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principle of squaring, averaging, and root extracting sections, as illustrated
in Fig.5.4.a. The Bruel & Kjcer RMS detectors, however, modify this general
principle by feeding back the voltage on the averaging capacitor to produce
a "variable squaring" characteristic, as in Fig.5.4b, removing the necessity
for a separate square root operation. The principle of the circuit and its
difference from conventional R MS detectors is discussed in the Bri.iel &
Kjcer Technical Review, 1969 No. 1 ("Impulse Noise Measurements" by
C. G. Wahrmann). The effective averaging time of the RMS circuit is approx-
imately equal to the RC time constant of the detector.
a
b
Input
Signal
Input
Signal
u
=
Ke
2
ill
S9uaring
Variable
~uaring
u
~
=;r;
Averaging
Averaging
Condenser
v
Fig.5.4. Principles of R.M.S. Rectifier Circuit
Meter
Meter
170326
A schematic diagram to illustrate the principle of the RMS detector of
the 2010 is shown in Fig.5.5. At the input of the detector the inverted AC
signal from the Output Section is amplified by 20 dB and split into two
components, one in phase with the Output Section signal and the other
180° out of phase. These two signals are then applied to the two rectifying
diodes 0
1
and 0
2
of the RMS Rectifier ( Fig.5.5) which are biased off by a
voltage on the averaging capacitor CA. When the instantaneous signal level
at either of the rectifier inputs exceeds the level of the averaging capacitor
bias, the diode 0
1
or 0
2
will conduct. If the instantaneous signal level is
increased further, the rectifier current will rise linearily to charge the averag-
ing capacitor via the resistor R
1
and to raise the voltage levels at the junc-
tions of the resistance chain. R
5 _ 8 .
The resistance chain determines the
43
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