Schematic
Schematic
Embedded Files
R1, R2
I2C pull-up resistors. These function to ensure that a consistent logic level was present along the SDA and SCL pins, which synchronize data over the I2C bus.
I used as high a value as necessary to ensure consistent performance, while keeping the resistance low enough to eliminate noise.
R3
The crystal is delicate, so this resistor acts to limit current to the oscillator circuit
R4
10k potentiometer to control the LCD screen backlight. One could also determine the best value and use a fixed resistance, however.
C1, C2
These are crystal load capacitors. Their value was determined by Silicon Labs such that the frequency of the oscillator remains stable.
Without these, the oscillator circuit would fail to start and fail to maintain the necessary frequency to act as the clock for the SI4732-A10 chip.
C3
This is a blocking capacitor, it will prevent low-frequency noise from the power mains from interfering with reception.
Fun fact: where I live, the FM broadcasts are so strong I don't even need an antenna to pick them up!
C4
In combination with the inductor L1, this capacitor forms a resonant (or tank) circuit.
C4 is a variable air gap capacitor whose capacitance ranges from 0 to 365pF. Combined with a fixed inductance of 121 uH, this circuit allows for reception of frequencies from 100kHz to around 7MHz.
The actual effectiveness of reception depends on a lot of factors, though.
X1
This is a 32.768 kHz quartz crystal resonator. In combination with C1, C2, and R3, it forms the oscillator circuit providing precise timing to the IC
32.768 kHz is a useful value because it can allow for a precise 1 Hz frequency, which the IC uses to clock its behavior
L1
Ferrite core inductor. The ferrite is material 65, and the inductance has been theoretically determined to be 121 uH.
SENB to GND
The SI4732-A10 only responds to a single I2C device address. The address can be controlled by tying the SENB pin to ground or VCC (3.3V). Tying to ground sets the IC's I2C address to 0x11 (please see the Silicon Labs document).
Crystal Oscillator Circuit
Crystal Oscillator Circuit
Here one can see the resistors in series making up R3 leading to the crystal oscillator. The two green capacitors are each 22pF, and the resonator itself is the cylindrical object in between the capacitors.
In order to make the oscillator breadboard-compatible, I soldered the resonator to two header pins.
The two resistors parallel to one another behind the oscillator are the I2C pullup resistors on the SDA and SCL lines.
Si4732-A10 Receiver IC
Si4732-A10 Receiver IC
This is the integrated circuit which does all the work. The Arduino communicates with the IC through the I2C protocol, where I have used an open-source library designed for this family of chips to control the actual IC.
I scavenged this chip from a broken radio by de-soldering it from its PCB and soldering it to an SOIC-DIP adapter board, as seen above. This was challenging, because the component is a very small surface-mounted component.
To solder this chip properly, I first soldered pins 1 and 9 (opposite one another) to hold the chip in place on the board, then I flooded the board with flux and performed a drag-soldering technique. I then tested each of the IC pins for continuity. Luckily, it worked well on my first try!
121uH Inductor
121uH Inductor
This inductor acts as the AM antenna. I salvaged the ferrite rod from an old radio, and I wrapped enameled copper wire around the rod until I had achieved the desired inductance. In order to make it breadboard-compatible, I soldered some header pins to the ends of the coil after scraping off the enamel coating.
Originally, I had used a fixed capacitance of 104pF, resulting in an LC resonant frequency of almost exactly 1MHz. I then came across a variable capacitor, which made it into the final circuit to allow for tuning the frequency.
Si4732-A10 Block Diagram
Si4732-A10 Block Diagram
Basic operation of the chip:
Radio waves generate an analog signal as they pass through the antenna
The signal is passed through a low-noise amplifier (LNA)
Analog-to-Digital Converters (ADC) allow for the Digital Signal Processor (DSP) to modify the data
DSP demodulates the signal, filters it, and performs mathematical operations
Signal is converted back to analog, before being sent to the audio amplifier/speaker setup
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