for the MW band, the numbers as those on pic.3.7 can also be
written. 5.5.3. Suppressing the Signal of the Local Transmitter From all the signals in the reception antenna, the one that is created by the local transmitter is by far the strongest one, due to the fact that it is hundreds, sometimes even thousands times closer than other radio transmitters. That signal can be so strong that it can jam normal reception of other stations. In case of simpler receivers its programme is heard,more or less, in all the positions of the variable capacitor. The solution for this problem is the so-called seal circuit, which serves to weaken the signal of the local transmitter, so that it doesn’t interfere (but is still strong enough for normal reception, when the receiver is tuned at it).
The seal circuit is a parallel oscillatory circuit which comprises the coil
L1 and capacitor C1, as shown on pic.5.14-a. By means of C1 the resonance
frequency of the circuit is set so that it corresponds to the carrier
frequency of the local station. On that frequency, this circuit behaves as a
huge resistor (see pic.3.2-b) and decreases the current that is created by
the local transmitter signal. For other signals it has very small resistance
and practically has no effect on them. The setup is done by tuning the
receiver on the local station, and the reception is weakened enough by
turning the C1. If the decay is too strong, a resistor should be added in
parallel to C1. Using a variable capacitor in the seal circuit (pic.5.14-a) isn’t an economical solution. It is much better, considering both economy and space, the solution given on pic.5.14-b. A block capacitor C1 and a variable inductance coil are used in the seal circuit. As shown on the framed part of picture, the coil is wound on the plastic body, with ferrite core. The number of quirks is found experimentally about couple of hundreds of quirks made with as thin copper wire as possible). The capacitance for C1 is also found experimentally (couple of hundreds of pF). The earlier mentioned IF transformer can also be used as a coil. With labelling acc. to pic.4.3-a, legs No.2 and 3 are used, the others are “hanging” (they are not soldered). C1 capacitance is also found experimentally. It is also possible to wind the coil on a piece of ferrite rod, as shown on pic.5.14-b, and setup to be done with trimmer Ct 5.5.4. Dual Tuning The author of this book, as great radio techniqe lover (amateur, in
French), owns great collection of over 150 pieces of various old-timer radio
receivers. There is one among them that is over 60 years old, at which the
tuning is being done by two knobs. With first one the receiver is set
roughly to the desired station, which is usually barely heard at that
moment. The second knob is then turned until the optimum reception is
achieved, which is significantly better than before, and in case of weak
stations - extremely better.
* Between the coils L1 and L a magnetic coupling should be prevented. This
is accomplished by mounting the coils to be as far from each other as
possible, and to position their axes mutually perpendicular. * Greater experimenting opportunities with dual tuning provides the diagram on pic.5.15-b. Once again, it’s the serial resonance (in circuit L1, C1), and parallel resonance (in circuit L, C), that are being used. The coils are placed side-by-side, in order to generate magnetic coupling between them. The tuning is done as previously explained, but now we also have a possibility of changing the amount of magnetic coupling between the coils by moving them closer or farther, which affects the antenna’s influence on the L, C oscillatory circuit, therefore changing its selectivity and sensitivity. 5.5.5. Separation of Stages - Preventing the Oscillation On of the significant problems that occur
at devices that comprise more cascade-linked amplifying stages is the
occurrence of the feedback over the conductors that connect those stages
with the positive pole of the battery, or the power supply. By the way, the
feedback is a phenomenon when part of the signal exiting an amplifier gets
on its input. Under certain conditions, this feedback causes the oscillation
of the stage, which in devices that have the loudspeaker on output,
manifests itself as strong whistling, squeaking and similar.
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