Stereo FM 88...108 MHz receiver based on CXA1238S chip. Scheme, description

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Stereo FM 88...108 MHz receiver based on CXA1238S chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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The receiver is assembled on a Sony CXA1238S chip. Possesses the increased sensitivity and profitability.

Stereo FM 88...108 MHz receiver based on CXA1238S chip

Receiver specifications on CXA1238S:

Operating frequency range 88...108Mhz
Current consumption 16mA
Supply voltage 2-6V

The design does not contain time-consuming IF filter coils with a frequency of 10,7 mHz and is assembled on an inexpensive universal microcircuit. The chip's built-in stereo decoder works with time division, using a PLL to synchronize with the pilot tone. VCO tuning is carried out by resistor R17. The signals of the left and right stereo channels are generated at pins 6 and 5 of U1, respectively. Chains R5 c8 and R6 C9 serve to compensate for the pre-distortion of the audio signal input on the transmitting side to improve the signal-to-noise ratio.

In the receiver, you can use resistors of any type with a tolerance of at least ± 20%, oxide capacitors are better than small-sized imported ones, the rest are ceramic capacitors. The proposed receiver is not critical either to the types or to the ratings of its constituent elements. Piezoceramic filters are any broadband, small-sized, used in all receivers with the VHF range. The appearance of the U2 U3 Filters is shown in the figure. The middle terminal is connected to a common wire, the rest - no difference. The U4 filter is similar to the previous ones, but has two leads and is colored light brown. Transistor Q1 - KT368A, KT368B, KT3102 with letter indices from A to E. Transistor Q2 - KT368A, KT368B, KT339 or KT399 with any letter indices. The inductance of the inductor L1 can be in the range from 22 to 220mkH (I set it to 100mkH). Coils L2, L4 and L5 are frameless with an inner diameter of 3mm. Coil L2 has 8 turns tapped from the middle (for easy installation I wound two coils of 4 turns each). Coil L4 contains 6 turns, L5 - 5 turns. The exact number of turns depends on the length and location of the tracks leading to the coils on the PCB and is specified during setup.

The variable resistor R21 is multi-turn, it tunes the receiver to the radio station. The transistor housings are connected to a common wire. The dimensions of the printed circuit board are 52X46mm. Resistor R8 and capacitor C21 are soldered on the side of the printed conductors. The receiver is connected to the antenna with a shielded wire. Near the antenna, the screen breaks, and the "hot" core is connected to the antenna.

During setup, connect the receiver to some kind of amplifier. If its installation is carried out without errors, when the power source is turned on, a characteristic noise should appear in the speakers. By stretching and compressing the turns of the L5 heterodyne coil, achieve a stable reception of any radio station. If this fails, try changing the number of turns of the heterodyne coil. A constantly working automatic frequency control (AFC) will let you know that you have tuned in to the mirror channel - the setting will be "floating", fuzzy. In this case, stretch the turns of the L5 coil or reduce the number of its turns until the same station appears with a clear tuning.

After you set up the receiver, fill the coils with paraffin - use a paraffin candle for this. In this case, the frequency range will shift slightly, but the coils will be protected from external mechanical influences.

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