7-12
|
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Chapter 7
Counters
Low Frequency with One Counter
For low frequency measurements with one counter, you measure one period of your signal using
a known timebase.
You can route the signal to measure (
fx
) to the Gate of a counter. You can route a known timebase
(
fk
) to the Source of the counter. The known timebase can be an onboard timebase, such as
100 MHz Timebase, 20 MHz Timebase, or 100 kHz Timebase, or any other signal with a known
rate.
You can configure the counter to measure one period of the gate signal. The frequency of
fx
is
the inverse of the period. Figure 7-12 illustrates this method.
Figure 7-12.
Low Frequency with One Counter
High Frequency with Two Counters
For high frequency measurements with two counters, you measure one pulse of a known width
using your signal and derive the frequency of your signal from the result.
Note
Counter 0 is always paired with Counter 1. Counter 2 is always paired with
Counter 3.
In this method, you route a pulse of known duration (
T
) to the Gate of a counter. You can
generate the pulse using a second counter. You can also generate the pulse externally and connect
it to a PFI or RTSI terminal. You only need to use one counter if you generate the pulse
externally.
Route the signal to measure (
fx
) to the Source of the counter. Configure the counter for a single
pulse-width measurement. If you measure the width of pulse
T
to be
N
periods of
fx
, the
frequency of
fx
is
N
/
T
.
fx
fk
G
a
te
S
o
u
rce
1
2
3
…
N
S
ingle Period
Me
asu
rement
…
Period of
fx
=
N
Fre
qu
ency of
fx
=
N
Interv
a
l Me
asu
red
fk
fk
fk
fx
Summary of Contents for DAQ X NI 634 Series
Page 1: ...PXIe 6349...
