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Input from
Output S.ction
Amplifier
Fig.5.5. Schematic of R.M.S. Rectifier Circuit
To
meter
d irect or
via Lin/log
Converter
instantaneous signal level for which the diodes 0
3 _ 5
will conduct. As each
diode conducts, the value of the averaging capacitor's charging resistance,
set by R
1 _ 4 ,
is reduced, causing the slope of the rectifier current v. instan-
taneous input voltage curve to increase. The magnitude of the resistors is
selected to form a parabola, which is the characteristic required from the
squaring circuit. Fig.5.6 illustrates this principle. As the diodes 0
3 _ 5
switch
in parallel resistors, so the slope of the inclined line will be increased in a
series of limbs to improve the approximation to the ideal
'
parabola at high
instantaneous values of the input voltage, e. The three diodes provide a total
of five limbs on the parabola approximation, permitting the detector to
measure signals with crest factors up to 5 with an accuracy of± 0.5 dB.
As the charge on the averaging capacitor varies so does the voltage across
the averaging capacitor. This alters the bias applied to the diodes, resulting
in a displacement of the RMS Rectifier's characteristic. The whole effect is
equivalent to a multiplication of the parabola's size by a constant factor
equal to that by which the charge on the averaging capacitor was changed.
From Fig.5.6 it can be seen that this corresponds to a root extraction
process, as the charging current is now proportional to the input voltage.
Consequently the RMS circuit's output will be linear even though the RMS
Rectifier's instantaneous voltage characteristic is a parabola.
At the same time as the averaging capacitor CA is charged a discharge
current proportional to the voltage across the averaging capacitor flows
through the averaging resistor R A. Therefore if the voltage on the averaging
capacitor remains constant for a period of time equal to or exceeding the
time constant of the averaging network (CA in parallel with RA) the charg-
ing current is proportional to the RMS value of the input voltage.
The averaging circuit of the 2010 is rather more complex than the
44
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