Measurement Computing CIO-DAS-TEMP - Manual (Page 11)

5.3 OPERATION OF THE COUNTERS
The 8254 counters 0 and 1 are used to make the frequency measurement of the V/F output. Counter 1 is
clocked by the 2 MHz clock source. A value loaded into counter 1 is used to count down to zero thereby
providing a precise time interval, known as the gate time. For example, if counter 1 were loaded with the
value 5,000 it would count down for 2.5 ms (5,000 / 2,000,000 = 0.0025 sec.).
While counter 1 is counting down, the output pin CTR 1 OUT goes high (a programmable feature
implemented in the Universal Library, or your software). This signal is used to gate counter 0.
Counter 0 is used to accumulate counts from the V/F converter. Recall that the V/F converter produces a
train of pulses proportional to the voltage on its input. At 0 volts the V/F produces zero Hz and at 10
volts, 2 MHz. If the V/F senses 5 volts on its input, it will output pulses at 1 MHz. If counter 1 has been
loaded with a value of 5,000, counter 0 will count pulses for 2.5 ms. 1,000,000 x 0.0025 = 2,500 pulses.
Note that the pulses counted from the V/F are exactly ½ the value loaded into counter 1; the time period
to count pulses. This is a general rule:
Voltage = 10 x (CTR 1 Load / CTR 0 Counts)
In general, a counter 1 period value of 5,000 is a g ood choice and w ill yield good results. The
relationship of degrees Kelvin to counts is:
Kelvin/Count = 500 / Full Scale Count
If the full scale count is 5,000 (as suggested) the relationship is:
500 / 5000 = 0.1K per count
5.4 LINE NOISE REJECTION
V/F converters will reject signal noise that is periodic and of a consistent am plitude because the V/F
converter averages the signal. Sinusoidal noise associated with line frequencies is an example. Since the
signal is averaged (equal areas over and under the mean cancel each other out) the measurement period
must be equal to one cycle, or a multiple of the cycle frequency if noise rejection is desired.
For example, if there is 60 Hz line noise on the signal, the measurement period would have to be:
1/60 = 0.1666
0.16666 x 2,000,000 = 33,333 counts Load this value or a multiple of it into counter 1
For 50 Hz, the calculations are similar. 1/50 x 2,000,000 = 0.02 x 2,000,000 = 40,000
5.4.1 Gate-Time, Resolution and Noise Rejection
The choice of gate time has an affect on the resolution of the measurement, line cycle noise rejection and
the rate over which multiple channels can be scanned. Increasing gate time improves both resolution and
noise rejection but it will decrease the scanning (multiple channel measurement) rate.
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