Table of Contents
Advertisement
4.5.3. B
Variabl~
Programme
This programme (position "1" on the B & T PROGRAMME selector) allows the ratio (B/f)
to be kept constant at the switchover frequencies.
If the case of two resonances with the same
Q
factor is considered where
Q =
fo/ .D.f (.D.f
=
3-dB bandwidth of the resonance), it can be seen that the time required for sweeping
each resonance will be the same. If the Effective Averaging Time is kept constant, the re-
cordings of the resonances will be identical. The relative resolution will also be the
same.
4.6. POWER SPECTRAL DENSITY MEASUREMENTS
Analysis
in
terms of Power Spectral Density (PSD) is subject to the same considerations
as those for a conventional frequency analysis discussed in sections 4.3 and 4.4. With
the 2010, however, the conversion to units of PSD is simplified by use of the BAND-
WIDTH COMPENSATION push button and the special meter scale SA 0055 cali.brated in
V
2
/Hz.
Calibration for PSD measurements should be made using the 50 mV reference voltage of
the 2010, adjusting the appropriate SENS potentiometer to give a reading to the "Ref"
mark on the scale, 6 dB below full scale deflection. The PSD levels can then be read di-
rectly from the meter scale or the Level Recorder paper (provided the Level Recorder has
first been adjusted to give a pen deflection 6 dB below full scale with the 201 0 refer-
ence voltage selected).
In
selective measurements, the BANDWIDTH COMPENSATION push button must be in
its inner position, selecting the 1
/vB
attenuator to factor the meter and recorder levels
according to the filter bandwidth selected. The compensation is automatically changed
with the filter bandwidth
if
one of the BT programmes is selected.
Conversion to units of sound pressure or vibration can most easily be made by working
on the decibel scale. For example, if the original signal was measured with a micro-
phone of sensitivity -36,6 dB re 1 V/Pa and a meter deflection of -62 dB re 1 V
2
/Hz
is obtained, then the
PSD
level is:
PSD
-62
-(-36,6)
-25,4
dB re 1 Pa2/Hz
4.7. LOWER FREQUENCY LIMIT
54
In a selective analysis, the analysis bandwidth used imposes a lower limit to the frequen-
cies measurable without introducing measuring errors. In practice, analysis of frequen-
cies lower than one bandwidth is not recommended.
The first mixer receives two signals, the input signal at a frequency f, and the signal
from the VCO, fr-f. Assuming the mixer is balanced, two frequencies are obtained at
the output: fr and fr-2f. When f is sufficiently large, the signal fr-2f is filtered out and
does not appear in the reading. Conversely, if f approaches zero, fr-2f is not filtered
and a 3 dB error can be introduced into the reading.
Advertisement
Table of Contents
