BH1417 USB FM Transmitter

Here's BH1417 USB FM Transmitter with built-in PLL circuit. Its low-frequency signal is converted into high-frequency, which can take any audio device with FM radio (stereo, car CD, MP3, DVD player, etc.), as a normal radio station. Transmitter power is sufficient for reliable reception of its signal within a few tens of meters. The basis of the device is a chip BH1417F, included in a typical scheme. This device contains all the necessary circuitry to generate a composite stereo signal c of the pilot tone, the RF generator with PLL and power amplifier. A detailed description is given in.

BH1417 specs and features chips:
- Supply voltage 4 … 6 volt;
- Current consumption 20 mA;
- Operating temperature range -40 … +85 ° C;
- A range of audio frequencies 20Hz … 15kHz;
- 40 dB channel separation;
- THD 0.1%.

Formation of carrier frequencies in the broadcast signal chip carries a stabilized quartz resonator Z1 (7.6MHz) PLL-synthesizer with 4-bit parallel interface control. Changing the position of switches SA1 … SA4 can get 14 discrete frequencies. Their values are listed in the table.

This device PLL is stable only works in one of the sub-bands (87.7MHz … 88.9MHz or 106.7MHz … 107.9MHz) without having to reconfigure L1. Structurally, the device is on the circuit board 50 x 23mm and is equipped with a USB connector, through which the supplied 5V. The photographs shows the appearance of printed circuit board unit.

The PCB is made of fiberglass-sided 1.0 mm thick. All resistors, inductor L2 and Ceramic Capacitors – SMD, 0805. As the VT1, VT2, you can use npn transistors for surface mounting with a static gain of at least 100. Crystal at 7.6 MHz can be purchased at the store “Quartz” 50m from the metro station “road enthusiasts” (Sh Enthusiasts, 31). You can also use less scarce, the frequency of 7.68 MHz. The coil loop oscillator L1 is wound on a frame of 4 mm diameter tinned copper wire with a diameter of 0.6 mm has 4.5 turns in the winding length of 6 mm. Within the framework of core trimming high ferrite.

It should be noted that the independent production and installation of such a board problem is rather complicated. A simpler version of the printed circuit board can be found in [2]. When first turned on and set power 5.1 V is supplied from a laboratory source. To get started, choose the sub-band switch SA4, which will operate the transmitter. Then, turning off the SA3 (on pin. 17 circuits DA1 to be a logical unit) and adding SA1 and SA2 (at the pin. 15 and 16 must be logic zeros) adjusting core coils L1, seeking to Pin 7 voltage 2.8 V. After that, the FM receiver check the stability of the PLL capture within the sub-band. To do this, changing the position of the switches SA1 … SA3, check for the receiver of the transmitted signal at all frequencies sub-band.

In conclusion, the maximum undistorted signal is selected value of resistors R1 and R2 (structurally they are located inside the input jack). The device is preferably placed in a screen of thin sheet metal, perhaps after this it is necessary to adjust the L1. As a piece of wire antenna and MGTF-0, 07 length of about 80cm. When choosing the operating frequency to avoid interference is desirable that it defended by no less than 200 kHz operating in your area FM radio stations and broadcast television channels.

BH1417 USB FM Transmitter Schematic
BH1417 USB FM Transmitter PCB Layout