The device's transmit channel consists of the following functional blocks: 138 kHz digital low-pass filter, bypassable 25.875 kHz digital high-pass filter, 14-bit high speed DAC, 138 kHz analog low-pass filter, transmit attenuator, and an echo cancellation channel. The receiver channel consists of two programmable-gain amplifiers, a frequency equalizer, a 1.104-MHz low-pass analog filter, a 14-bit high speed ADC, and a 1.104-MHz low-pass digital filter. The clock circuit divides a 35.328-MHz frequency from an external VCXO down to the necessary frequencies used throughout the device. The frequency of the external VCXO is controlled by a 12-bit onboard voltage output DAC. An onboard reference circuit generates a 1.5-V reference for the converters.