BRUEL & KJAER 2305 User guide

Level Recorder Type 2305
AC and DC level Recording.
logarithmic and linear Recording.
.polar and Rectilinear Recording,
RMS, Average and Peak Recording.
50 and 100 mm Width of Recording Paper.
Ink and Sapphire Registering.
High Writing
Speed and Paper Speed.
Synchronous Drive with Scanning of
B & K Oscillators, Spectrometers, Analyzers and
Filter Sets.
BAUEL& K~AC.R
N~RUM , DENMARK. Teleph .: 800500 . Cable : 8RUKJA. Telex: 5316. COPEN':tAGEN
B B 230 5 - 0300036

Level Recorder
Type 2305 .
Reprint december 1967

Contents
Page
Description 4
Controls, Terminals and Mechanical Drives . ......... . 21
Operation . .... ... . . .. . . . ..... . .. .... . ...... . . ... ... 30
Accessories. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Combined Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Appendix ..... ... ....... ... .... . . .. . . ... . ... .. .. . ... 118
Specification . . . . ... . ... . . ... . . . ...... .... . .. . ....... 125

Description
General
Ii
Electronic System . ... .. . . . . ..... . .. ... . . . . . ... .... ..... ...... ........ 6
Input Circuit ... .. ........ .. ... .. ............ ............ .. . ... . . 6
Range Potentiometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Chopper . ..... . ... .. . .. . .. . .... ..... .. . ... . ..... ... .. .... ... ..... 6
A. C. Input Amplifier .. . ....... . . ... . ...... ........ .. . . ....... . .. 6
Rectifier Response Circuit ......... .. ... .. . . .... .. ............ .... 7
Lower Limiting Frequency Circuit . .. . . ........ .. . .. ......... . . .. . 7
First Limiter .... . .............. ...... ........... .. .. . ..... .... .. 8
Potentiometer Range ........ . ... ...... . ... .... . .... ....... .. .... 8
Second Limiter ..... . ..... ........ .. . . . ........ ........ .. ... .... . 9
Output Amplifier Section .. ... .. ..... .... . ..... . .... .. ... ........ 10
Writing Speed ..... . ..... ....... . . .. . ... .... . . .. .. . .. ... ....... . 10
Mechanical System . . .. . .. . . . . . ..... ...... . . ... .. ...... . .. . .......... 11
Principle .. . . ......... ... .. .. .... . ...... . ...... ... . .. . ......... . . 11
Gear-Box ..... . ............ .. ...... . .. . .. .. .. . ...... . ........... . 12
Paper Speed ... . ..... ... . . . .. .... .. . .. . .. ... ...... ... ..... . . .. .. 14
Sy nchronization Gear 1: 10 . . . . .. . ....... .... . .. . .... . ...... . .... 14
Drive Shaft I . .. . . . . . • ....•. . .... . ... . . . ... .. .......... . . . . .... .. lIi
Friction Clutch .... . ..... . ............. ... . ........ .. .... ........ 15
Single Chart .............. .. . . ... . ......... .... ... .. .. .. .... .. .. 15
Drive Shaft II . .. .. . . ....... . • . ...... . .. . .. . . . . . ... ... .... . . . . . ... 15
Two - Channel Selector . ............... .. .. . ..... ..... .... . . . .. .. . . 16
Electromechanical System .... . . .. ... . . . .. . . .......... ...... .. . .. ..... 16
Synchronous Motor . .... ... ...... . . . ... ... ... . ..... .. .. .. .. . ... . . 16
Electromagnetic Clutch . . .... . ........ ... . ..... . .. ... .... . . . ..... 16
Lifting Magnet . . .. . ... . .... . ..... . ......... ..... .... . .. ... . . .. .. 17
Event-Marker ..... . ........... ... ... . . .. . .... .. .... ... .. . . ... . . . . 17
Writing System . . .. ..... ..... . .. . ................ . . ... .... ..... . ... .. 17
Magnet System . . ... . . ........ .. . .... . . .. . .... . . .. .. .... .. .. . .. . . 17
Mechanical .... . .. . ..... ....... .. ...... . .. ... . ..... . ... ...... . . .. 18
Paper Holder System ..... ... .... . .... . . .. . ...... .... . . . .... .... .. . .. 18
Power Supply ........ . .... . . .. . . ..... .. .... .... .. .. . ...... . .......... 19
Rectifiers . . ......... . .....•........... . ...... . ...... . . .. .. ... ... • 19
100 mV Ref. ....... . ....... . . .. . . ... . .... .. ... . . ... . . ........ .. .. 19
Mains Frequencies and Voltages ........... ...... .... .. ....... ... . 19
Fuses . . .... . .. . . . ..... . ... .... .. .. .. .. ........... . . .... .... .... 20

Description
General.
The Level Recorder Type 2305 has been designed for accura te recording of
signal levels in the fr eq u ency range 2 Hz (c I s) to 200000 Hz (c I s) . Typical
fields of application are the recording of frequ e ncy r esponse characteristics,
reverberation decay curves, noise and vibration levels, spectrograms, and
polar diagrams .
Potenti ometer
db-Ranll'
Fig. 1.1. Block diagram of the Level Recorder.
f59J98
Record' ings can be made by means of ink , either on lined paper or on
frequency calibrated paper, or by means of a sapphire stylus on wax-coated
paper. Different types of ink and preprinted recording paper are available .
A
synchronous motor is used for the paper drive, and by means of a gear-
box 12 different paper speeds are available.
The operation of the Recorder is based upon the servo principle . When the
magnitude of the voltage applied to the input terminals is changed, a current
will flow through the driving coil of the writing system, thus moving the
stylus, which is mechanically coupled to the Range Potentiometer. By the
movement of the stylus the voltage delivered from the Potentiometer to the
input amplifier will be altered until a stable condition is regained. In this
way it is possible to obtain recordings for different ranges of voltage
variations by employing different Range Potentiometers .
Ii

Electronic System.
Electronically the Recorder Type 2305 mainly consists of an input circuit,
a Range Potentiometer, a direct-coupled AC input amplifier, the specia l
B & K
signal rectifier arrangement, and an unconventional DC output ampli-
fier section which drives the electrodynamic writing system (see block dia-
gram Fig. 1.1).
tnput Circuit. The input of the Level Recorder is unsymmetrical. If desired
it can be made symmetrical by a special Input Transformer, see chnpter
Accessories. The signal is first fed to the continuously variable input potentio-
meter and then to the calibrated input attenuator. Great care has been
taken in the design of the input circuit to obtain an approximately constant
input impedance of the Recorder. The impedance varies between 16 and
18 IdJ, depending upon the position of the INPUT POTENTIOMETER, and
reaches its minimum value in the middle and end positions of its 12 dB
range. Due to stray capacitance in the input potentiometer the frequency
response will be influenced to a small degree at the highest frequencies.
Vide Fig. 1.2 where the highest deviations for the different positions of the
potentiometer are illustrated.
The calibrated INPUT ATTENUATOR attenuates the input signal in six steps
of 10 dB with an accuracy of approximately ± 0.2 dB.
Range Potentiometer. From the input attenuator the signal is fed to the
Range Potentiometer. Different interchangeable Range Potentiometers are
available, logarithmic (dB potentiometers) as well as linear, see also page 45.
Each potentiometer consists of precision resistors connected between 216
silver lamellae on which the slider of the driving arm moves, again refer
to the block diagram Fig. 1.1.
Chopper. The voltage developed be twe en the slider and ground is fed to an
electromechanical chopper. When it is desired to record AC signals, the
c hopper arm is continuously in one of its two contact positions. However,
if the Level Recorder is switched to re co rd DC levels , the c hopper will
operate at twic e the mains frequen cy, g iving an AC input to the following
amplifier.
AC Input Amplifier. Th e non-chopped or chopped s ignal is applied to the
balanced direct-coupled AC amplifier which , in s tead of a conventional
AC amplifier has two distinct advantages; unw a nted blocking of the
amplifier due to overdriving (and thus overshoot in the recording) is eli-
minated, and the innuence of variations in the supply vollage is decreased
to a minimum. There are several other reasons why this type of circuitry
is used. Firstly, the halancing enables the output from the signal rectifier,
which follows immediately after the amplifier, to be symmetrical wilh
respect to ground. Secondly, by using AC as well as DC negative feed-
6

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.
7

This circuit determines the lowest frequency to which the Recorder responds
correctly. It can be set to five values 2, 10, 20, 50 and 200 Hz (cis) by the knob
LOWER LIMITING FREQUENCY . In Fig. 1.2 can be seen the response at the
five different settings . It should be noted that at the various frequency
limits marked around the knob the reading is decreased by approximately
0.3 dB only . As the stability of the complete writing system is dependent
on the combined setting of the LOWER LIMITING FREQUENCY, WRITING
SPEED and POTENTIOMETER RANGE the reader is referred to the com-
me nts under "Writing Speed", pag e 10 .
~~ SOCIS
ODets
5dB
10 100 1000 10000
Highest dev i otl ' OO a n ~=
to be found f:lr E~
any setting of ====
" Input Potentiometer"
5 100 000 2 CIS
16(807
Fig. 1.2. Typical frequency characteristics of the Level Recorder for different
settings of LOWER LIMITING FREQUENCY switch. The two dotted lines at
the high frequencies give the greatest deviation to be found for any setting Of
INPUT POTENTIOMETER.
First Limiter. To avoid overdriving (heavy charging of the condensers in
the Lower Limiting Frequency circuit) by high and very rapid fluctuations
in the input signal level, an amplitude limiter is introduced. The threshold
of the limiter is designed so that the drive system has full power when the
signal fluctuations reach its limit.
Potentiometer Range. The output signal from the first limiter network,
which is now DC, is compared with a built-in balanced DC reference
voltage, the signal difference then being used to drive the output amplifier
via a second limiter.
The difference signal obtained by comparing the rectified and limited input
signal with the buill-in reference voltage is attenuated in a step attenuator
marked POTENTIOMETER RANGE. By the different positions of the atte-
nuator the resolution of the Recorder is determined. (See also "Controls,
Terminals, and Mechanical Drives", page 22) . The resolution of the Recorder
means the degree by which the Recorder is able to record the details in
signal level variations. Fig . 1.3 illustrates the effect of the resolving power
for different settings of POTENTIOMETER RANGE. The four measurements
8

were made by means of a step voltage on the input of the Recorder and with
a 50 dB Range Potentiometer inserted. It is seen from the figure th at with a
POTENTIOMETER RANGE setting of "10" the resolving power is too high
for the Range Potentiometer used, thus the writing system tends to over ·
shoot. By choosing position "50" it is seen that the "corners" of the step
voltage are sharp, i.e. the correct position. In position " 80" the resolving
power is too low, the "corners " are thus rounded off .
.
OO OOOOOOOOOOOOOOOOOOOGOOOOOOOOOOOOOOOOOOOOOOOOOOO
_ ...
_ ...
_ ...
·~O · db
'20 " db 'SO " db "eO " db
Paper Speed 10 mm/nc . _
Fig. 1.9. Effect of resolving power (POTENTIOMETER RANGE setting). A
50 dB Range Potentiometer was used, WRITING SPE ED on "500", LOWER
LIMITING FREQUENCY -60 Hz (cIs" and measuring frequency 600 Hz (cIs).
I
When employing logarithmic Range Potentiometers the signal voltage differ·
ence between any of the lamellae is equal in percent, i.e. the optimum resolv·
ing power setting will be the same for any deflection of the stylus. With
linear Range Potentiometers, however, the signal voltage difference in percent
between the lamellae at one end of the lamellae face is not the same as the
difference at the other end, which means that the optimum resolving power
will not be identical. A 10-35 mY linear Range Potentiometer requires for
about full deflection of the stylus an optimum resolving power setting of
"10" and for about zero deflection a setting of "25", whereas a 10-110 mY
Potentiometer requires a setting of "10" and "80" respectively. In practice,
the resolving power (POTENTIOMETER RANGE setting) is mostly chosen to
a value between the two limits covering most deflectio ns of the stylus, refer
to part " Control, Terminals and Mechanical Driv es", page 22.
Second Limiter, From the Potentiom e ter Range altenuator the signal is
fed
to the second amplitude limiter. The limiter ensures that the drive signal
to the output amplifier section is independent of the magnitude of the servo
9