Tektronix 11801C User Manual page 416

Appendix D: Algorithms
D 20
RMS
n*1
[W(j ) 1)
RMS +
[2(n * m)]
j+m
where:
m = index of left most measurement zone sample
n = index of right most measurement zone sample
W(j) = sampled data point
The summation extends over the interval of time corresponding to one
period when Data Interval is set to one period, or the entire measurement
zone when Data Interval is set to the entire zone.
Spectral Frequency
See Frequency Domain Measurements on page D 16.
See also Table D 2 on page D 16.
Harmonic Mode
1. Scan measurement zone on FFT magnitude waveform to find the high
est peak.
2. Determine the largest adjacent spectral component.
3. Using FFT window characteristics, find the fundamental components,
S( ) and S( ) 1) .
4. The nth frequency harmonic of the waveform is determined by multiply
ing the harmonic number by fundamental frequency, and then searching
the waveform for the nearest spectral peak. The interpolation algorithm
is applied again in order to prevent the increase of the error rate by the
harmonic number.
where:
| s( ) 1) |
, is the ratio of the two highest peaks
s +
| s( ) |
icoeff (0) s * icoeff (1)
D +
icoeff (2)
where icoeff(i) is an array of interpolation coefficients which depend on the
type of FFT windowing function (refer to the table on the next page).
fundamental frequency + ( ) D)
where fresolution is the frequency resolution.
) W(j) ]
2 2 1 2
1 2
, is the interpolation value.
s ) 1
fresolution
Appendices