Trying to understand why LimeSDR mini RX goes silent

Hey all,

I’m working on a project that uses a LimeSDR mini to receive a GFSK modulated signal at 169MHz, with 15kHz channel bandwidth. The limeSDR mini is connected via USB 3.0 to a Raspberry Pi 4 running balena OS, and the gnuradio pipeline that processes de signal runs in a balena container.

The testing I’m doing involves having another device doing TX with 20dBm on the same channel, and connecting that to the limeSDR mini with an SMA cable and 80dB of attenuation (4 x 20dB attenuators).

The testing setup sends one message per second, and most of the times this goes all well. But for some reason, sometimes the limeSDR mini just goes silent and doesn’t receive anything. This situation is extremely random; sometimes it happens very quickly, sometimes it needs hours to happen. And when the limeSDR mini gets stuck in this state, the only way to recover it is via a full power off (or physical replug).

The interesting part is that if I add an extra 20dB attenuator, making it 100dB, the system is way more stable and doesn’t experience the issue. Using physical antennas instead of the cable and attenuators also works much better.

I also have another limeSDR mini, a much older and used/tested one, that showcases the same symptoms, but really with any kind of attenuation or even with antennas. I’ve deemed this specific device broken and not usable, but the fact that the fully new limeSDR mini shows the same symptoms in some scenarios makes me think they’re related.

What could be happening here? is the signal received after 4x20dB attenuators still too much for the device and it’s saturating RX somehow? Any other explanation that could fit this scenario?

P.S.: I’ve also tested the same with two different limeSDR USB devices and they both work as expected without any issue; this problem seems to be specific to the limeSDR mini.

This is odd because they both use the same RFIC and there are only matching networks between this and the RF connectors. Tagging @Zack in case he has any ideas.