1K meter small stereo FM transmitter circuit diagram

Filter 18.432M

Today, I introduced to the FM enthusiasts a BA1404 FM stereo transmission circuit. The BA1404 is an FM dedicated IC. Because of its excellent working performance, it is also often used for radio remote control. The required power supply voltage of the BA1404 is 1.5V~3V. Small, the circuit structure is perfect, less external components are required, and the work is stable and reliable. However, if you want to use BA1404 well, you should pay attention to the following items:

(1) In order to make the frequency characteristics of the transmitter consistent with the frequency characteristics of the FM broadcast receiver, a pre-emphasis circuit with a time constant of 50 μs is connected in series to the input terminals of the left and right channels, as shown in the figure. When used in a radio remote control transmission circuit, the left and right input terminals can be combined and connected to the signal input terminal with a 0.1 to 1 μF capacitor.

(2) The external R and C components of pins 12 to 14 are used to ensure that the output signals of the stereo modulator and the pilot signals are combined into a composite signal. The phase characteristics of the two are the same, and the channel separation is not deteriorated. Generally, it cannot be changed. . If changes are required, they should be determined after actual testing.

(3) In stereo modulation, the upper limit of the audio frequency of the modulated signal is 19 kHz. If the input signal is accompanied by a pulsed signal, in order to prevent the buzzer and channel separation from deteriorating, a low-pass filter should be connected in series at the input end.

(4) Pins 16 to 17 are pins for adjusting the balance. Even if the two legs are not connected, a satisfactory resolution can be obtained. In normal use, a 50kΩ potentiometer is always connected, and the DC balance is adjusted to further improve the resolution.

(5) The DC output voltage of pin 11 is fixed to VDD to 0.7V by the internal circuit.