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DE-M060304E • DEWE-ORION-1616-10x • Technical Reference Manual • Printing version 1.2.0 • March 19, 2014
DEWE-ORION-1616-10x
3.2.1.10 Frequency Measurement
In general it is possible to take the inverse of a period measurement to get the frequency of the input signal.
If the period time measurement is done an inaccuracy of counted internal time base cycles of ±1 cycle ap-
pears, because the counted cycles of the internal time base depends on the phase of the input signal with re-
spect to the internal time base. For long period times, and therewith low frequencies, the measurement error
is negligible. At high frequencies, and therewith short period times, few cycles are counted. In this case the
error of ±1 cycle becomes significant.
Input Frequency
Number of internal
time base cycles
Measurement error
of -1 cycle
Measurement error
of +1 cycle
Calculated
frequency with error
of -1 cycle
Calculated
frequency with error
of +1 cycle
40 kHz
2000
1999
2001
39,98 kHz
40,02 kHz
10 MHz
8
7
9
8,75 MHz
11,25 MHz
Accuracy at period time measurement
For higher precision result the frequency measurement is done with two counters. At the ORION-EXP-CNT8-
TTL/ADJ in each case two counters are paired, i.e. it have to be used counter 0 and counter 1 or counter 2
and counter 3 or counter 4 and counter 5 or counter 6 and 7 for the frequency measurement. The first coun
-
ter counts the rising edges on
Counter Source
. The second counter counts the rising edges of the internal
time base. At every rising edge on
Counter Source
the counter value of the second counter is stored in a
register. At every
Sample Clock
(
1
,
2
, …,
6
)
the values of both counters are read out.
2
3
4
1
5
6
7
8
0
0
2
185
185
30
15
60
220
5
30
6
15
7
60
7
0
1
2
3
4
5
6
Sample Clock
Counter Source
Counter
(First Counter)
Value
- Counter
0
2.3
5.8
6.3
7.3
7.9
Recalculated
measurement result
- Event counting
Synchronized
counter result
- 80 MHzCounter
Two edge separation
(Second Counter)
Figure 23: Frequency Measurement
With these both measurement results not only the frequency can be calculated in a precise way. Also the
event counter result can be show in fractions because the exact time when the event occurs at the input is
known. The event counting result is recalculated with interpolation to the exact sample point like shown in the
diagram above.
On the next page, the difference of the measurement result is shown. While a standard counter input shows
the value up to one sample delayed, the counter input of the ORION calculates the counter result at the
exact sample point.
Summary of Contents for DEWE-ORION-1616-10 Series
Page 56: ...C2 Notes...
