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RESOLUTION WITH AN FM SIGNAL

TAT designs use vibrating quartz crystals, which produce an FM signal. Most loadcells use an analog strain gage, which produces an analog load signal.

In theory an analog signal which has no noise has virtually infinite resolution where resolution is defined as the smallest detectable signal change. (Refer to the tech note on accuracy versus resolution.) With an analog signal, noise is the practical limit of true resolution. Unfortunately, the analog signal directly out of an analog transducer is very low in voltage. That voltage is generally millivolts full scale. Such a small signal must be amplified and digitized to be used in the digital world. The digitization is also noise sensitive and a digitized analog signal has a noise limited resolution which cannot be gotten below with longer digital signal sampling and signal averaging times. Internal electronic noise filters can help but there is still a lower limit to resolution.

With an FM signal, on the other hand, the longer the sample time, the lower the noise and the lower the resolution. Simply counting the pulses in an FM signal over a time period is essentially an integration process and integration reduces noise. For that reason, long sample periods with FM signals allow very high resolution in a digital signal without introducing electronics which are significant sources of signal instability over time and temperature.

 

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