FM DSP PLL radio receiver module, 87–108 MHz with LCD, Campus mode 76–108 MHz, stereo output 2x3W

This tutorial uses the FM radio receiver module DSP PLL, 87-108 MHz, with LCD and stereo output 2x3 W, together with the FM radio transmitter module DSP PLL with LCD. We do not use any development board or program, because both modules have their own controller, their own LCD display and their own commands and work independently, each powered by a battery holder.

The receiver has an integrated stereo amplifier and can play directly on two 4 ohm speakers. The transmitter sends audio from a phone, from a computer or from its own microphone into the air. Together they form a wireless audio link over short distances. They can be used for a wireless speaker, a wireless microphone, a camera monitor or for sound reinforcement in a classroom.

How it works:

The transmitter receives the audio signal, processes it digitally with the DSP circuit and modulates it in frequency (FM) at a frequency you choose. The PLL synthesizer keeps the frequency stable, without drifting with temperature or over time. The transmit power is 100 mW, and the transmitted audio band is 50 Hz - 18 kHz.

The receiver does the reverse. The antenna brings in the FM signal, the DSP circuit demodulates and decodes it stereo, and the integrated amplifier delivers 2 x 3 W to 4 ohm speakers or 500 mW to headphones. A digital squelch, adjustable in 20 steps, cuts the sound when the signal is too weak, so you don't hear the background hiss between stations.

The frequency is adjusted in 0.1 MHz steps. In the standard band 87.0 - 108.0 MHz there are (108.0 - 87.0) / 0.1 + 1 = 211 possible frequencies. With Campus mode enabled the band becomes 76.0 - 108.0 MHz, that is (108.0 - 76.0) / 0.1 + 1 = 321 frequencies. The transmitter and receiver must have the same frequency and the same band setting (C0 or C1), otherwise they won't communicate.

The Campus setting matters even if the chosen frequency is in the standard band. One module with C0 and one with C1 are not configured the same way, so for the first test set C0 on both modules, and choose the frequency between 87.0 and 108.0 MHz.

Connection:

Each module has its own 3 x AA battery holder, which provides 4.5 V, within the allowed range of 3.0 - 5.0 VDC. Batteries are a good choice because they don't introduce switching noise in reception. Do not use phone chargers or other unfiltered switching power sources. Do not exceed 5.0 V and respect the polarity. For the receiver choose a power source that can provide at least 1 A, because at high volume with speakers the current consumption increases significantly. The audio connections of the modules do not use logic levels, so there is no need for a level converter, resistors or separate signal power.

Do not connect the receiver's SP- pins to ground and do not link them to each other. The speaker outputs are in bridge mode, and an SP- connected to ground can damage the amplifier. Each speaker uses both pins of its pair, SP+ and SP-. Check the pad names on the board silkscreen before soldering, because the order in the diagram may differ slightly from that on the board.

Configuration:

The receiver is controlled from two rotary encoders with a button, marked FRE and VOL. The settings are automatically saved when power is disconnected, so at the next startup the module returns to the same frequency, with the same volume and the same options. The table below lists the receiver actions.

To change the Campus mode on the receiver, disconnect power, press and hold the VOL encoder, reconnect power and wait for C0 or C1 to appear on the LCD. Repeat the procedure to return to the previous value. The setting takes effect on restart. The backlight is switched the same way, with the FRE encoder.

The transmitter is controlled from buttons, and the table below shows their functions.

The transmitter automatically selects its audio source. A cable in the 3.5 mm LINE jack provides stereo input. A USB cable to a computer turns it into a sound card named CD002, the LCD shows PC and the module is powered from USB. Without either of them it uses its own microphone, in mono.

Usage example:

We will transmit music from a phone to a stereo speaker, on frequency 87.5 MHz, with Campus mode off on both modules.

Insert the batteries into the two holders, but don't power up the transmitter yet. Power up the receiver and check that the LCD shows C0. Rotate the FRE encoder and look for a free frequency, that is one where you hear only background noise. If 87.5 MHz is occupied by a local station, choose another free frequency and use it instead throughout the example.

Power up the transmitter and check that the LCD shows C0. Press FRE+ or FRE- until 87.5 MHz appears on the display. Connect the phone to the LINE jack with a 3.5 mm jack cable and start playing music, with the phone volume at approximately three quarters.

On the receiver, tune to 87.5 MHz with the FRE encoder and increase the volume with the VOL encoder to approximately 15 out of 30. After a few seconds the music should be heard on both speakers, and the receiver's LCD should indicate stereo reception.

Then adjust the final volume from the VOL encoder on the receiver. Adjust the transmitter volume only if the sound is distorted or too low. For the first test keep the modules a few meters apart from each other, then gradually move them away to see how far the clean reception goes.

Frequent problems:

A humming or crackling sound is heard from the speakers. The usual cause is power from a phone charger or another unfiltered switching source, whose noise passes through to reception. Switch to batteries or a stabilized and filtered power source.

The receiver produces no sound, even though the transmitter is transmitting. First check that the frequency and C0 or C1 setting are the same on both modules. Then check that there is no headphone jack in the Earplugs jack, because the speaker output automatically turns off, and the sound is not muted. If squelch is active, lower the threshold or turn it off, because a high threshold cuts weak signals.

The sound distorts or the module resets when you turn up the volume. The power source cannot provide the current required by the amplifier at high volume. Use fresh batteries or a source that can provide at least 1 A and lower the volume.

One speaker is not working or the amplifier gets hot. Check that both pins of each SP_R and SP_L pair are connected only to their speaker and that no SP- touches ground or another SP-. Use 4 ohm speakers, not lower impedance ones.

Reception is weak or there is a lot of noise. Fully extend the 75 cm telescopic antennas on both modules and move them away from cables, switching sources or other electronic devices. Increase the distance between the two modules only if reception is saturated at very close distances.

The backlight turns off by itself. This is normal behavior in B0 mode. For permanent light, switch to B1 with the procedure in the Configuration section, then restart the module.

Limitations and tips:

FM transmission has 100 mW and a range of tens of meters in open field, smaller in buildings with thick walls. The quality is suitable for music and voice, but not for high fidelity playback, with an audio band of 50 Hz - 18 kHz and a signal to noise ratio of at least 30 dB. Choose a free frequency and comply with local radio transmission regulations.

The receiver amplifier delivers 2 x 3 W to 4 ohm speakers and 500 mW to headphones. For a more powerful speaker, use the headphone output as a signal source for an external amplifier.

The receiver also has a serial UART interface at TTL level, with TX, RX and GND pads, speed of 38400 bps and AT commands for frequency, volume, Campus mode and squelch threshold. You can use it with a microcontroller or with a USB-TTL adapter, for example Arduino, ESP32, STM32 or Raspberry Pi, if you want to automate tuning. Check the logic level of the board you are connecting to, because the module's interface is TTL.

For a transmitter that runs continuously, power it from USB from a computer or from a good power adapter, and for the portable receiver use batteries. For a wireless microphone, leave the transmitter without a LINE cable and without USB, so it uses its own microphone, and adjust the volume for clear sound.