BRUEL & KJAER 2305 - Appendix

Appendix
Particular Recording Characteristics of Potentiometer
Type Recorders.
Many physical quantities which follow the laws of nature will be more or
less
randomly distributed around a mean, therefore the fluctuations present
in the measured quantities have to be taken into consideration . Dependent
on the type of investigation it may be of interest to obtain details of the
fluctuations or to smooth them out to a lesser or greater degree in relation
to a certain characteristic. The following, therefore, deals with the averaging
time characteristics of the Level Hecorder when switched to measure the
RMS va lue of a fluctu ating input signal. Only the RMS condition of the
Level Recorder is discussed as this normally is the one of greatest interest
when recording and inve s tigating complex signals . The fluctuations, which
it may be of benefit to analyse in detail will mostly be those of the lower
frequencies, for in such cases as noise investigations the averaging time of
the human ear will limit the distinguishing of fluctuations comprising the
higher frequencies .
When looking at ~he various types of signals which in practice will be
recorde$l by the Level Recorder, one comes to the conclusion that bands of
random noise will cover most properties of such signals. *) If a wide band
of white random noise is taken the low-frequency fluctuations will contain
relatively little energy alild will therefore be of little or no interest.
By making the band of the random noise narrower, the significance of the
low-frequency fluctuations will be increased. The narrow band of white
random noise can be looked upon as an amplitude modulated signal where
the carrier frequency is equal to the center frequency of the band and where
the modulating signal (fluctuations) cont ai ns frequenci es from zero to that
of the bandwidth (presuming that the filter is id ea l ).
Two circuits in the Level Recorder which display integrating effect mainly
determine the possibility of detailing or smoothing out fluctuations in the
signal under measurem en t. Th ese circuits are the LOWER LIMITING FRE-
QUENCY and th e WRITING SPEED circuits.
Lower Limiting Frequency.
This circuit caters for the first integration of the signal and averages the
" higher frequency" fluctuations. As the frequency of the fluctuations de·
*) See B &: K Technical Review No. 4-1960.
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