However, any system will have a frequency response function associated with it and will
amplify some frequency components and attenuate others. So in some sense all systems are
filters. A thermocouple behaves as a first order system. If we plot its frequency response we
will see that the magnitude becomes very small at high frequencies. So this is an example of a
system that removes high frequencies from the signal. Very fast temperature fluctuations may
not be measured with a thermocouple. We say that the thermocouple behaves like a low pass
filter; it only allows low frequency components in signals to pass through. Some systems
behave like high pass filters, they attenuate low frequencies and pass high frequencies. Some
accelerometers made of quartz crystal have a frequency response that is small at very low
frequencies, is flat for a range of frequencies, peaks in the region of the natural frequency of
the crystal and then becomes very small at high frequencies. This type of system is a second
order system combined with a high pass filter.