Stereo FM radio design



These are the use MB1404 as a stereo composite signal transmitter, you can use the internal oscillator with high-frequency amplifier and can not! According to my experience it is recommended that beginners use the typical recommended the use of internal high-frequency part of the three-point oscillator circuit, if you are already with a certain strength, a high-frequency master it, even it is recommended that you use high-stability crystal oscillator device to make transmitter frequency stability, but the general crystal oscillator with frequency deviation is too small the problem, if the proposed conditional use or PLL frequency synthesizer, so if you have a professional radio station to the same individual (illegal) radio Ha! But to use your legal and easy to get started novice, I will most simple applications to share with you, you want to get more information if you can communicate with me directly.
BA1404 is one of the few FM transmitter ICs, which make up the past to design using discrete components circuit less than FM, but also has a stereo modulation functions. Only a few external components can be beautiful stereo FM signal. Therefore, in wireless FM stereo transmitter and the microwave has important application value.

1. BA1404 The main features of

BA1404 The main features are as follows:
● the use of low voltage, low-power design, voltage between 1 ~ 3V, typically 1.25V, maximum power 500mW, quiescent current of 3mA;
● stereo modulation, FM modulation, the RF amplifier circuit integrated on a chip;
● the required external components;
● two-channel separation is high, typically 45dB;
● Input impedance of 540Ω (fin = 1kHz), the input gain of 37dB (Vin = 0.5mV);
● Typical RF output voltage is 600mV.

2 pin functions and working principle

BA1404 mainly by the pre-audio amplifier (AMP), stereo modulator (MPX), FM modulator and RF amplifier.
Stereo pre-class are two-channel audio amplifier. 0.5mV input, the gain up to 37dB, bandwidth is 19kHz. If the input signal frequency is higher than 19kHz components exist, you must add an input low-pass filter, or two-channel separation will decrease.
In the stereo modulation group, the oscillator output signal at 38kHz stereo modulation. Usually 16, 17 feet to an adjustable resistor to get the best channel separation.
Stereo mixed-signal (MPX output signal) output signal with the pilot (PILOT OUT) synthesized by the modulation signal 12 feet into the RF carrier oscillator and FM modulator, the RF amplified output RF signal, RF signal is typically between 600mV or so.
BA1404 interior also provides a reference voltage unit VREF. Designers can use this external voltage signal to change the capacitance value of varactor diodes, and then change the oscillation frequency of the carrier. Therefore, as long as the control value of a resistor divider can achieve the purpose of changing the transmitting frequency, which is a more unique design.

3 Typical Application

The picture shows a typical application circuit BA1404. The figure, left and right channels through a pre-emphasis circuit of the audio signal input to the internal BA1404. Use of internal reference voltage changes the capacitance value of varactor diodes, in order to achieve emission frequency adjustment.
Design should pay attention to the following:
(1) In order to make the transmitter and FM receiver, the frequency response of each match, the input to be added pre-emphasis network, the time factor of 50μs.
(2) 13, 14 feet, stereo mix stereo modulator output signal and pilot signal synthesis, may cause deterioration of stereo channel separation, it should be noted that the value of external components 12, 13 feet.
(3) OSC oscillation frequency range of output of the network if within 76 ~ 108MHz, can? 5mm on the core with? 0.5mm enameled wire around the circle about 2.5, so that the capacitor C11 is 47pF. 7 feet of the RF matching network should also be the case.
(4) In order to simplify the application, you can take the following measures:
● the 16 and 17 feet vacant. Because the internal manifold has to ensure a high channel separation, adjustable resistor is connected in order to optimize.
● do not fine-tune the emission frequency of the varactor, varactor at the direct short circuit, so that eliminates the need for R3 and D1.
● 7 feet can be omitted for RF matching network, connecting directly with VCC.

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