BRUEL & KJAER 2305 - Page 8

back the lower limiting frequency of the amplifier is very easy to change.
Becaus e of the feedback arrangements the DC amplification of the ampli-
fier is approximately 270, while the AC amplification is in the region of
4500.
Rectifier Response Circuit. The rectification circuit is the specially designed
Briiel & Kj!er type, which can be switched to me as ure the true RMS, the
arithmetical average and half the peak-to-peak value of the input signal·).
A bridge-type full-wave rectifier for detection of the signal allows both the
positive and negative parts of the input signal to be treated in the Recorder .
The three different rectifier characteristics can be selected by the switch
marked "Rectifier Response". The last position of Ihis switch is marked
DC, when in this position, peak detection of the chopped DC Recorder input
signal takes place .
The Rectifier Response networks are symmetrically arranged with respect
to ground, whereby the output signal contains no superimposed fundamental
AC component . This type of circuitry is also advantageous because of its
insensitivity to mains voltage fluctuations.
The amplifiers and Ihe Rectifier Response networks have been designed 10
measure the true RMS value of signals with crest factors up to 5 (crest
factor = peak voltage / RMS voltage). Correct measurements of arithmetical
average values are possible for signals with the product: form factor X crest
factor = 5, i.e. peak voltage / average voltage = 5. The peak rectifier
measures half peak-to-peak value which should Le horne in mind when
unsymmetrical signals are being measured. Further, as a certain amount
of energy is dissipated within the rectifier circuit, the true peak value of
very narrow pulses cannot be correctly measured . It has therefore been
found convenient to calibrate the peak rectifier to accurately measure
sinusoidal signals, whereby the peak values measured on square-wave signals
(crest factor = 1) and very narrow pulses will in the first instance be slightly
too high a nd in the second too low . Refer also to Appendix.
When non-sinusoidal signals containing important frequency components in
the neighbourhood of the lower or upper limiting frequency of the recorder
are being measured, the recorded peak level will be incorrect, due to phase
distortion in the amplifiers . The recorded average level will only be in-
fluenced to a very small degree by the phase shift, whereas the RMS
recording is Independent of any phase distortion. Curves concerning this are
found in the Appendix.
Lower Limiting Frequency Circuit. Following the Rectifier Response circuit
is the Lower Limiting Frequency circuit, which allows for a first averaging
of the level of the measured signal. (Refer part Appendix in this book).
*) A th o rou gh description of the functi oni ng of thi s circuit is given in the B & K Tec hnic al
Review No. 3- 1958, 4-1960 a nd 1-1 96 1.
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