Buffered Time traces with SR830 Lockins

[paulsurrey]

The class that implements the logic for a buffered time trace requires a timestamp dimension. But the SR830 do not return such a dimension.
(The SR830 are the fixed Lockins for one of our measurement stations.)

[THuckemann]

I guess one can get around that by calculating the timestamps from the sampling rate and adding them manually.
I'll try to fix it next week.