National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 8 Counters
NI USB-621x User Manual 8-10 ni.com
Yo u can config u re the co u nter to meas u re one period of the gate signal.
The freq u ency of F1 is the inverse of the period. Fig u re 8-10 ill u strates
method 1.
Figure 8-10. Method 1
• Method 1b: Measure Low Frequency with One Counter
(Averaged)
—In this method, yo u meas u re several periods of yo u r
signal u sing a known timebase. This method is good for low to medi u m
freq u ency signals.
Yo u can ro u te the signal to meas u re (F1) to the Gate of a co u nter. Yo u
can ro u te a known timebase (Ft) to the So u rce of the co u nter. The
known timebase can be 80MHzTimebase. For signals that might be
slower than 0.02 Hz,
u se a slower known timebase.
Yo u can config u re the co u nter to make K + 1 b u ffered period
meas u rements. Recall that the first period meas u rement in the b u ffer
sho u ld be discarded.
F1
Ft
F1
Ft
G a te
S o u rce
1 2 3 … N
S ingle Period
Me asu rement
…
Period of F1 =
N
Ft
Freq u ency of F1 =
N
Ft
Interv a l Me asu red