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About 50mW BH1417 Stereo PLL FM Transmitter
This is an excellent Hi-Fi Stereo PLL FM Transmitter, the audio source can be the computer, sound card, game consoles, CD, DVD, MP3, stereo mixer for stereo audio signal modulated emission transmission, the board also contains two mic amplifier, with the common receiver can achieve high-fidelity FM stereo radio FM stereo transmission. Suitable for the production of stereo wireless speakers, wireless microphone, wireless headphones, CD, MP3, DVD, PAD, notebook computer and other wireless audio adapter development and production. ROHM's new Japan has BH1417 is one of the most simple and practical integrated circuits, which combines phase-locked loop circuit, stereo encoder circuit, transmitter circuit, as well as other additions. Pre-emphasis circuit, limiter circuit and low pass filter can significantly improve the sound quality. The total harmonic distortion up 0.3%, stereo separation to 40dB, RF output level is 100dB. BH1417F is an excellent new IC chip, this circuit improves signal to noise ratio (S / N) of pre-emphasis circuit to prevent signal over emphasized limiting circuit, the control input signal frequency low-pass filter circuit (LPF), generate stereo stereo composite signal modulation circuit, FM transmitter phase-locked loop circuit (PLL) component. BH1417F excellent frequency characteristics, it can achieve 40dB of isolation, transmitted sound quality is similar to local FM radio stations. External power is filtered by 78L05 5V DC regulated output for BH1417 chip. DIP switches M1, M2 turn on dual microphone amplifier, microphone MIC1 the sound signals into electrical signals by the C3 sent by the V1, R6, R7 microphone amplifier circuit composed of enlarged RP1 adjust the volume by the C5 to C6 into BH1417 later by the left channel signal input. DIP switches D0, D1, D2, D3 used to set the firing frequency. BH1417 Stereo FM carrier signal from the output of the first 11 feet, coupled by the C36 into the V3, R21, L3, C37 composed of high-frequency power amplifier, amplified signal coupled through C38 to launch transmitting antenna ANT1. BH1417 FM Transmitter chip includes a lot of features in one small package. It comes with pre-emphasis, limiter so that the music can be transmitted at the same audio level, stereo encoder for stereo transmission, low pass filter that blocks any audio signals above 15KHz to prevent any RF interference, PLL circuit that provides rock solid frequency transmission (no more frequency drift), FM oscillator and RF output buffer. There are 14 possible transmission frequencies with 200KHz increments that users can select with a 6-DIP switch. Lower band frequencies start from 88.7 up to 89.9 MHz, and upper band frequencies start from 107.7 up to 108.9 MHz. BH1417 can be supplied with 4 - 6 voltage and consumes only around 30mA, providing 20mW output RF power. BH1417 provides 40dB channel separation which is pretty good, although older BA1404 FM Transmitter chip provides slightly better 45dB channel separation. BH1417 is only available in SOP22 IC case so this may be an inconvenience for some folks. On the other hand, because the chip is smaller than regular DIP-based ICs it is possible to fit the entire transmitter on a small PCB. BH1417 chip may also be used a stand alone stereo encoder. The advantage of that is that you have full freedom of using a transmitter & amplifier of your choice. You will still have a pre-emphasis, limiter, stereo encoder and low pass filter in one small package because very few external components are required for these blocks. PIN 5 is MPX output that can be directly connected to an external FM transmitter through a 10uF cap. |
BH1417 Block Diagram
Main blocks of BH1417 chip are; pre-emphasis, (audio level) limiter, stereo encoder (MPX), PLL circuit, oscillator and RF buffer.  |
Frequency Selection / Calibration
Frequency selection is very straight forward. Simply select transmission frequency at which you would like to transmit, set the combination for 6-DIP switch and BH1417 will immediately tune to that frequency. If you can't hear the transmitted audio signal on your FM receiver then re-adjust 2.5 turn variable coil until you can hear the signal. As a test to see how PLL is working you can use a laboratory power supply to vary the voltage supply from 4 to 6V. While doing that BH1417 will automatically vary the voltage for MV2109 varicap diode making sure that there's no frequency drift.
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Frequently Asked Questions
This is list of answers to questions that I have received through an e-mail so far. If you should have any other questions please don't hesitate to e-mail them to me. What is the range of BH1417 Stereo FM Transmitter? Range of the transmitter depends on many factors. a) Length and type of the antenna (the longer the better, copper will radiate radio waves better than aluminum) b) Supplied voltage (6V Max power output) c) Weather conditions (clear skies & no humid, greater distance) d) Elevation above the ground (the higher the better) e) Line of sight (no buildings or trees, greater distance) f) Whether external amplifier is used I would like to be able to transmit the music to greater distances. Can you provide any suggestions on how to modify the circuit? You can connect an external amplifier based on the common 2N3866 / 2N4427 RF transistors. Please keep in mind that it is illegal to operate higher power transmitters in most of the countries without the proper license. |
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