Cirrus Logic AN336 User Manual
An336, Introduction, Afc - automatic frequency calculation
Copyright
© Cirrus Logic, Inc. 2008
(All Rights Reserved)
CS5463: Using the Epsilon Register & AFC Function in
Reactive Power and Line Frequency Measurements
1. INTRODUCTION
In the CS5463, average reactive power (Qavg) is generated by averaging the voltage multiplied by the current with
a 90° phase shift between them. The 90° phase shift is realized by a digital filter. This filter will give exactly -90°
phase shift across all frequencies, and utilizes Epsilon (
ε
) to achieve unity gain at the line frequency. To attain ac-
curate Qavg result over line-frequency variation, the Epsilon register has to be updated in accord with the line fre-
quency.
Figure 1. Average Reactive Power Data Flow
2. THE RELATIONSHIP BETWEEN THE EPSILON REGISTER AND LINE
FREQUENCY
The value in the Epsilon register is the ratio of the line frequency to the output word rate (OWR).
Epsilon = Line Frequency / OWR
By default, Epsilon is set as 0x1999F or 0.0125 decimal for 50 Hz line frequency and 4000 Hz OWR. If the
CS5463 is used in different line and OWR frequencies, Epsilon must be reconfigured accordingly. For ex-
ample, if the line frequency is 60 Hz and OWR is 4000 Hz, Epsilon should be reconfigured as
60 / 4000 = 0.015.
3. AFC - AUTOMATIC FREQUENCY CALCULATION
Epsilon can be calculated automatically by the CS5463 by setting the AFC bit in the Operational Mode
(Mode) register. The Frequency Update bit (FUP) in the Status register is set every time the Epsilon register
has been automatically updated.
To achieve 0.1% resolution, the Epsilon update period is fixed at 1000 zero-crossings or 10 seconds for
50 Hz line frequency. In other words, the delay between the line frequency change and the correct Epsilon
output will be around 20 seconds (worst case) for a 50 Hz system.
-90° Phase
Shift
π × Epsilon
V[n] from V -HPF
I[n] from I -HPF
Q[n]
÷N
Σ
N
Q
AVG
AN336
DECEMBER '08
AN336REV1