Cradlepoint Atrocities: SDR as a Modem

So you may know I have a bit of a Cradlepoint obsession by now. In doing some of my hardware hacking, I realized that the MC400 expansion modems are pretty much “just” a USB carrier for a mPCIe modem. Meaning, it exposes regular ole USB signals to the modem for us to play with.

My mind was running wild, and I made the first atrocity my mind came up with: an RTLSDR in an MC400 modem slot. Theoretically, this was going to be pretty easy - remove the SMA and USB connectors off of an RTL-SDR, cut the traces on the modem board for the USB connector, solder fly wires to the RTL for USB, and re-solder the coax directly to the RTL. Turned out it was just that easy, and worked the first time!

The MC400-LPE I have (there are multiple revisions - LPE, LP6, 600M, 1200M, etc) has an old Sierra MC7345 modem so I didn’t feel to bad tearing it apart. There was a ferrite choke on the data lines right by the USB connector so I used my hot air and torched it off rather than cutting the traces with an x-acto or something. Then, I used a combination of solder wick and flush cutters to remove the SMA and USB connector from the RTL board. From there, I soldered on some short lengths of silicone 22awg wires for the USB lines, trying to length match the USB data pairs as best I could.

Then I cut the u.fl off of the coax going to the MAIN antenna connector and soldered it directly to the SMA connector of the RTL. I know it’s not perfect, and I suspect if I were to take shots with my VNA it would look like a nightmare, but it does in fact work!

Connecting it to the Cradlepoint directly I was able to see the RTL2832, so I was in business. I might try to make a board that has a Zigbee, Matter/Thread, and Bluetooth radio in it (for the Main/Aux/GPS connectors) for an all-in-one HomeAssistant hub. Stay tuned!