I acquired a Keithley 238 SMU a while ago, which appeared to work well before I connect it to a scope. It seems it's somehow generating way more noise at the output then the specification (15 mVpp DC-20M), regardless with whether it's in standby or CV operation. And interestingly, a majority of the noise are spikes that have a regular pattern, appearing "semi-randomly" on a grid of integer microseconds. The FFT in attached screenshots confirms this. The appearance of the spikes is also correlated with the CPU activity, when it enters the memory test, the noise shows reproducible patterns.
Does anyone have seen a similar problem, have any clue on the cause or can give some generic suggestions on troubleshooting? Failing decoupling capacitors somewhere?