The above noise waveform shows the simulated voltage noise on the output of a CCD buffer amplifier over a period of 650ms. The noise has a "white" or Gaussian component that has a flat spectrum. Added to this can be seen a "1/f" component that increases at lower frequencies. 1/f noise has the characteristic that the spectral power density in the waveform is proportional to the reciprocal of the frequency. Such noise is universal and also found on the linear buffer transistor in a CMOS-pixel unit cell. Other low frequency sources can add to the noise when reading a pixel voltage, for example 50Hz mains pickup and temperature coefficients in external signal processing electronics.
Gaussian noise can be reduced by increasing the time over which a signal is measured i.e. by reducing the bandwidth of the measurment. This does not work with 1/f noise where increasing the measurement time (in the case of an imaging system effectively reducing the frame rate) can actually increase the noise.