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FM Radio is a compact, gang capacitor – tunable 88 to 108 MHz with 20 Watt Audio amplifier. FM Receiver described here uses very economical and high quality receiver IC. This receiver covers FM range 88MHz to 108MHz. The IC integrates the following functions :
The detailed circuit diagram is shown in figure.
About the Circuit : The circuit is divided into two parts as FM section and Audio Amplifier section.
FM Section : The FM section consists of an FM tuner, an FM-IF amplifier and the detector. The FM frequency is selected by FM trimmer Gang Capacitor. The IF tuned circuit provides high selectivity to the receiver. Antenna is directly coupled through capacitor C5 to pin12 of IC1. IC1 contains super heterodyne receiver. The FM section receives the incoming signal via a wideband antenna circuit and pin12 from there, the signal is amplified and then applied to a mixer via a parallel- tuned circuit. In the mixer, the signal is mixed with an oscillator signal, which is also controlled by a parallel-tuned circuit. The oscillator is combined with an automatic frequency control circuit- AFC - which only needs an external buffer capacitor.
The output of the mixer is applied to an external filter and from there to the first (internal) FM IF amplifier. From there, it is again filtered externally and then applied to the FM discriminator. Finally, the signal is demodulated in an FM discriminator. The resulting audio frequency signal is output via pin23. The amplified RF signal is applied to a mixer together with the output of an appropriate oscillator. The mixer is followed by IF filters and an IF amplifier. The output of the IF amplifier controls the automatic gain control - AGC - circuit. The AGC holds the output of the IF amplifier and mixer substantially constant in spite of variations in the RF signal. The output of the IF amplifier is demodulated by a suitable detector and the consequent audio signal is applied to pin23.
AF Section :The AF section consists of IC2, which is low power audio amplifier IC specially designed to deliver 5 to 8W output power at 4W load at 9-12V input supply. The output of this amplifier exhibits low distortion and low noise. The input is fed at pin23 of IC1 through variable pot VR1 to control the input signals to the amplifier. The ICTBA810 (IC1) which is an audio amplifier IC has been in production for several years. It has built in thermal and short circuit protection circuits, so it should have a reasonable life. The main functions of IC2 are, 1.A. F. Preamplifier, 2. A. F. Driver, 3. A. F. Output.
Without any additional cooling, the IC can deliver 4W into a 4ohms Loudspeaker with a 9V supply. The input sensitivity in this case is 240mV. If 8Ohms Loudspeaker are used, the maximum output power is about half. The frequency response is 3db from 40Hz to 18KHz. Note that the pin numbers in the circuit do not take account of the cooling fin; the IC has a total of 12 pins .The pin-out diagram is shown in figure. The Speaker output is taken from pin12 of the TBA810 amplifier.
PCB Fabrication :First of all you should place on the board the resistors the capacitors , the diodes & make sure that the electrolytic capacitors & diodes are connected the right way round because they are polarized. Make a careful visual inspection for mistakes, shorts across adjacent tracks etc. and if everything seems to be all right. In our kit the secondary connection of the transformer are connected. All the inductors are hand-made. The coils are wound closely so that there is no gap between the turns. The end of the coils L1/L2/L3 should be scratched off with the help of a blade or an iron scrapper so that the enameled cover is removed. The gang capacitor use for tuning the frequency between 88 to 108 MHz.
Dilip Bhoyar (Elektrokit)
Keshav Imperial,Shop Nop B-101,B Wing First Floor,Nagpur, , Shani Mandir, Near Shani Mandir, Sitabuldi, Nagpur-440012, Maharashtra, India, Shankar Nagar
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