At present, MP3 with FM transmission function in the market has attracted much attention from consumers. This kind of function looks quite novel, and it can also add selling points to MP3 players. In fact, it is not difficult to implement. We radio enthusiasts can also make a small FM transmitter by themselves. Here we introduce a new type of transmitter, which is easy to manufacture and has excellent sound quality, suitable for high-fidelity wireless audio. ![]()
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![]() This circuit uses only one chip (MAX2606), and the actual size of the finished product is 45mm × 27mm (Figure 3). Compared with the transmitter circuit usually made of BA1404 chips on the market, it is more compact, simpler to manufacture, and cheaper. The main features of this chip MAX2606 include low noise, low power consumption, wide frequency range (45MHz to 650MHz), etc. It is a voltage-controlled oscillator with integrated varactor diode, its average oscillation frequency is set by L1, and the inductance of 390nH The frequency can be preset at around 100MHz. Adjust the potentiometer R1 to select a channel in the FM band of 88-108MHz. The input left and right audio signals are added by R3 and R4, and attenuated by the potentiometer R2, and the signal at the sliding end of R2 is modulated by the volume to modulate the RF frequency. When the input signal is higher than 60mV, distortion will occur. Therefore, the potentiometer should attenuate the signal below this voltage. If there is no standard FM transmitting antenna, a 1-meter wire can be used as the transmitting antenna. In order to ensure the best receiving effect, it is best to install it in parallel with the receiving antenna. The chip can work from a single power supply of 3V to 5V, but it is best to use a stable voltage power supply to reduce frequency drift and noise. The electrical principle of this transmitter is shown in Figure 1. In the actual production process, we made a battery box (see Figure 2), which is more convenient, using three ordinary No. 5 dry batteries, you can work for more than a month. ![]() Next, let's talk about the problems related to antennas. In actual production, we tried two antennas, one is a self-made loop antenna, and the other is a whip antenna. The method of self-made ring transmitting antenna: Use copper enameled wire with a diameter of 1 ~ 1.2 mm, wind it into a circular ring with a diameter of about 10 cm and a number of turns of 2 ~ 3, and fix it on the support, as shown in Figure 4. The whip antenna is the kind commonly used in the market. The two antennas have their own advantages and disadvantages. The loop antenna is small in size but has a small coverage area, and can generally reach a radius of about 30 meters. The coverage area of ​​the whip antenna is large, using a 1 meter whip antenna, the radius can reach 100 meters, and the sound quality is good. However, more space is required when working. Readers can choose according to their needs, and can also use wires to replace the transmitting antenna. ![]() There are some issues that need to be paid attention to in the circuit manufacturing process. The first is when making PCB boards, because it is a high-frequency signal of about 100MHz, so you should pay attention to some skills when wiring, for example, the wiring is as short as possible to avoid parallel wiring The corner of the connection line is greater than 90 degrees, etc. If you do not pay attention to these problems, the overall effect of the system will be reduced, and it may not even work properly. Figure 6 is a printed circuit board diagram of the system. In addition, we must pay attention to three points when welding: First, the MAX2606 is highly integrated, so the volume is very small, which brings certain difficulties to welding, at the same time, there are welding temperature (300 ℃) and welding time (less than 10 Seconds), you should pay special attention when soldering. If it is not operated properly, it will cause permanent damage to the chip and bring difficulties to the debugging of the circuit. Therefore, we need to pay attention to the welding method. The second is to pay attention to the coupling between the antenna and the circuit (Figure 5), which will make your system work well. In addition, it is necessary to fix the antenna so as not to move during work and affect the effect. Third, the size of the PCB board is very small, and it is not easy to solder. It requires the welding personnel to have good soldering skills. When necessary, two people can cooperate. In addition, the transmitter has many uses. For example, audio signals output from audio output devices such as PCs, TVs, CDs, and DVDs are modulated by the machine and transmitted, and can be received using ordinary FM radios. The sound quality is good, which is equivalent to wireless headphones. The most important thing is that the cost of this circuit is extremely low and it is extremely convenient to implement. In this way, you can easily realize the transmission and reception of audio signals. Wearing an ordinary FM radio, you can walk around the room or backyard as you like while enjoying sports news broadcast on TV in the living room! |
