7
From the second pin of the ASW header, the alternating current passes through the 10µH
inductor (L1) and the 33pF capacitor (C4) as well as the 1N4148 diodes (D1 & D2) in order to limit
the pin voltage to about 0.3V to protect the chip from damage. This signal enters the HEX3653
chip at pin 4 (FM IN). This incoming signal is then compared to the reference signal received from
the on-board antenna which is a 32KHz crystal connected to pin 9 (RCLK).
The RCLK signal is controlled by the SEEK+ and SEEK- push buttons. The tuning, detection
and amplification of the signal is all done inside the HEX chip.
The audio output is at pin 12 (Lout) and 13 (Rout). Both signals are connected to
the 10KΩ
resistors (R2, R3 and R4) and the DC blocking 100µF electrolytic capacitors (C1 & C2) to prevent
any DC signals with 0Hz frequency from passing through and causing distortion. This RC network
(resistor-capacitor) also functions as a high-pass filter that only allows the audio signal to pass
through and removes any unwanted noise.
