Satellite Modem

Overview

A satellite modem is the indoor unit of a VSAT terminal: it turns IP packets into a radio carrier the dish can transmit, and turns the faint carrier coming back down from the satellite into packets again. The hard part is that the received signal is weak and Doppler-shifted, and the satellite link is expensive, so the modem leans on heavy forward-error-correction and adaptive coding to wring every usable bit out of the channel. It connects to the outdoor electronics over a single coax cable that carries the L-band signal, a reference tone, and DC power all at once.

How it works

On receive, the L-band carrier comes in on the Port Module coax and enters the L-Band RF Front-End. A Low-Noise Amplifier sets the noise figure, then the Down-Converter mixes the carrier down to an intermediate frequency using a tone from the Frequency Synthesizer, with RF Filter stages rejecting images and out-of-band energy. The conditioned IF reaches the Modem Mainboard, where the IF ADC digitizes it and the Signal-Processing FPGA runs the demodulator: it recovers carrier and symbol timing, demaps the constellation, and decodes the LDPC/BCH code to recover the original bits. The Compute SoC Module reassembles those into IP frames and pushes them onto the LAN.

Transmit runs the chain in reverse: the SoC frames outbound packets, the IF DAC generates the IF waveform, the Up-Converter translates it to L-band, and the Driver Amplifier feeds it out to the dish-mounted up-converter.

Everything hinges on frequency stability, so the Reference Oscillator disciplines the synthesizers and sample clocks. The Front Panel reports carrier lock and signal quality, and the modem runs from its internal Power Supply.