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Capacitive transducer We know that : C=kЄ° (A/d) Where : K=dielectric constant Є° =8.854 *10^-12 D=distance between the plates A=the area over lapping plate Hence the area is different Since , the capacitance is inversely proportional to the spacing between the two plates , any variation causes a corresponding variation in the capacitance . A force apply to the diaphragm (DF): That functions as one plate of a simple capacitor changes the distance between (DF), and the static plate . The result of change in c is usually measured with an oscilloscope circuit . The transducer as a part of the oscilloscope circuit cause a change in the frequency of the oscillator . This change in frequency is a measure of the magnitude of the applied force . The capacitive transducers has an excellent is a measure of the magnitude of the applied force The capacitive transducer has an excellent frequency response it’s disadvantages are sensitivity to temperature variation and possibility of erratic distorted . Digital meters : The digital voltmeter displays measurement of AC or dc voltages as discrete values , instead of a pointer deflection , which reduces error of interpolation of a (Dvm ) and analog device . Some specification of (Dvm) “when you need to use” are : 1- input voltage range 2- accuracy as high as + or - 0.005% 3- stability 4- resolution or calibration So (dvm) are specified as : A) B) C) D) Ramp type Dvm integrating Dvm continues balance Dvm Successive approximation 1- ramp type Dvm : The Dvm is based on the measurement of time that takes for a linear ramp voltage to rise from zero volt to the level of the input voltage . (or the decrees from the level of the input voltage to zero volt ) The resultant time is measured with an electronic time interval counter , where this count is displayed as a number of digits , on the Dvm tube . As the gate open on oscillator generate clock pulses a counter will totalize the number of pulses passed through the gate which indicates the magnitude of the input voltage . There is also a sample rate – multi vibrate , which can be adjusted by front panel , controlling rate from few cycles per second , to as high as 1000 ‘cycle per sec’ and grater . It’s also either provides an initiating Pulses for the next ramp voltage , or generate a rest pulse which returns all the DCU : (Decade Counting Unit ) to their zero state . Signal analyzer : DC input voltage The first instrument to measure any set of spectrum content of signal was a device called “harmonic distortion analyzer. These early distortion analyzer measured total harmonic distortion , this is a problem , because it didn’t indicate which harmonic was responsible for the measured part of the signal . New analyzer called wave analyzer could separate harmonics distortions and evaluate each one . A periodic signal may consist of a sum of elemental sinusoidal . A plot of the Fourier coefficients as a function of the frequency index , displays the signal spectrum . The word “spectrum” implies that the variable (k) corresponds some how to the frequency . Ak One can find the spectrum from the signal ‘decomposition’ . The signal from the spectrum known as composition . k A wave analysis is an instrument designed to measure the relative amplitude of signal frequency component in complex or distorted wave forms . The instrument acts as a frequency acts as a frequency selection meter which is tuned to the frequency of one signal components while rejecting all other components . The wave to be analyzed in terms of it’s separate frequency component is applied to an input attenuator . The bandwidth (BW) of the instrument is very narrow and typically 1% of the . selected frequency . The wave analyzer is applied industrially in the field of reduction of sound and vibration generated by machines and appliances . The source of noise or vibration generated by a machine must first be identified before it can be eliminated . A fine spectrum analysis with wave analyzer will show varies discrete frequencies that can be related to motion within the machine . Relative response in (dB) 0 -20 -40 -60 -80 0.3 0.5 1 2 Normalized frequency Attenuation characteristic 3