BRUEL & KJAER 2305 - Page 120

creases an incomplete integr-ation takes place, thereby introducing an error
in the measured RMS level of the complete signal (e.g. the carrier frequency
plus the fluctuations in random noise signals).
Random Noise. From Fig. A.1 can be seen the effect of incomplete integra.
tion on recording the level of random noise bands. The error is plotted
against the noise bandwidth (narrower bandwidths correspond to fluctua-
tions of lower frequency). The obtained characteristics are independent of
the
band center-frequency ("carrier") as long as this frequency is higher
than the selected LOWER LIMITING FREQUENCY and l ower than the
highest limiting frequency of the Recorder. Five characteristics are shown,
one for each LOWER LIMITING FREQUENCY setting 2, 10, 20, 50, and 200
Hz (cis). If an error of 0.5 dB in the reading can be tolerated, it can be seen
from the figure that in the instance of the "2 Hz ( cis)" characteristic the true
RMS of the complete signal can be read corresponding to a bandwidth as
narrow as 10 Hz (ci s). (The voltage on the condensers in the Lower Limiting
Frequency circuit may be considered constant). Taking narrower bands than
10 Hz (cis) for the same position of LOWER LIMITING FREQUENCY the
+1
dB
I-- r-
-
L.L.
F= 2 1~ 20 5~ 20 ~
1 \1
~ Pk- ICI ~
::;;;.-
L-)
/
/'
\
~b( '
I>
..........-:
>-
V
I-f- V
V
K
.......----
I
J v
1- 1
~
IT
C-
o 0 "'--
--
theriY i l' lI' i
10 20
50
100 200
500 1000
2000 Hz 5000 10000
161 860
-Bandw idth of random noise
Fig. A.t. Curves illustrating the deviation ff'om tlle correct RMS level of a
comp lete random noise signal as function of the noise bandwidth. All five
settings
of LOWER LIMITING FREQUENCY 2, 10, 20, 50, and 200 Hz (cis) are
presented.
deviation from the true RMS reading of the complete signal will reach a
value of approximately - 0.6 dB*). (The voltage on the condensers in the
Lower Limiting Frequency circuit then cannot be considered constant) .
From the above can be seen that when the 50 and 75 dB Range Potentio-
meters are employed in the Recorder, the error in recording narrow bands
*) In this r egion the va ri ation of the RMS va lu e of the "c arri er " signal will be recorded,
provided that the "Writing Speed" is high enough to follow. (See next paragraph).
t 19