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Rejerenct <br />r~ <br />~J <br /> Peak PamUe Veixuy <br />~ <br />0 <br />~. <br /> Tm~ <br /> Y~~1 <br /> T, Zero Cresslnp <br />Figure 5.10 Calculating the Zera Crossing <br />Frequency. <br />(2) Zero Crossing Frequency Limitation <br />The Zero Crossing Frequency calculation is limited because it azsumes a single predominant <br />frequency at the peak, typically represented by sinusoidal waveforms. In practice, the peak may be <br />the result of two or more major frequency components representing compound waveforms az <br />illustrated in the figure below. It is therefore only an approximation of the frequency of the Peak <br />Panicle Velocity. <br />Waveforms may have the same Peak Panicle Velocities but different Zero Crossing Frequencies <br />depending on the shape of the waveforms involved. With reference [o the figures above and below; <br />both waveforms have the same Peak Panicle Velocities however their Zero Crossing Frequencies <br />differ. In the figure above, the zero crossing frequency uscs the 1f2 period indicated by Tt• In the <br />figure below, the zero crossing frequency uscs the 1/2 period indicated by Tp. Notice that Tr is less <br />• than T= because of the different waveform shapes, therefore the Zero Crossing Frequency in figure <br />above is greater than the Zero Crossing Frequency in the figure below. It is for this reazon, the <br />Zero Crossing Frequency may differ for peaks having the same Peak Particle Velocity. <br /> <br />~. / Peak Parlide Vebcay <br />/ <br /> <br />Tr Tkn <br />Zaro Cnssinp <br />Figure 5.11 Zero Crossing Frequenty Calculation <br />Limitation. <br />• <br />(3) Sample Rate Error <br />The Zero Crossing Frequency requires the period of a wavelength before it can calculate the <br />wavelength's frequency using the formula 1/period. A sampling error occurs for higher frequencies <br />when wavelength periods become relatively small and the sampling rate begins to miss zero <br />crossing points. In other words, the wavelength periods occur much faster than a BlaztMate III can <br />sample and use in the calculation. <br />At higher frequencies there aze fewer sample paints per cycle and therefore greater error. The <br />following table illustrates how error increases with frequency. <br />B1aztMate III Operator Manual 5-9 <br />