The most widely used car MP3 transmitter is the FM stereo transmitter integrated circuit BHl414-1417 series launched by the Japanese RHOM company. Its radio frequency part uses a frequency synthesis circuit, and the frequency is very stable. The audio signal processing integrates the pre-emphasis circuit, limiter circuit, and low-pass filter circuit (LPF), so that the quality and frequency stability of the audio signal are greatly improved compared to the BA1404 without a phase-locked loop circuit. The surface-mounted BH1417F chip creates conditions for miniaturization of the circuit.
However, in order to reduce costs, some manufacturers produce transmitters that look just like ordinary car MP3 transmitters with cigarette lighter plugs and headphone plugs, as shown in Figure 1. But cut corners on the internal circuit, did not use BHl414-1417 series FM stereo transmitter integrated circuit, which greatly reduced the performance of the entire circuit, such as the two-channel stereo mentioned in the product overview, it is nothing. The selection of the output frequency point cannot be found in the internal circuit. The integrated circuit BHl417 can be used to set 14 frequency points with a code switch or other circuits.
Analysis of car audio MP3 transmitter Figure 1
However, we also admire these manufacturers. They have made great efforts to reduce costs. They can still pass the test by simply listening to the test.
Below I try to analyze a car MP3 transmitter I recently got:
According to the circuit diagram drawn on the circuit board as shown in Figure 2, the circuit principle analysis is as follows:
Analysis of car audio MP3 transmitter Figure 2
The car battery 12V power supply obtained from the car cigarette lighter plug, through the TO-92 packaged 5V three-terminal voltage regulator integrated circuit 78L05, output 5V power supply, for the complete circuit needs. Oscillation circuit uses quartz crystal X1 to stabilize frequency, crystal transistor Q2 is preamplifier, Q3 is frequency doubling power amplifier, Q1 is SOT-23 surface mount transistor 1AM, performance is the same as 1N3904, equivalent to varactor diode function, its overall equivalent The capacitor, based on the level after the voltage drop of R1 and R2, is affected by the change of the audio signal output from the MP3 headphone jack end, so that the crystal oscillation frequency swings up and down at the center frequency, that is, the FM signal is generated, because the center frequency of the FM signal Only 17.734475MHz, can not be received by the FM band of the car radio, it needs frequency multiplication amplification processing, according to the calculation, 17.734475MHz 5th frequency (fifth harmonic) is 88.672375MHz; 6th frequency (sixth harmonic) is 106.40685MHz . Near the upper and lower limits of the FM band respectively, it has little effect on the broadcast of common FM radio stations. It is consistent with the frequency output data of 88.7MHz ～ 106.4MHz marked by this car MP3 transmitter shell, as shown in Figure 3.
Car audio MP3 transmitter analysis Figure 3
It can be seen that the selection of the oscillation frequency of the quartz crystal is still very accurate.
The connection method of the Q3 power amplified RF output antenna is different from the receiving antenna of the FM radio with headphones. The receiving antenna of the FM radio is a lever type; the receiving antenna of the FM radio with headphones is to use the headphone cable, and the FM will be FM It is isolated from the audio signal; and 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 use surface-mounted components. See Figure 4 and Figure 5 on both sides of the printed circuit board.
Anatomy of the MP3 transmitter of Manbile car audio Figure 4
Car audio MP3 transmitter analysis Figure 5
According to the analysis of the above circuit principle, as a product does not meet the requirements, but as an electronic enthusiast, the production is still good. In terms of component selection, it is only difficult to find quartz crystals that meet the requirements.
Readers who need to purchase car MP3 transmitters should choose MP3 transmitters with FM stereo transmitting integrated circuit BH1414-1417 series, or MP3 transmitters with audio modulation wireless transmitting integrated circuit of GFT808 in Korea. From the appearance, there should be multiple frequency settings; if it can be disassembled, it is best to see the FM stereo transmitter integrated circuit to be at ease.
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