Convenient and practical electric guitar wireless transmitter

The electric guitar transmitter uses the FM stereo transmitter chip BH1417 from Japan's RHOM company. It can transmit FM signals by adding only a few peripheral components. BH1417 works in the 87MHz~108MHz frequency band. The signals it transmits can be received normally by an ordinary radio, and there is no need to design a special receiving circuit. It is very convenient to use.

1. Principle and characteristics of BH1417

The frequency stabilization circuit of BH1417 adopts a phase-locked loop system, which can ensure that the error between the transmission frequency and the reference frequency is almost zero. The transmission frequency is very stable, which solves the frequency drift problem in the wireless transmission system. During the operation of the wireless transmitter, even if the antenna is touched by hand, there will be no frequency drift, which basically meets the requirements for stability on the stage. The whole machine circuit is shown in the attached figure.

Pins (1) and (22) are audio input terminals. DC current input is strictly prohibited, otherwise it will affect the static operating point of the chip. In addition, the quality of the coupling capacitor directly affects the sound quality. The author uses a 1uF tantalum electrolytic capacitor, which has a good listening effect. The input audio signal must be pre-emphasized to obtain a more balanced sound quality. The pre-emphasis effect can be obtained by using an RC parallel circuit. Pins (3) and (20) are adjustable low-pass filter terminals. Connecting a 150pF capacitor can filter out signals above 15kHz. Connecting a 10μF electrolytic capacitor to pin (4) can improve the ripple factor of the reference voltage. The stereo signal is output from pin (5). C28, a 10μF coupling capacitor, also plays a very important role in sound quality. Pin (7) is the PLL phase-comparison output terminal.

The high-frequency oscillator is composed of an LC loop outside the (9) pin and an internal circuit. The oscillation signal is output from the (11) pin through a high-frequency amplifier and sent to the phase-locked loop circuit for comparison. A signal is output from the (7) pin. After being amplified, the value of the high-frequency oscillator is corrected through a varactor diode to ensure frequency stability. The stereo signal output from the (5) pin is directly frequency modulated through a varactor diode. The (13) and (14) pins need to be connected to an external 7.6MHz crystal oscillator to provide the BH1417 with a stable clock required for internal phase detection, stereo signal modulation, and other parts. The (15)-(18) pins are parallel data setting terminals. The corresponding frequencies are shown in the attached table. The (11) pin is the FM signal output terminal, which can be directly connected to the antenna for transmission.

2. Circuit Component Selection

The capacitors, resistors and varactor KV1417 required in the circuit can be easily purchased. The varactor KV1417 can be replaced by BB910 (already verified). The price is very cheap. The Darlington tube 2SD2142K can be replaced by the ordinary low-power triode 9014. All resistors and capacitors are made of SMD to make the transmitter smaller. The audio coupling capacitor is recommended to use a 10μF, 0805 packaged tantalum electrolytic capacitor, which saves space and has good sound quality. The crystal uses a nominal frequency of 7.6MHz.

When making the board, I only bought 1μF tantalum electrolytic capacitors in I206 package, so the 1μF capacitor on the author's PCB board uses 1206 package. The four-position DIP switch for setting the frequency point is composed of four pairs of empty pads (corresponding to C40~C43 in the schematic diagram) and four pull-up resistors (R17~R20). In actual application, if the corresponding two pads are welded together, it is low (L), otherwise it is pulled high (H). The transmitting frequency set by the author is 107.1MHz, and the corresponding four-bit parallel data is set to LHLH. The inductor is made of 0.8mm diameter enameled wire, which is tightly wound 3-4 times on a 4mm diameter cylinder. The antenna uses a half-wavelength soft wire (I originally wanted to use guitar strings as antennas, but because I have to keep touching the strings while playing the guitar, it will affect the normal operation of the transmitter, so I finally chose to use a half-wavelength soft wire as the transmitting antenna).

When drawing PCB, special attention should be paid to separate the high-frequency and low-frequency parts. The wires of the high-frequency part should be as thick, straight and short as possible. A 104 decoupling capacitor should be connected near the chip at each power supply end of the chip to eliminate self-excitation. There should be no wiring or vias under the chip to avoid interfering with the chip. The crystal oscillator should be as close to the chip as possible, and the feed line of the crystal oscillator is prohibited from having vias. There should be no wiring under the crystal oscillator to avoid interference. If the crystal oscillator is not easy to start, a 1MΩ resistor can be connected in parallel with it (when drawing the circuit board, space can be drawn for the crystal oscillator to be connected in parallel with a 1MΩ resistor); the power line should be as thick as possible. It is best to be more than 1mm, and the entire PCB board should be grounded over a large area.

3. Welding and debugging

Make a blank PCB board according to the PCB board drawing. When soldering, it is best to use a soldering iron below 35W. When soldering chips, do not let the soldering iron contact the chip pins for a long time to avoid burning the chip. When soldering capacitors, the soldering iron should be in contact with the capacitor pins for no more than 108 seconds. The contact time with the varactor KV1417 pins should not exceed 5 seconds. When soldering crystal oscillators, inductors, power interfaces and audio interfaces, the solder should flow from the solder holes to the other side as much as possible to increase the reliability of solder joints and component soldering.

After soldering, carefully check whether there are any cold solder joints on the circuit board.

Before powering on, use a multimeter to check if the two ends of the power supply are short-circuited. This is an effective way to protect the power supply, otherwise the consequences will be very serious. If the set transmission frequency is different, the multimeter reading will be different, but as long as there is no short circuit, it will be fine. If everything is in place, you can power on for debugging. During the power-on process, make sure that the experimental bench is very clean, without exposed wires or extra pins of the cut components, otherwise it is likely to cause a short circuit in the circuit and burn out the chip or circuit board.

After powering on, check whether there is smoke on the circuit board and smell carefully for any peculiar smell. You can touch each component with your hand to see if there is any heat to ensure that everything is safe.

Tune the FM radio to 107.1MHz. Use a non-inductive screwdriver to adjust the coil spacing (you can also use a toothpick) until you hear the background noise in the radio suddenly become smaller, indicating that the phase-locked loop has locked the frequency at 107.1MHz. At this time, input the audio signal, and you can hear the FM signal emitted by the transmitter in the radio. Fine-tune the coil spacing to make the transmitter work stably, and then fix the coil with hot melt glue. Note: The debugging process requires patience, and the coil spacing must be adjusted repeatedly to adjust the time for the phase-locked loop to lock the frequency within the time that the human ear can distinguish.

The size of the transmitter is equivalent to a mobile phone battery. Install the transmitter together with the antenna into the electric guitar, and input the output signal of the pickup directly into the FM transmitter. A 3.7V lithium battery can make it work normally.