Meander line patterned infrared detectors, with values of resistanceRonsetat the onset temperatureTconsetof 3–5 k&OHgr;, were fabricated from YBa2Cu3O7−xsuperconducting material on MgO and SrTiO3crystalline substrates. Noise voltages from the samples were measured versus bias current, radiation modulation frequency, and temperature, in both the normal and superconducting states. Four major types of voltage noise were identified according to where they occurred in temperature relative toTconsetand the zero resistance temperatureTczero, and their dependence on frequency and bias current. They were also associated with the granularity of the superconducting film, which is related to the substrate material used. From these observations a specific cause for each type of noise is suggested. The results are as follows. (i) In the normal state with temperatureT≳Tconset, noise with a magnitude that is consistent with thermal (Johnson) noise is seen, but it depends linearly on bias current above a threshold value, at low frequencies.The suggested noise source is conductivity fluctuations due to Cooper pairs. (ii) Noise was found to occur belowTczeroin granular films. With increasing bias current its magnitude increases, and it shifts to a lower temperature range; however, the noise magnitude becomes constant as the current goes to zero. It is weakly dependent on frequency above 400 Hz. Suggested cause is voltage fluctuations in superconductor–normal–superconductor junctions at grain boundaries. (iii) This noise also occurs belowTczerowith peaks at various temperatures. With increasing bias current the peaked noise spreads to lower temperatures, but the noise goes to zero as the bias current goes to zero. Its suggested cause is magnetic flux tube motion. (iv) This noise occurs betweenTconsetandTczeroand is present in all samples, but lowest on samples prepared on SrTiO3substrates. Its suggested cause is fluctuations in the volume fraction of the superconducting phase along the current path. While the measured detectivityD* of our samples at a wavelength of 20 &mgr;m was only 106cm Hz1/2/W, engineering changes can be expected to raise the value to above 1010cm Hz1/2/W. ©1996 American Institute of Physics.