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4.2.2.1 Mini FM Receiver The electronic diagram of the monophonic FM
receiver made with TDA7088T is shown on Pic.4.12. If built with SMD
components it can be placed in a matchbox, altogether with two button-type
batteries. The operating principle of this device is given in the previous
chapter. The only thing new is a very simple audio amplifier made with BC547
transistor, which is loaded by cheap 16-Ohm headphones. The telescopic
antenna is used, as on Pic.4.8. The AFC (Automatic Frequency Control) of
the local oscillator is accomplished with BA483 diode, obtaining that
station’s position on scale does not “walk” over the scale. If higher voltage battery is used, the voltage stabilizer, comprised by the aforementioned components, has to be activated. D2 is a Zener diode with 3 V Zener voltage. The optimum value of R4 is found experimentally: in order to make the power consumption as low as possible it should have the resistance as big as possible, while simultaneously keeping the voltage on Pin 4 about 3 V and the device working well within the entire reception bandwidth (One should start with, say, R4=1.5 kOhm, and if the receiver operates well bigger resistance should be tried out, and if not smaller one, until the optimum value is found). * Pic.4.16-a shows 3x enlarged picture of the IC and the surrounding lines.
The soldering procedure for SMD is as follows:
* Any audio amplifier described so far can be used, e.g. the one with LM386, as on pic.4.8. * Instead of the antenna, a 20 cm piece of wire can also be utilized. 4.2.2.2. Stereophonic Receiver Built with TDA7088T Stereophonic radio broadcast is performed in the ultra short waveband, from 88 MHz till 108 MHz. All radio transmitters operating in this range are stereophonic, but their signal is designed so that monophonic receivers can also read it, performing the compatibility. The readers that wish to get acquainted in more details with the stereophonic broadcast basics can refer to the “Radio Receivers” textbook, for the IV grade of the Electrotechnical Highschool. Making an introduction to this part, a operating principle of the stereophonic radio receiver shall be considered, its block diagram shown on pic.4.18. Comparing this diagram with the one of the monophonic receiver given on pic.4.6, one may notice that they are identical, up to the block called "The Decoder". It means that, as already described, exiting the FM detector the LF signal is obtained, i.e. the information that was used to perform the frequency modulation in the transmitter. However, this is not an ordinary LF signal, but the one, called the "composed" (KS) or "multiplexed" (Mpx) signal. Besides the full-scale LF signal used by the monophonic receiver, it also contains the so-called auxiliary
signal which allows the separation of left (L) and right (R) channels in the
stereophonic receiver. E.g. if a direct broadcast of some band music is
performed, the left part of performers is being recorded with one microphone
(the signal marked as L), whilst the right side is recorded with the other
one (it’s a R signal). These two signals are being led in the FM transmitter
in the stage called “the coder”. Exiting the coder we have the multiplexed
signal Mpx which contains, in an indirect manner, both left (L) and right
(R) signal. Frequency modulation of the transmitter is being performed with
the Mpx signal. In the receiver, Mpx signal is obtained on the output of the
FM Detector and is then led to the decoder. This stage plays a role
complementary to the one of the coder in the transmitter, therefore two
signals are exiting it, the L and D signal. They are being amplified over
two identical audio amplifiers, then reproduced over two same loudspeakers.
The listener can now hear the left half of the performers from the
loudspeaker placed on its left, and the right half from the loudspeaker that
is placed on its right. The performers that are situated in the middle of
the orchestra are being equally reproduced from both loudspeakers, making an
impression to the listener as if there’s a third loudspeaker, located in the
middle, between the left and right one. Based on all this, the listener has
a picture about the layout of the performers in space, which significantly
improves the total musical impression.
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