FM radio transmitter module, DSP PLL, 87–108 MHz, LCD, campus mode 76–108 MHz

This tutorial uses no development board and no software. The FM transmitter module DSP PLL (board marked LCD_FM_TX_V2.2.0) and an FM receiver module DSP PLL with LCD and 2 x 3 W output each work on their own, immediately after power-up, and are adjusted from buttons or rotary knobs. Here we use them together as a wireless audio link. The transmitter receives sound from the computer through the micro USB cable, and the receiver plays it through two speakers.

The pair is used in projects for short-distance audio broadcasting, for example music from the computer in another room, a wireless microphone or a simple intercom system. The transmitter has 100 mW output power, and the receiver has its own amplifier for 4 ohm speakers.

How it works:

The transmitter generates a radio carrier at the selected frequency and modulates it to the rhythm of sound, that is it uses frequency modulation (FM). The PLL circuit (phase-locked loop) keeps the carrier exactly on the displayed frequency, so the signal does not drift. The digital signal processor (DSP) prepares the sound before modulation. The receiver does the inverse operation. It tunes to the same frequency, extracts the sound from the carrier, amplifies it and sends it to the speakers.

The antenna is 75 cm long because the simplest antenna is a quarter wavelength. The wavelength in meters is calculated with the formula 300 / f, where f is the frequency in MHz. At 100 MHz it gives 300 / 100 = 3 m, and a quarter is 0.75 m, or 75 cm. At 87 MHz the quarter wave is about 86 cm, and at 108 MHz about 69 cm. The 75 cm antenna fits well across the entire band. Use it fully extended on both modules.

The operating band has two variants. In the standard variant (C0) the modules cover 87.0 - 108.0 MHz. In the extended campus variant (C1) they cover 76.0 - 108.0 MHz. The transmitter and receiver must be in the same variant, otherwise the receiver cannot tune to frequencies below 87.0 MHz where the transmitter broadcasts.

Power consumption is calculated simply. The transmitter draws 35 mA at 5 V, so about 0.175 W, power easily provided by the computer's USB port. The receiver draws about 60 mA at minimum volume and up to 1000 mA at maximum volume. That is why its power supply must deliver 1 A. A 18650 cell of 2500 mAh (approximate value) would theoretically last about 41 hours at 60 mA, but only about 2.5 hours at 1000 mA.

Connection:

The transmitter is powered at 5 V from the computer's USB port via the USB-A to micro USB cable, and the computer sees it as a sound card. The receiver is powered by a single Li-ion 18650 cell (3.0 - 4.2 V) mounted in a holder, a voltage in the 3.0 - 5.0 V range of the module. The signals are analog and radio, so there are no logic levels, level converter or pull-up resistors to choose. The serial pins TX, RX and GND of the transmitter are not used in this setup.

Respect the - and + markings silkscreened on the receiver board, because the terminal order in the diagram may differ slightly from your board. Do not use two Li-ion cells in series, as 7.4 V exceeds the 5.0 V limit and destroys the receiver. Do not use an unfiltered phone charger as a power source either, as its noise gets into the sound.

If you want to power the transmitter from a battery, connect a 3.0 - 5.0 V battery to the - and + pads on the top left of the board, respecting polarity. The audio source, for example a phone, is then connected to the LINE input.

Configuration:

Each module is adjusted separately from its buttons. The transmitter has five buttons (VOL-, VOL+, FRE-, FRE+, PAUSE), and the receiver has two rotary push buttons (VOL and FRE). The last frequency and last volume remain in memory even after power-off, so they do not need to be adjusted at each start.

Band and backlight changes on the receiver are applied after restart and stored in memory. Repeat the procedure to return to the previous setting. The factory default is standard band C0, that is 87.0 - 108.0 MHz.

Usage example:

The purpose of the example is to play the computer sound on two speakers in another room. Mount the 18650 cell in the holder and connect everything as in the diagram above, but leave the holder without the cell for now.

Connect the micro USB cable between the transmitter and the computer. The transmitter LCD lights up and displays the frequency. Verify that both modules are in the same band. For this example leave C0 on both, that is 87.0 - 108.0 MHz.

Choose a free frequency, for example 100.0 MHz. On the transmitter use long presses on FRE+ until you get close, then short presses for 0.1 MHz steps. Set the transmitter volume to about 20 out of 30 steps with VOL+.

In the computer's sound settings select the newly appeared USB device as audio output, then play a song. Insert the cell in the holder. The receiver starts, and the LCD shows the frequency. Rotate the FRE button until the display shows 100.0 MHz. After the song is heard, gradually increase the volume with the VOL button.

If the sound is weak, increase the receiver volume and not the transmitter volume above 20 out of 30 steps, as too high a level on the transmitter can distort the sound. If you want to temporarily stop the sound, press PAUSE on the transmitter or press the receiver VOL button.

Common problems:

The receiver plays only noise. The usual cause is a different frequency or a different band. The solution is to check that both LCDs show the same frequency, then check the letter C0 or C1 on restart so the modules are in the same band. If the frequency is occupied by a local station, choose another.

Nothing is heard, even though the frequencies match. The computer may be playing sound on another output, and the transmitter may be muted. Select the USB audio device as output in sound settings and press PAUSE on the transmitter until the mute state disappears. Also check the volume on both modules.

Sound cuts out or noise appears. Usually the antennas are loose or missing, or the power supply produces noise. Fully extend the 75 cm antennas and keep them away from metal objects. On the receiver use a battery or a regulated and filtered power supply, not a phone charger.

Weak signal disappears completely. The receiver's automatic squelch cuts weak signals if the threshold is too high. Press the receiver FRE button to turn off the function or reduce the threshold between 0 and 20.

Receiver turns off or restarts at high volume. At maximum volume current rises to 1000 mA, and a discharged cell or poor holder no longer maintains voltage above 3.0 V. Use a charged cell and short wires, and if the problem persists, reduce the volume.

Speakers remain silent. If something is inserted in the headphone jack, the receiver automatically stops the speakers. Remove the plug from the jack. Also check that the speakers are 4 ohm and connected to terminals SP+_1, SP-_1, SP+_2 and SP-_2.

Limitations and tips:

The transmitter has 100 mW power, so the range is small and depends heavily on walls, antenna and radio noise in the area. The frequency response is 50 Hz - 18 kHz, and the signal-to-noise ratio is at least 30 dB. It is sufficient for voice and everyday music, but not for high-fidelity listening. FM broadcasting in the band is regulated, so find out about local rules and use the setup over short distance on a free frequency.

The receiver provides 3 W per channel only on 4 ohm speakers. For this it uses a power supply that can deliver 1 A and a Li-ion cell with voltage between 3.0 and 4.2 V. Keep the transmitter and receiver speakers apart if you use the transmitter microphone, as sound from the speaker enters the microphone and produces echo.

The setup can be extended. The transmitter accepts sound through the LINE input from a phone, and the MIC microphone turns it into a wireless microphone. The transmitter also has serial TTL pins (TX, RX, GND) on the back for control from a microcontroller. The receiver in turn has a TTL serial interface (UART_RX, UART_TX, GND) at 38400 baud with AT commands for frequency, volume and squelch. The level is TTL, so a USB-TTL adapter is used from a computer.