Making an FM receiver with a high-frequency head

The author has an unused V-shaped built-in information card, and wants to make good use of the high-frequency head on the card. After consulting relevant materials, he removed the high-frequency head and added peripheral components, and successfully made a full-channel FM receiver with good performance, and the use effect is satisfactory. Because the high-frequency head on the card has its own intermediate amplifier circuit, it can output independent audio and video, which reduces the debugging points in the production process and simplifies the circuit. As long as the components are welded correctly, the production can be successful and good performance can be obtained. The production method is introduced as follows for reference by the majority of enthusiasts:

1. The original high-frequency head is an FC interface integrated high-frequency head. To change it from PC control to manual control, some small changes need to be made to the high-frequency head. Carefully pry open the shielding shell of the high-frequency head to see three integrated circuits. Their names, functions and modification instructions are shown in Table 1. Because the pins of the integrated circuits are dense, you need to be careful when making changes. The pin definition of the modified high-frequency head is shown in Table 2, the frequency band selection principle is shown in Table 3, and the entire circuit schematic is shown in the attached figure.

2. The tuning potentiometer uses a 100kΩ multi-turn special tuning potentiometer. If it is difficult to buy on the market, you can use the pre-selection board of an old TV. The author uses a 12-bit pre-selection board with built-in frequency band selection. Its potentiometer rotates through gear transmission, has a high tuning accuracy, and can also achieve a good tuning effect. For ease of use, except for one for main tuning, the remaining 11 bits can be used to store channels; although the number of mechanically stored channels is a bit small, it is better than nothing.

3. In order to know the receiving frequency intuitively, you can use a meter with a full-bias current of 100μA to indicate the tuning voltage by dividing the voltage of the potentiometer.

4. The entire circuit can be installed on a general experimental circuit board. In order to obtain higher performance, the power supply part needs to be shielded. The shield can be welded by thin copper sheets. It is best to use a single-core shielded wire for the tuning voltage input line.

There are no special requirements for other components.

The receiver can receive FM signals in the range of 49MHz to 865MHz. Due to the wide receiving frequency range, it can be used as a frequency indicator under undemanding conditions. The author made a wireless microphone that uses an ordinary FM radio to receive signals. With the FM receiver introduced in this article, the transmitting frequency of the wireless microphone can be easily and quickly adjusted to the frequency range received by the ordinary FM radio.