AN3322 datasheet(28/38 Pages) STMICROELECTRONICS

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Computational issues and compensation algorithms

AN3322

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DocID18304 Rev 1

After detection of the zero crossing position i, the finer zero crossing is computed according

to Figure 15, Equation 29 and Equation 30.

Figure 15. Finer calculation of zero crossing

Equation 29

di = bi / (abs. ai + abs. bi)

Equation 30

c + d = 1

where ai is the amplitude of the signal prior to a zero crossing, and bi is the amplitude

of the signal after the zero crossing. The sub index position di computed is in the range from

0 to 1. The difference of the two subsequent zero crossings is di - di+1.

The value Tupdate that can be added or subtracted from the trigger timer reload constant

must be an integer number bigger that one. The difference can be divided into 20 steps by

the rounding of Tupdate = [20 x (di - di+1)]. This leads to a division of the position index of the

zero crossing to 20 sub values.

The trigger timer reload constant is updated by this difference to stabilize the zero crossing

in this finer position. This makes initial locking faster (by factor 20).

As described in Section 2.6, the signal used for zero crossing detection is filtered so that

only fundamental frequency is preset. This avoids errors when computing the zero crossing

position finer that one sample index.

2.5

Frequency measurement

The frequency value is derived from the coherent sampling trigger period. The sampling

trigger period is measured using a precise external low speed oscillator (LSE). From that

value the frequency is computed.

In order to get a higher resolution of the measured frequency value, more values must be

accumulated and averaged. The accumulation of ten values brings the resolution that is

sufficient and can be displayed.

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