Car audio MP3 player transmitter analysis
Car MP3 transmitters have been popular for one or two years, and their functions are also increasing. The internal circuit, the most widely used is the FM stereo transmitting integrated circuit BHl414-1417 series launched by RHOM, Japan. Using a frequency synthesis circuit, the frequency is very stable. The processing of the audio signal integrates the pre-emphasis circuit, the limiter circuit and the low-pass filter circuit (LPF), so that the quality and frequency stability of the audio signal is much improved compared to the BA1404 without the phase-locked loop circuit. The surface-mount BH1417F chip creates the conditions for miniaturization of the circuit. This article refers to the address: http:// However, in order to reduce the cost, some manufacturers produce the same kind of transmitter from the outer packaging as the ordinary car MP3 transmitter with cigarette lighter plug and earphone plug, as shown in Figure 1. However, the internal circuit is cut corners, and the BHl414-1417 series of FM stereo transmitting integrated circuits are not used, which greatly degrades the performance of the whole circuit. For example, the two-channel stereo mentioned in the product overview is purely false. The selection of the output frequency point is not found in the internal circuit. With the integrated circuit BHl417, 14 frequency points can be set by using a code switch or other circuits. However, we also admire these manufacturers, have made great efforts in reducing costs, simply listening to the test, or can pass. Below I am trying to analyze a car MP3 transmitter I recently got: According to the circuit diagram drawn by the circuit board, as shown in Figure 2, the circuit principle Analysis is as follows: The 12V power supply of the car battery obtained from the car cigarette lighter plug is outputted by the 5V three-terminal voltage regulator integrated circuit 78L05 in the TO-92 package, which is required for the whole circuit. The oscillating circuit uses quartz crystal X1 to stabilize the frequency, the transistor Q2 is preamplifier, Q3 is the doubling power amplifier, and Q1 is the SOT-23 surface-package triode 1AM. The performance is the same as that of 1N3904, which is equivalent to the varactor function, and its overall equivalent. Capacitance, based on the level of R1 and R2 after the step-down, is affected by the change of the audio signal output from the MP3 headphone jack, so that the crystal oscillation frequency swings up and down at the center frequency, that is, the FM signal is generated, due to the center frequency of the FM signal. Only 17.734475MHz, can not be received by the FM band of the car radio, requires multiplying amplification processing, according to calculation, 17.734475MHz 5 times (5th harmonic) is 88.672375MHz; 6 times (sixth harmonic) is 106.40685MHz . In the vicinity of the upper and lower limits of the FM band, it has little effect on the broadcasting of common FM radio stations. It corresponds to the frequency output data of 88.7MHz~106.4MHz marked on the car MP3 transmitter housing, as shown in Figure 3. It can be seen that the selection of the oscillation frequency of the quartz crystal is still very accurate. The RF output antenna connection of the Q3 power amplifier is different from the FM radio receiving antenna with headphones. The receiving antenna of the FM radio is a lever type; the receiving antenna of the FM radio with headphones is a headphone cable, which is frequency modulated by an inductor and a capacitor. Isolated from the audio signal; this unit adds a separate fourth line to the headphone plug line as the transmit output antenna, and the other three lines are the left and right channels and the common ground. All resistors, inductors, and capacitors in the circuit are surface-mount components, and both sides of the printed circuit board are shown in Figure 4 and Figure 5. According to the above circuit principle analysis, as a product is not in line with the requirements, but as an electronic enthusiast's production is still good. In terms of component selection, it is hard to find a quartz crystal that meets the requirements. For readers who need to purchase a car MP3 transmitter, they should choose an MP3 transmitter with FM stereo transmission integrated circuit BH1414-1417 series, or an MP3 transmitter with audio modulation wireless transmission integrated circuit of GFT808 of Korea. From the appearance, there should be multiple frequency settings; if you can open it, it is best to see the FM stereo transmitter integrated circuit. Compact Substation,Isolation Transformer,Compact Prefabricated Substation,Compact Transformer Substation Hangzhou Qiantang River Electric Group Co., Ltd.(QRE) , https://www.qretransformer.com
