34
DC Average
AC Peak-to-Peak
DC
+
AC Peak
Figure 14 – Time Varying Signal Measurements - Many different measure-
ments can be made on different portions of a time varying signal. The most
common are: DC power, peak power, and peak-to-peak power.
The Model 2835-C can only measure periodic AC peak-to-peak signals within
the frequency range: 50 - 7,000 Hz. Above 7,000 Hz, bandwidth limitations
significantly effect the accuracy of the peak-to-peak measurement. Below 50
Hz, the AC coupling attenuates signals and measurement accuracy. However,
low frequency measurements can be quantified in DC CONT measurement
mode, see note below.
NOTE
Peak-to-peak measurements of frequencies below 50 Hz can be accomplished
by observing the MAX-MIN value of the STATS buffer in DC CONT measure-
ment mode, Sections 2.3.13 and 2.3.15. To do so, disable the analog filter,
adjust the sample rate, Section 2.4.7 and the STATS buffer depth, Section
2.3.15, so that the actual maximum and minimum values will be captured by
the stream of DC measurements.
3.8
Integration of Detector Signals
The Model 2835-C provides for making measurements that integrate incoming
power detector signals to obtain an energy via the INTG mode, Section 2.3.13.
In INTG mode, the display units indicate Joules since energy is the time
integral of power:
E
P
dt
S t
R
dt
t
t
t
t
λ
λ
λ
( )
=
( )
=
( )
( )
∫
∫
0
1
0
1
As the detector signal actually consists of a stream of digitized values, the
integral becomes a numerical approximation using the trapezoid method,
Figure 15. In order to maintain the highest accuracy for this numerical
process, the sample rate is set at its maximum value of 400 Hz.
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