Most of the time:
Schematics and PCBs for systems with data paths up to 10Gbps,
High-speed digital design,
Analogue,
PSUs,
FPGAs, etc.

Some of the time:
Data links to 26Gbps, from multiple ADCs, via optical,
Signal integrity,
Power integrity.

The rest of the time (the fun bits):
Analysis of the Space-radiation environment,
measuring the effects of high-energy particles on semiconductors.