App-24
IM 701240-01E
Appendix 11 Measurement Principles (Measurement
Method and Update Rate) of the
Frequency Module
Measurement Principles of the Frequency Module
The measurement principles of period, frequency, pulse width, and duty cycle on the
frequency module (701280 (FREQ)) are described below.
Period and Frequency Measurement
The frequency module updates the waveform at a rate of 25 kHz (40-
µ
s interval). The
measurement method differs for frequencies above 25 kHz and below 25 kHz.
• When the input signal is less than or equal to 25 kHz, measurement is made as
described in (1).
• When the input signal is greater than or equal to 25 kHz, measurement is made as
described in (2).
• The sequence of processing described below is performed simultaneously through
pipeline processing. Thus, the period (t) and frequency (f) are updated every 40
µ
s.
(1)When the input signal is less than or equal to 25 kHz
Measures the time of occurrence of the pulse edge (T1) in measurement interval A.
Measures the time of occurrence of the pulse edge (T2) in measurement interval B.
Calculates the period (t) = T2 – T1 in measurement interval C.
The frequency (f) is calculated as 1/period (t).
When the period of the input pulse spans over multiple measurement intervals, computation is
performed at the measurement interval following the interval in which the edge is detected.
Input pulse
Measurement
interval: 40
µ
s
Time: T1
Time: T2
A
B
Period (t)
Period (t) = T2–T1
Frequency (f) = 1/period (t)
40
µ
s
C
40
µ
s
(2)When the input signal is greater than or equal to 25 kHz
Measures the time of occurrence of the last pulse edge (Te1) in measurement interval A.
Measures the time of occurrence of the last pulse edge (Te2) in measurement interval B.
Measures the number of pulses between the last pulse edge in measurement interval
A and the last pulse edge of measurement interval B.
Calculates the period (t) = (Te2 – Te1)/the number of pulses in measurement interval C.
Period (t) is the average value of multiple pulses.
The frequency (f) is calculated as 1/period (t).
If the input pulse period is short, the SL1400 automatically takes the average of
multiple pulses and calculates the period and frequency. Therefore, the resolution does not
degrade even when the input pulse period is short, and highly accurate measurement is
possible.
Time: Te1
Input pulse
Period (t) = (Te2 – Te1)/3
Frequency (f) = 1/period (t)
Number of pulses: 3
Time: Te2
Measurement
interval: 40
µ
s
A
B
40
µ
s
C
40
µ
s