Campbell CR9000, Instruction Manual

Page: 71 / 218

Section 3. CR9000 Measurement Details
3-27
trigger must rise from below 1.5 volts to above 3.5 volts in order to trigger an
output. Due to the attenuation caused by the filter on the front side of the
Schmitt circuit, a larger input voltage transition is required for higher
frequencies. The transition required for the input of the Schmitt trigger can be
viewed as 2.5 volts
±
1 volt (from below 1.5 volt to above 3.5 volt). The
equation to calculate the amount that the signal is attenuated by the front end
filter is:
( ) ( )
( )
2
2 1
1
f
V
V
In
Out
πτ
+
=
V
Out
is the voltage level leaving the filter (level into the Schmitt circuit) when
V
In
is the input voltage. V
Out
must be at minimum 1 volt for the Schmitt
circuit to trigger an output.
Chart 3.6 Required Transition Voltage for High Frequency Pulse
Chart 3.6 plots the trace for the minimum transition voltage about 2.5 volts
against the input signal frequency. To demonstrate how to use this plot, for a
input frequency of 1 MHz, the voltage signal, centered about 2.5 volts, must
have a transition of
±
1.6 volts in order to trigger the Schmitt circuit. In other
words, the signal must rise from below 0.9 volts (2.5 volts minus 1.6 volts) to
above 4.1 volts (2.5 volts plus 1.6 volts) for a pulse to be counted.
0.6
0.8
1
1.2
1.4
1.6
0 100000 200000 300000 400000 500000 600000 700000 800000 900000 1000000 1100000
Signal Frequency ( Hz)
±V T
ran
si
sti
o
n
(Cen
tered
at 2.
5 Vo
lt
)
Vin
Required Input Voltage