Turning a Cable Modem into an SDR

In this really interesting and well-documented post, the author stdw, explains how he reverse-engineered a cable modem and turned it into a software-defined radio.

The cable modem stdw reverse-engineered was the Motorola MB7220. The very versatile Raspberry Pi was used to analyze the serial console. The functionality of the SDR was demonstrated by capturing and demodulating FM signals at 100 MHz.

The author was able to achieve streaming bandwidth of just under 500 kHz which is much less than that achievable by the RTL-SDR at around 2.4 MHz. The DOCSIS standard specifies that each downlink channel can be up to 8 MHz wide so the Motorola MB7220 is in fact capable of processing much wider bandwidths. The frequency range for the DOCSIS downlink is between 50 MHz and under 1 GHz so it’s narrower than the RTL-SDR.

All in all a very interesting post and we love the process that the author has documented very clearly.