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
Figure 1 BA1404 pin functions
● 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 pin functions shown in Figure 1, Figure 2 shows the block diagram of the structure and function BA1404. It is mainly from the pre-audio amplifier (AMP), stereo modulator (MPX), FM modulator and RF amplifier.
Stereo pre-class are two-channel audio amplifier. Input is 0. 5mV, 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 38kHz oscillator output signal for 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 (PILO T OU T) synthesized by the modulation signal 12 feet into the RF carrier oscillator and FM modulator, the RF amplified output RF signal, RF signal typical value of about 600mV.
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
Figure 3 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, you 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.
Figure 3 shows the application circuit BA1404 transmission range can reach a radius of several hundred meters, if you think about increasing their firing range, the RF output can be coupled with an RF amplifier, you can use discrete components can also be used directly MAXIM's RF power amplifier MAX2611 or MAX2650, they are suitable for matching with the BA1404.