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Latetly we recognized a parasitic signal in our voltage measurements. It is a perfect sine wave with a period of a few hours (f = uHz). What could be the origin of such a signals?

Ok, a few more details:

We do Hall-Effect measurements in a cryostat. The temperature is held constant at a few hundred mili Kelvin. We use a standart six terminal measurement to obtain the hall resistance and the magnetoresistance. We use standart lock-in amplifiers at a detection frequency of ca 22 Hz to measure the voltage along and transverse the current path through the sample.

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    \$\begingroup\$ How about you tell us the signal size and actual frequency. Also how you are measuring it. \$\endgroup\$ – Andy aka Dec 2 '13 at 11:28
  • \$\begingroup\$ Also, what is your measurement frequency? If it is very close to 50 or 60 Hz there is potential for aliasing, though aliasing with AC mains is unlikely to resemble a pure frequency! \$\endgroup\$ – Brian Drummond Dec 2 '13 at 11:50
  • \$\begingroup\$ Temperature is the usual culprit. Even though you are using a cryostat, do you have any wires connected from the outside? It could also be from light/windows or perhaps tidal forces if you are measuring something extremely sensitive (oceanservice.noaa.gov/education/kits/tides/media/…). \$\endgroup\$ – Justin Dec 2 '13 at 16:14
  • \$\begingroup\$ @justin No we are in a air conditioned lab. Temperature is quite stable, especially in the cryostat(< mK).. Also we don't see this behaviour in other cryostats in this lab measuring similar stuff. \$\endgroup\$ – P3trus Dec 2 '13 at 16:36
  • \$\begingroup\$ Is your temperature stable to less than a millikelvin (like you say in comments) or less than a few hundred millikelvin (like you say in the question)? Seeing effects from 100's of mK is not too suprising. \$\endgroup\$ – The Photon Dec 2 '13 at 17:04
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Speculation: a temperature dependency of the voltage measurement instrument, the circuit, or the voltage source. Disprove the speculation by placing the system in a temperature controlled enclosure.

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    \$\begingroup\$ Or by deliberately manipulating the environment at a known rate. \$\endgroup\$ – Brian Drummond Dec 2 '13 at 11:47

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