Miniature high-efficiency FM transmitter module
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1. FM transmitter module FM1 19 yuan each  
The volume of the FM transmitter module FM1 is only 10X36X6 mm (the height only refers to the highest point in the device), which is about the size of a little finger. The operating voltage is 3 to 8 volts, the optimal operating voltage is 6 volts, and the corresponding operating current is 4 to 50 mA. The transmission distance: at 9 volts, the open area is greater than 600 meters. The operating frequency is pre-adjusted to 92 MHZ. By adjusting the inductance coil (the leftmost coil sealed with high-frequency wax in the figure), the transmission frequency can be adjusted between 90 and 100 MHZ. The transmission frequency increases when the coil is stretched, and the transmission frequency decreases when compressed. FM1 comes with a high-sensitivity microphone. When using it, you only need to weld a 50-cm wire as an antenna to use it normally.
When you want the transmitter module to transmit at the longest distance, you need to pay attention to the following: the transmitter module should be at least 5 meters above the ground, which can significantly increase the transmission distance. If the antenna uses a half-wave coaxial antenna made of a cable TV feeder and is installed at a high place outdoors, the transmission distance can be significantly increased.
The monitoring sensitivity of the module is higher than that of the human ear. In a quiet environment, you can hear the "tick-tick" sound of a quartz clock 1 meter away. The monitoring sensitivity of the module is determined by the bias resistor of the microphone. The module uses 3.3K (332) with the highest sensitivity. Because there is a sound limiting circuit inside the module, it can avoid the phenomenon of sound blocking and distortion under a certain degree of strong sound source. If you need a lower monitoring sensitivity, such as: the teacher uses a wireless microphone, you can remove the module's microphone bias resistor micro chip resistor 332 and replace it with a resistor within 200K. The resistance value of the bias resistor can be selected between 3.3K and 200K. The larger the resistance value, the lower the sensitivity of the microphone. Alternatively, sticking a piece of tape on the microphone head can also reduce the microphone sensitivity and improve the signal-to-noise ratio.
If the FM1 module is powered by two AA batteries, it can be used continuously for 1 week. If two AG13 micro button batteries are used, it can work continuously for more than 10 hours. The FM1 module has been sealed with epoxy resin, so it works reliably and the frequency drift is very small (condition: the transmitter power supply voltage is stable). The receiver is best to use a hand-tuned FM radio to compensate for slight frequency drift. When used at close range, it must be monitored with headphones, otherwise it is easy to form self-excited howling due to sound feedback. Actual test: The FM1 module can work normally as long as it is connected to an external DC power supply and a transmitting antenna. It is best to use a battery with a voltage of less than 9 volts as the power supply. The battery has a low internal resistance and will not produce the AC modulated sound that is easy to appear in the AC power supply. It is best to use a 120 mm long TV telescopic antenna. The transmitting antenna pad is on the copper foil of the circuit board at the lower right side of the coil on the far right of the FM1 module. It is recommended to weld the root of the telescopic antenna directly to the transmitting antenna pad, which can greatly improve the transmission efficiency. If you consider ease of use, you can also use multi-strand soft wires as the antenna. Test location: Living room on the 4th floor of a residential new village, separated from the outside by a brick-concrete wall. Working voltage 9V, test sound source: the ticking sound of a quartz clock, test results: monitoring signals can be received within 200 meters, and there are dead spots (silent spots) in a few locations. When the receiving position changes, the receiving frequency will be affected (it may be that the transmission path of the radio signal to the receiving point is different). If the receiving location is fixed, the receiving frequency is very stable. People walking near the transmitter will not affect the transmission frequency, but the transmission frequency will change slightly when the transmitting antenna is pinched by hand. If there is no obstruction between the transmitter and the receiver, the distance is very far, greater than 600 meters. If there is an obstruction, it generally cannot exceed 2 to 3 new village buildings. Note: The receiving sensitivity of the FM receiver greatly affects the transmission distance of the FM transmitter module. The common electric tuning radios on the market do not receive FM1 very well, mainly because the automatic lock cannot lock the station, so it is recommended to use manual tuning. The sensitivity of ordinary FM receivers is not very high, and they can generally be received within 200 meters in an open area. After testing, the stationmaster found that the following high-performance SONY head-mounted FM receiver works very well. In order to maximize the receiving sensitivity, the stationmaster replaced its original small antenna with a soft antenna of more than 1 meter long, which can be received within 600 meters. The stationmaster uses a high-performance car FM radio and has received FM1 signals at more than 800 meters. The factors that increase the FM transmission distance are as follows:
1. The transmitter is powered by a battery, and the voltage is preferably a regulated 9V.
2. The transmitting antenna uses a 120cm telescopic antenna, and the bottom of the antenna is directly welded to the transmitting pad of the module. The shorter the better, and the transmitting antenna is best placed vertically.
3. The transmitter is as high as possible above the ground, the higher the better.
4. The receiver is preferably a highly sensitive hand-tuned FM receiver, such as: car radio, imported FM radio, the antenna is replaced with a 120cm telescopic antenna, and placed vertically.
5. The receiver is as high as possible above the ground, and the fewer obstructions between it and the transmitter, the better.
6. Make sure that there is no FM radio station working at the transmitter operating frequency, otherwise adjust the coil to change the transmission frequency to avoid strong stations. If the above conditions are met at the same time, FM1 can achieve a transmission distance of 1 km.
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2. FM2 FM transmitter module is out of stock!   
The FM2 FM transmitter module consists of a transmitter and a receiver. The transmitter is a milky white plastic box with a bayonet structure on the upper and lower covers. The size is 38x70x22 mm. Inside, there are a full set of accessories such as a power switch, a microphone sensitivity selection switch, a microphone, a soft antenna, a transmitter module, and a battery clip. The power supply is two No. 7 batteries, which can work continuously for a week. The microphone sensitivity has two levels, high and low, which can be switched by toggling the switch near the microphone. The microphone sensitivity of this machine is very high, and it should be kept at a distance of more than 2 meters from the sound source, otherwise the sound is easily blocked and distorted. This set of equipment can transmit 200 meters in an open area and 40 meters in a building area. 
The receiver is a dedicated high-sensitivity receiver with a receiving frequency of 142.5~152.5MHZ. It can be monitored with a speaker or with headphones.
The performance of this module is more stable, the frequency does not drift at all, the continuous working time is long, and the receiver's receiving range is only 10MHZ, so tuning is very convenient. Since its working frequency is not in the working frequency of FM broadcasting, ordinary FM receivers cannot receive it, and it avoids the interference of FM transmitting stations. FM2 can greatly increase the transmission distance by increasing the working voltage of the transmitter, not exceeding 8 volts. Directions: Open the back cover of the receiver, install two No. 5 batteries, turn the band switch to the "FM" side, turn on the volume switch on the right, and adjust the channel switch to around 94 to receive the transmitter's signal. Note that the transmitter and receiver should be more than 10 meters apart to avoid howling. If you want to transmit over a longer distance, you can clip the included crocodile clip to the receiver antenna to increase the receiving distance. 3. FM transmitter module FM4 29 yuan each volume is only: 61x25x11 mm 
FM4 is a low voltage, high efficiency stereo FM transmitter module, which consists of a single-chip stereo modulation transmitter integrated circuit BA1404 and a high-frequency amplifier circuit. It can process and modulate various stereo sound sources and receive them through a stereo FM radio. It contains MPX multiplexer, pilot oscillator, RF circuit and other functions. It can be used in teaching, stage and proportional remote controllers.   
The working voltage of FM4 is 1V-3.6V. If it is lower than 1V, it will stop working. If it is higher than 3.6V, the chip of the circuit will be burned, causing permanent damage. The amplitude of the audio input voltage is 20-200mv. When it is greater than 200mv, a resistive attenuator should be added to the input end of the circuit. For example, when the input sound source is CD or VCD, (its output is 1V). R1 is selected as 10K and R2 is selected as 1K. At this time, an audio signal of about 85-100mv will be obtained at the input end of FM4. The transmitting frequency is marked on the FM4 module. If you need to change it, just slightly turn the coil near the 10th foot of the BA1404 chip.
Notes:
1. The sound source of FM4 must be L, R stereo, and the radio must be able to receive FM stereo broadcasts.
2. The positive and negative poles of the FM4 power supply must not be connected in reverse, and must not exceed 3.6V, otherwise the circuit will be damaged. | Operating voltage | 1.5-3V | | Current consumption | <10mA | | Pilot Oscillation | 38KHz | | Channel separation | 45dB | | Frequency stability | >10 -4 | | RF Output | >600mV | | Operating temperature | -20℃-+50℃ | | volume | 61×25×10mm | 4. FM crystal oscillator transmitter/receiver module JFM1 86 yuan per set
  | JFM1 transmitter module performance parameters | JFM1 receiver module performance parameters | | Operating voltage | 3～9V limit value 10V | Operating Voltage: | 3-8V, limit value: 2-9V | | Working current | About 20 mA | Quiescent Current: | 8-10mA | | Transmit power | 15 mW | Sensitivity: | Better than 5UV | | frequency | 79.000MHz | frequency | 79.000MHz | | Modulation | 3KHz | Output power: | 0.3W at 3V, 1W at 8V | | Distortion | 0.15% | Output Impedance: | 8 Ohm | | Operating temperature | -10～65 degrees | Reference voltage: | 1.25V | | volume: | 30x60x12mm | Distortion: | 0.3% | | | IF frequency: | 10.7MHz | | | Operating Temperature: | -10℃to65℃ | | | volume: | 44×31×10mm | | | Green Line: | Soft Antenna | | Green Line: | Soft Antenna | Red Line: | Connect to the positive pole of the power supply | | Red Line: | Connect to the positive pole of the power supply | Black line: | Connect the negative pole of the power supply and the negative pole of the speaker | | Black line: | Connect to the negative pole of the power supply | Brown line: | Connect to the positive pole of the speaker | | | R: | Volume Adjustment | | | LED1: | Power indicator | | | LED2: | Squelch indicator | The JFM1 FM crystal oscillator module is exquisitely crafted. It consists of a transmitter and a receiver. Both are bare circuit board structures. It can be used to transmit audio signals. The signal modulation method uses crystal oscillator frequency stabilization, and the frequency is very stable. The transmission frequency uses 79MHZ and other frequencies outside the FM broadcast band to avoid mutual interference with normal broadcasting. JFM1 can be used in many fields such as lectures, teaching, toys, anti-theft monitoring, etc.
The JFM1 transmitter module uses the dedicated transmitter chip MC2833 of Motorola in the United States. The chip already contains a crystal oscillator circuit, an audio modulation circuit, and a radio frequency amplification circuit. The module uses a base frequency oscillation circuit with the best starting effect, and transmits the 13.1667 base frequency on the transmitter module after 6 times the frequency. When the user uses it, it can work normally as long as it is equipped with a 3-9 volt DC power supply.
The JFM1 receiving module uses the latest Radio chip CXA1691S from SONY and adds the peripheral control circuit of the multiplication shirt. It includes high frequency, mixing, intermediate frequency, low frequency and all its auxiliary circuits, and completes all signal processing functions from antenna input to audio output. It only needs a power supply and a speaker to form a highly sensitive playback device. The JFM1 receiving module is very compact in design, a bare board, the size of a matchbox.  
The transmission distance of the JFM1 FM module is about 30 meters. If the 15 cm soft antenna on the transmitting and receiving modules is replaced with a 50-100 cm telescopic antenna, the distance can be increased. If you want to transmit over a longer distance, you should use a higher transmitting voltage and a longer telescopic antenna, and the transmitter can be placed at a higher place. The operating frequency of JFM1 is 79MHZ when it is supplied. If you pre-order, you can provide the operating frequency of 80, 81, 82, 83, 86MHZ. The upper adjustable inductance of JFM1 module has been precisely adjusted by professional instruments, so users should not change it by themselves. Actual trial: The JFM1 module transmitter is powered by a 9-volt battery, the receiver is powered by two No. 5 batteries, the speaker is an 8-ohm small speaker, and the antenna is erected with a 1-meter soft wire. When the transmitter is about 5 meters from the ground, it can transmit 80 meters in an open area. The sound quality is very clear, without frequency drift or instability, and the transmitted sound is very clean, without interference noise, only a slight rustling high-frequency noise. It can transmit 20 meters in residential buildings and can penetrate about 2 to 3 floors. The JFM1 module's low power consumption, high stability and high-quality sound quality can be widely used in the fields of wireless karaoke microphones or wireless telephone recording and anti-theft monitoring. This is the detailed information of MC2833 chip 5. High-fidelity digital phase-locked loop FM transmitter module FM6 68 yuan each Japan's ROHM company has newly launched BH1414~BH1417 FM stereo transmitter ICs. The high-frequency oscillation uses a frequency synthesis circuit, the oscillation frequency is very stable, and the transmission frequency can be easily changed. Among them, BH1414 and BH1415 are controlled by an external MCU, and BH1416 and BH1417 can change the transmission frequency by the parallel data setting terminal. BH1416 is suitable for the Japanese frequency band, while BH1417 is suitable for China's 88~108MHz frequency band.
BH1417 is a simple and practical integrated circuit. It integrates a phase-locked loop circuit, a stereo encoding circuit, and a transmission circuit. With a few peripheral parts, it forms a high-frequency fixed multi-frequency HI-FI FM stereo transmitter. In addition, it is equipped with a pre-emphasis circuit, a limiter circuit and a low-pass filter, which can significantly improve the sound quality. Its total harmonic distortion reaches 0.3%, stereo separation is 40dB, and RF output level is 100dB.
It can transmit stereo audio signals from computer sound cards, game consoles, CDs, DVDs, MP3s, mixers, etc. in stereo modulation, and can achieve high-fidelity wireless FM stereo transmission with ordinary FM stereo receivers. It is suitable for the development and production of wireless audio adapters for stereo wireless speakers, wireless headphones, CDs, MP3s, DVDs, PADs, notebook computers, etc. 
BH1417F is a newly launched excellent integrated circuit chip, which is composed of a pre-emphasis circuit to improve the signal-to-noise ratio (S/N), a limiter circuit to prevent signal overmodulation, a low-pass filter circuit (LPF) to control the frequency of the input signal, a stereo modulation circuit to generate a stereo composite signal, and a phase-locked loop circuit (PLL) for FM transmission. The frequency characteristics of BH1417F are very good, it can achieve a separation of 40dB, and the sound quality transmitted can be comparable to that of local FM radio stations, and it is suitable for use in some systems with high audio indicators. BH1417F adopts SOP22 chip package, and the internal structure is shown in Figure 1. The functions of each pin are as follows:
{1} and {22} are the input terminals of left and right channel signals. {2} and {21} are connected to the pre-emphasis circuit, and the time constant (T = 22.7 kΩ × C) can be changed by an external circuit. {3} and {20} are the adjustable terminals of the low-pass filter. An external 150pF capacitor can limit the input of signals above 15 kHz. {4} is the filter terminal, and an external capacitor can improve the ripple factor of the reference voltage. {5} is the output terminal of the stereo composite signal. {6} is grounded, and {7} is the output of the PLL phase detector. {8} is the power supply terminal, connected to a +5V power supply. {9} is the RF oscillator terminal, which forms a voltage-controlled oscillation circuit with external components. {10} is the RF ground terminal. Pin {11} is the RF signal output terminal, which is connected to the antenna or the post-stage power amplifier through a bandpass filter. Pin {12} is the PLL power supply terminal. Pins {13} and {14} are connected to an external 7.6MHz crystal oscillator. Pins {15} to {18} are parallel data setting terminals, which control the output frequency of the transmitter, and pin {19} is the pilot signal adjustment terminal.  
The board is equipped with:
1. Power input socket: The upper right side of the circuit board is the power socket (positive inside and negative outside) with 8~15V DC power input. The LED on the board is the power indicator. When there is power input, the LED will light up and emit red light.
2. Audio input socket: The socket on the lower right side of the board can input L, R two-channel stereo signals. You can directly input CD; VCD; CD; MP3; TV audio output signals; but don't connect to the amplifier output easily, otherwise the sending circuit will be burned out due to the large signal amplitude.
3. Audio volume adjustment: Turn the blue square potentiometer to adjust the audio modulation volume. (Note: There will be no sound when it is turned to the minimum).
4. Antenna output terminal: The ANT end can directly lead out the RF signal, which is transmitted through a pull-rod antenna (self-matched) or a soft wire. It is equipped with a yellow hard antenna, and the appearance is shown in the figure.
5. Frequency setting terminal: The transmission frequency can be changed by changing the position of the short circuit, see the table below: | Terminal location | Output frequency | | D1 | D2 | D3 | D4 | | - | - | - | - | - - - - 87.7MHz | | + | - | - | - | + - - - 87.9MHz | | - | + | - | - | - + - - 88.1MHz | | + | + | - | - | + + - - 88.3 MHz | | - | - | + | - | - - + - 88.5 MHz | | + | - | + | - | + - + - 88.7 MHz | | - | + | + | - | - + + - 88.9MHz | | + | + | + | - | + + + - PLL stop | | - | - | - | + | - - - + 106.7 MHz | | + | - | - | + | + - - + 106.9 MHz | | - | + | - | + | -+ -+ 107.1 MHz | | + | + | - | + | + + - + 107.3 MHz | | - | - | + | + | - - + + 107.5 MHz | | + | - | + | + | + - + + 107.7 MHz | | - | + | + | + | - + + + 107.9 MHz | | + | + | + | + | + + + + PLL stop | The following are the performance parameters of FM6: (for reference only) | project | index | | Input voltage | 7～15V | | Current consumption | About 20mA | | FM transmission power | 10mW | | Transmission distance | 5-8 meters | | Audio frequency input range | 20～15KHz | | Audio input sensitivity | -10dBV | | Audio input voltage (recommended value) | 1.0V | | Frequency output range | 87.7 MHz～88.9 MHz | | 106.7 MHz～107.9 MHz | | Operating temperature | -15°C～50°C | | volume | 76.5x37x18mm | Features of FM6: 1) The pre-emphasis circuit, limiter circuit, and low-pass filter circuit (LPF) are integrated to greatly improve the quality of the audio signal compared to the circuit with discrete components.
2) Stereo modulation circuit with pilot mode.
3) The phase-locked loop frequency is adopted and integrated with the FM transmission circuit, and the combined transmission frequency is very stable. A 4-bit three-row jumper array is used for frequency setting, and 14 frequency points can be set, which is very convenient to use. In order to ensure the restoration of the sound quality, the circuit does not make any modification to the tone to ensure the original flavor. All coupling capacitors in the audio circuit are tantalum electrolytic capacitors. There are precision devices on the circuit, please do not adjust them at will under amateur conditions.
When it comes to the power supply of the circuit, it is necessary to pay attention to the selection of a pure power supply. The clutter on the power supply will have a negative impact on the sound quality of the circuit. Since the transmission frequency of the circuit is in or near the FM broadcasting segment, the transmission power shall not be increased without authorization during actual use, otherwise the consequences will be at your own risk. In actual use, the frequency setting should avoid the local FM radio station. 6. High-fidelity car MP3 FM transponder FM8 32 yuan each This is a car-mounted MP3 FM repeater developed using the BH1417 chip. It can easily convert MP3 music into high-definition FM radio. It is an audiophile-grade product suitable for users with high requirements for sound quality.  
Transmitting distance: 30 meters
Working current: 45-60 mA
Frequency response: 20-15KHZ
Working frequency: 7 fixed frequency points can be switched This product has a USB port, which can power and charge an MP3 with a lithium battery. In order to prevent the transmission frequency from overlapping with the radio station, this product can also switch between 7 working frequencies by flipping the switch on the side, and it does not have the disadvantage of the market that the frequency will return to the original position once the electronic switch is turned off.  
The picture above shows the internal structure of FM6. The red one is the dip switch used to switch the operating frequency. The LM7805 in the picture is used to generate 5V voltage to power the USB port. The circuit also has a specially designed inductor and capacitor filter circuit to purify the car power supply to prevent interference. Directions: Plug one end of the transmitter into the car cigarette lighter and the other end into the headphone jack of the MP3 player. Play the MP3 player and adjust the volume to 70-90%. Tune the car radio to the frequency set by the transmitter and you can enjoy the wonderful MP3 music. This is the best solution for car audio to support MP3.
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