Campbell Scientific CR9000X Measurement and Control System - Manual (Page 125)

Section 3. CR9000X Measurement Details
shift or delay into the stored data. For instance, if a POption of 2000 (2 second
average) were used on a vehicle speed measurement, and the vehicle came to a
sudden stop, the output from the instruction would stay at the frequency from
the last pulse edge for the 2 second running average interval after the vehicle
stopped. If an over-range condition occurs when the running averaging is in
use, the over-range value will be included in the average for the duration of the
averaging period (e.g., with a 1000 millisecond running average, the over-
range will be the value from the PulseCount instruction until 1 second has
passed.
3.4.2 CR9071E PulseCount Resolution
At the beginning of each scan, the CR9000X interrogates the accumulators'
registers for the number of pulses (N) since the previous scan and resets the
counters. The CR9071E also returns the time of the last pulse before the start
of the previous scan, as well as the time of the last pulse during the previous
scan. The CR9000X calculates the time period (P) between these edges with a
40 nanosecond resolution. It then calculates the frequency by dividing the
number (N) of pulses by the time period over which the pulses took place.
For example, refer to Figure 3.4.2-1. Let us assume that the Scan period is 1
mSec. At the beginning of Scan 3, The time (P)eriod between the falling edge
of the last pulse in Scan 1 and the last pulse in Scan 2 would be calculated (lets
say P = 1200 uSec). The (N)umber of edges, which equals 3,would be divided
by P. So we would get 3/(0.0012) to get a frequency result of 2.50 kHz.
4 2 1
P E R I O D ( P )
S C A N #
3
E 1 E 2 E 3
FIGURE 3.4.2-1. Frequency calculation for the CR9071E
The resolution of the CR9071E's PulseCount frequency option, rather than
being tied to the Scan interval or the duration of the instruction's running
average (POption parameter), is dependant on the input signal frequency and
the 40 nanosecond timing resolution. The resolution can be determined using
equation 3.4.2-1.
Eq. 3.4.2-1
)
) (
(
E
R
P P
E
R
FR
+ ×
=
where:
FR = Resolution of the frequency measurement (Hz)
R = Timing Resolution of the period measurement = 40 x 10 -9 seconds
P = Period of input signal (seconds); for a 1000 Hz signal P = 1/1000 = 0.001 S
3-37