Spectrum 1746sc-CTR4, Owner'S Manual, Page: 31 / 66

Chapter 4: Channel Configuration, Data, and Status
31
Rate - Instant:
When this bit is reset to “0” the rate detection circuit operates in “Instant”
mode. Instant measurements are fast, in that they calculate a rate based
on one cycle. However the accuracy of the measurement degrades as
the input clock rate goes up to 50KHz, and any jitter within one cycle will
be measured.
Figure 4.3 - One Period Rate Measurement
One Period
The accuracy while operating in the average mode is ±1 count. When
using the Instant mode the accuracy is ±1% at 50kHz and improves to
±0.002% at 100Hz. The graph below shows the rate accuracy in Hertz vs.
Frequency.
Frequency Error
-50
0
50
100
150
200
250
0 10,000 20,000 30,000 40,000 50,000 60,000
Frequency
Error
Frequency Error
Frequency
Error
Input Rate
Error in Hz
Instant Rate Mode Error
Input Range: (Configuration Bits 4-5)
This group of 2 bits selects one of 4 input ranges. Each range is selected
for a given system voltage level. Each range has its own counter trip
level. Refer to the Specifications section of this manual for input limit
information.
Table 4.2 Input Range Selection Bits
Mode Bit Setting Range
AC 00 50 mV to 30 VAC
DC 01 0 to 5 VDC
DC 10 0 to 12 VDC
DC 11 0 to 24 VDC
Note: The input range must configured in channel pairs to operate
properly. Pairs are channels 0-1, 2-3, 4-5, 6-7.
Note: You should allow at least 1 scan time for input range
information to be updated at the PLC.