BRUEL & KJAER 1405 User manual

Noise Generator
Type 1405
A highly stable random noise generator pro-
ducing white noise up to 100 kHz, white noise
up to 220 kHz (both with a standard spectral
density of 10 -4 V2 /Hz) and pink noise up to
50 kHz. All three random signals can be
stopped
without extraneous noise either
manually or remotely.
The instrument contains a - 3 dB /Octave filter
and a Compressor circuit both of which can be
used independently.
BRilEL& K\.IAER
Power
Compr. Voltage
Compr. l np ut
~
.Compr.~
dS/,
1405
Noiie Generator
Type 1405
Compression
"
WI'>,leNo. .. Pinl<No.u
20 kJot.t . 50 ~HI
Gen. StOP
Output
Output
@

RANDOM NOISE GENERATOR
TYPE 1405
January 1974

CONTENTS
1. INTRODUCTION AND SPECIFICATIONS (PRODUCT DATA) ........ ............. ........ .. ............. ..... 1
2.
CONTROLS... .. ......... ....... ..... ....... ............. ................. ........... .. ........ ........... ..................... ...... 3
2.1 . FRONT PANEL ........ .. .. .. .... .. .. .. .. .... .. .. ...................... .... .. .... ......... .. .. .. ....... 3
2.2 . REAR PANEL .. ..... .. .......... .. .... .. .... .. .. ................. .. . .. .......... .. .. ........ ......... .. 4
3.
OPERATION AND APPLICATIONS ........... ......... .. ......... . .. ........... ........... ............... .. ............ ... 6
3.1. PRELIMINARY ADJUSTMENTS .. .. .. ......... .. .. .. ....... . ....... .. ........................... 6
3.2 . BATIERYPOWER .. ...... ........ .. .. .. ................ .. .. ... ........ ... ......... . .. .. .. .. .. ........ 7
3.3. OPERATION AND APPLICATIONS
.............. ... .. .. . .. ..... .. ..... .. .......... .. ............ 7
Mode1; Wh ite Noise, 100 kHz ~..... ............. .. .. .. . .. ................... . .. ...... .... .. ..... 7
Mode 2; White Noise, 20 kHz ............. ... .. .. .. ..... ... .. .. ... .. ...... ...................... 8
Mode 3; Pink Noise, 50 kHz .. ... ...... .. ... .. .. ....... ........ .. ....................... ..... .. ... 9
Mode 4; Pink Noise, -10 dB .......... .. .. ...... ......................... .. ...... .. ........ ..... 12
Mode 5; Filter, -3 dB/Octave .... .. . .. .. .. .. .. .. ... .. ...... .. ... .. .. ... .. ...... .. . ....... .. .... 13
Calibration pf Power Spectral Density Levels .. .. .............. .. ....... .......... ....... .. 13
Mode ; Compressor Only .............. .. .... .. .... ..... .. .. .. . .............. ............ .. .. ....... 15
Setting up of the Compressor Loop .. ... .......... . .. .... .. ...... ...... .. . ............. ........ 15
4. DESCRIPTION ..... .. ......... ..... .. ............ ... .......................................... ........ . .. ..... ............ ....... .. 17
4.1. RANDOM NOISE OUTPUTS ..... ........................... . .. . .. .. ..... .. .... ................... 17
4.2. THE -3 dB/OCTAVE FILTER .. ................. .. ..... ....... .. ........ .. .. ...... .. ........ .. .. . 19
4.3. THE COMPRESSOR CIRCUIT ............ .. ........................ .. . .. . ....... ................. 20

FEATURES:
• White Noise in the
frequency range 20 Hz to
100 kHz
• Uniform spectral density
10-
4
V2 /Hz
• Pink Noise in the frequency
range 20 Hz to 50 kHz
• Built - in -3 dB/octave filter
can be used separately
• Bui lt - in compressor
amplifier with meter
• Six compressor speeds
• Signal / hum ratio better
than 90 dB
• Manual and remote
generator stop
• Mains or ext. battery
operation
USES:
•
Sound insulation
measurement
•
Frequency response
measurement
•
Vibration testing
• Electro
acoustic
measurements
• Reverberation
measurement
•
Sound distribution
measurement
• Cross
talk measurement
•
Signal to noise ratio
measurement on
commun i cation lines
13 - 1 09
type 1405
Noise Generator
Noise Generator
Tvpe 1405
Power
Gen. Stop
Campr. Voltage Compression
dB
Output
Compr. I nput
10 .
3 ·
Compr . Speed
dB ls
100 JOO
.. Whlt, No,,. P
,nk Noist
20 kHz 50 kHI
..
Comp-,
Only
Output
@
Brliel& Kj<ef"
The Noise Generator Type 1 405 is
designed to supply well defined
white noise in the frequency range
20 Hz to 100 kHz. The generator has
a built - in - 3 dB/octave filter which
is used to weight the white noise in
order to produce pink noise in the
frequency range 20 Hz to 50 kHz.
The
filter may also be used sepa-
ra tely for other weighting purposes
such as automatic bandwidth com-
pensation . The generator also con-
tains a compressor amplifier which
is useful wh en performing frequency
response measurements in various
applications within the fields of
sound and vibration. In such cases
the noise generator is often com-
bined with a filter in order to obtain
a narrow band signal which is then
used to control the compressor . Also
the compressor amplifier may be
used
separately .
The Type 1405 is built - into a
standard B & K cassette which en -
ables easy combination with other
instruments and mounting in racks
(for
example with Power Amplifier
Type 2706 for high output power) .
Description
The white noise is made in the
generator (see block diagram Fig.1)
which is built up around two zener
diodes kept at
constant temperature
in an oven. It has a uniform spectral
density of 10 -4 V2/ Hz. The signal
from the generator is passed on to
the first of two filters via an
attenuator which sets the output
level. The signal from the attenuator
can be short - circuited to stop the
output , a feature necessary for
reverberation measurements . The
generator stop can be controlled
manually as well as automatically .
The two low pass fi lter / amplifier
sections limit the upper frequency of
the white noise signal, the first to
100 kHz and the second to 20 kHz.
Both filters are 3 pole filters, the

second is of the Chebichev type. The
output signals from the filters are
available, via the mode switch, at
the output of the generator .
The
20 Hz to 100 kHz signal from
the output of the first filter is also
I
fed to a -3 dB/octave weighting
network in order to make a pink
noise signal in the frequency range
20 Hz to 50 kHz.
The mentioned output signals are
also fed to a compressor amplifier
system which compresses them
according to the signal present at
the compressor input .
The degree of compression can be
read-off the built-in meter.
Mode Switch
2 1 Gen.Output
Output LP
LP
Generator
---
Voltage
r---<r--
Filter
Amplifier
r---<
~
Filter Amplifier ~
3 / 0
R egu lator
100 kHz
20 kHz 4
~ °6
/ IExt.
Gen.
/
I /
Input)
/
~---------
/
/
! oV
r--
Ma nual
Pink Noise
Remote Generator
Filt er
0
~ontr ol
Control Stop - 3
dB /
f------01
°
oc t ave
@Compressor Input
IFilter Input)
Working Modes:
1. Output of white nois e in the
fr equency range 20 Hz to 100 kHz
with uniform spectral density of
10 -4 V2 /Hz (Up to 50 kHz)
2. Output of white noise in the fre -
quency range 20 Hz to 20 kHz. Level
within ± 1 dB in the range.
Fall off slope above 20 kHz:
> 18 dB/octave
3.
Output of pink no ise (white noise de-
creasing 3 dB/octave) in the fre -
quency range 20 Hz to 50 kHz within
± 1 dB
4.
Same as mode 3 but at a 10 dB l ower
level. (Used when 1405 is comb in ed
with 1/ 3 Octave Filter Set 1616)
I I
Compres-
sor
Int eg r ator
Volta ge
r--
Rectifier r-- R
efe r ence
Regulator
/
Fig. 1. Block diagram of Type 1405
Specifications 1 405
5. Use as a - 3 dB/octave filter
6. Use as a compressor amplifier .
Amplitude Distribution:
Symmetrical Gaussian with crest factor
up to five
Output level :
Calibrated output : 3, 16 V RMS in
100 kHz pos.
Continuously variable down to 0 V
Output load Impedance:
Output: '" 5 kO
Compressor Output : '" 5 kO
Signal to Hum Ratio:
> 90 dB for "w hite noi se" output
> 70 dB for "pink noi se" ou.!put
Stability of Output level:
Be tt er than ± 0,3 dB in the range - 10 °
to 40 °C
Compressor :
Speed: 3 - 10 - 30 - 100 - 300 -
1000 dB/ s
Range: >
80 dB
Min . input signal: 0,5 V avg
Max . input signal: 9,5 V avg
r----
I--
Compre
ssor
Amplifier
r-
AGC
-::-@
Amplifier Output
I +
Compres- Compres-
sor
sor
Speed
Meter
/
473012
Input imp e danc e: > 18 kO on compressor
input
82 kO on ext. gen. input
Frequency range: 20 Hz to 200 kHz
Meter : Calibrated in dB compression (min .
compr. at 0 dB)
Distortion : approx. 1% with 3 V RMS on
ext.
sine gen.
Generator Stop Function :
Operated manually or re motely .
Remote
operation : 5 V positive logic,
contact or a DC voltage
o to 2,5 V: Stop , 2,5 to 20V : signal
Power Supply :
110 , 115 , 127 , 150 ,220, 240V AC
± 10 % 50 - 60 Hz , approx. 5 W
Batteries : 2 x 22 to 30V, 2 x 2 W
Dimensions (KK 0025 cassette):
He ight : 132 ,6 mm (5,2 in)
Width : 209 ,5 mm (8,3 in)
D
epth : 200 ,0 mm (7,9 in)
Weight:
2 kg (4 ,5 Ib)
Accessories Included :
Spare lamps , fuses, power cord

2.1. FRONT PANEL
Power
Power ___ ..... __ -I
Compr. Voltage
Compr. Voltage--+---1
Compr. I nput
Compr. InpIJt ___ + __ -H~
POWER:
COMPR. VOLTAGE:
COMPR. INPUT:
COMPR. SPEED:
MODE SWITCH:
10 .
3 ·
2. CONTROLS
Compf. Speed
dBls
100 300
..
Noise Gene rato r
Typ e 1405
Compression
dB
White Noise Pmk HolM
20 kHz SO kHz
..
Compr. Sp ee(j ----'
Fig.2. 1. Front Panel
Gen. Stop
~ __ -+ ____ Gen. Stop
Output
t---t-----output Control
Mo(je Swi
tc h
113581
Toggle switch for turning the instrument on. When the instrument is
turned on, the meter lamp should light.
Knob for adjustment of the compressor voltage. Turning the knob anti-
clockwise will increase the compressor voltage, increase the degree of
compression and decrease the output signal level.
B & K co-axial socket which accepts plug JP 0101 for input of the com-
pressor control signal. This socket is also used
for input to the
-3 dB/Octave filter.
Knob for adjustment of the compressor speed. In the position "Compr.
Off" the compressor voltage will be removed from the system.
Allows selection of the instrument mode, six being available. These
are:
" White Noise 1 00 kHz" : the instrument outputs a white noise signal
from 20 Hz to 1 00 kHz.
3

OUTPUT:
OUTPUT CONTROL:
GEN. STOP:
COMPRESSION METER:
2.2. REAR PANEL
"White Noise 20 kHz": the instrument outputs a white noise signal
from 20 Hz to 20 kHz.
"Pink Noise 50 kHz": the instrument outputs a pink noise signal (fail-
ing at 3 dB/Octave) from 20 Hz to 50 kHz.
"Pink Noise -10 dB": the instrument outputs the pink noise signal as
above but decreased in level by 10 dB.
"Filter -3 dB/ Oct ." : allows the instrument to be used purely as a
-3 dB /Octave filter.
"Compr. Only" : allows the instrument to act purely as a compressor
amplifier .
B & K co-axial socket which accepts plug JP 0101 for output of the
compressed signal.
For adjustment of the random noise output level. In the position
"Cal.", fully clockwise, if the instrument is in either of the two white
noise modes, the instrument will output a signal of spectral density
10-
4
V2 /Hz.
Push-button operated noiseless switch for turning the instrument's
Random Noise Generator off.
Indicates the degree of Compression between 0 and aOdB.
2: Wh ite No ise 20 kHz
3: Pink Noise 50 kHz
4: Pmk
No ise ·- 10 dB
5: Filter - 3 dB /Oct.
6: Compr. Only
Gen. Remote Control
Output Ou tput
Made in Denmark
ComDress,or I nput Output Gen. Remote 1
73622
POWER SOCKET:
MAINS VOLTAGE SELECTION:
4
Fig. 2.2. Rear Panel
For connection of the AC mains supply. For connections see Fig.2.3 .
To select
the correct AC mains supply, remove the central fuse holder
and turn the selector with a wide-bladed screwdriver or small coin.

Blue N
eu t ra l
AC Power I"J
B row n
Su pp ly
Li v
[
I~
J
Gr een a nd Ye llow Grou
nd
172246
Fig.2.3. Connections to the POWER socket (External View)
REMOTE CONTROL: 7 pin DIN socket which accepts plug JP 0703 for remote control of
generator stop and application of external battery power . For connec-
tions see Fig.2.4 .
GEN . OUTPUT: BNC socket for output of random noise signals, output from
-3 dB/Octave filter, and input of signal from an external generator for
compression, as selected by the MODE SWITCH.
OUTPUT: BNC socket
in parallel with B &K socket on front panel for output of
the compressed signal.
COMPRESSOR 'INPUT: BNC socket in parallel
with B & K socket on front panel for input of
compressor control signal.
22-30 V DC 2 2-3 0V DC
173600
Fig. 2.4 . Connections to the REMOTE CONTROL socket (External
View)
5

3. OPERATION AND APPLICATIONS
3.1 . PRELIMINARY ADJUSTMENTS
6
Before the instrument is connected to the mains supply the voltage selector on the rear
panel should be checked for the correct voltage setting . If adjustment is required, push
and turn the central fuseholder, remove it and adjust the voltage selector to the correct
mains voltage using a small coin or wide-bladed screwdriver . Replace the fuse and fuse-
holder .
In any
measurement system using several instruments grounding considerations are im-
portant . In general to avoid ground loop and noise problems, all the instrument signal
grounds should be connected, and the signal ground of only one of the instruments
should be connected to mains ground.
The 1405 has an internal switch which enables the connection between signal ground
and chassis ground to be removed if necessary. The switch is shown in Fig.3.1.
Fig. 3. 1. The Signal Grounding switch
The switch can be located by standing the instrument on its left side as viewed from the
front, releasing the knurled nut on the front panel and removing the bottom panel of the
instrument. The switch will then be visible close to the mains connection socket.
As delivered, and as shown in Fig.3.1 the switch makes the connection between signal
ground and chassis ground. This configuration can be used in the following circum-
stances:
when none of the instruments in the system are connected to mains ground,
when the 1405 is the only instrument connected to mains ground e.g. via a three
pin mains socket.

The connection should be removed in the following circumstance :
when any of the other instruments in the system have their signal grounds con-
nected to
mains ground.
3.2. BATTERY POWER
To power the 1405 with batteries, two 22-30 V DC batteries are required, the power
consumption from each being approximately 2 W. Connection of battery power is made
to
the 8 pin DIN REMOTE CONTROL socket on the rear panel. The necessary connec-
tions are shown in Fig.3.2.
22-30V
+ +
22-30
173582
Fig. 3.2. Connections to the REMOTE CONTROL socket for External
Battery Power (External View)
3.3. OPERATION AND APPLICATIONS
As the Noise Generator Type 1405 is a versatile instrument. its operation will be de-
scribed
in six set-ups, each corresponding to one of its modes of operation. The applica-
tions given are intended to show the full capability of the 1405. The instrument is ne-
vertheless suitable for applications that are less demanding than those given in this sec-
tion .
3.3.1. Mode 1; White Noise, 100 kHz
The application given for this mode is the determination of the response to random noise
of an electronic circuit. A typical set-up is shown in Fig. 3.3 .
The
1405 controls should be set as follows :
POWER
COMPR. SPEED
MODE SWITCH
OUTPUT
"On"
"Compr . Off"
"White Noise 1 00 kHz"
"Cal."
The response of the test circuit to random noise is shown analyzed in Fig. 3.3 by a Heter-
odyne Analyzer
which is a narrow constant bandwidth analyzer. The 1405 output in this
mode is flat within ± 1 dB from 20 Hz to 75 kHz, allowing the test circuit response to be
obtained in
this frequency range. In this mode the 1405 has a standard output of 10 -4
V2 1Hz, allowing the gain of the test circuit to be immediately calculated knowing the ana-
lysis bandwidth and voltage output. For instance if 100 Hz bandwidth is used, then a vol-
tage output of 0,1 V (v, 0 - 4 . 100) will correspond to unity gain through the test cir-
cuit . Alternatively if the power spectral density (in V2 1 Hz) of the analysis is required, the
bandwidth compensation facility of the Heterodyne Analyzer and meter scale SA 0055
can be used.
7

In Test Out
~::..:.:--:------~ Circuit t='-'------t-;i ~ ". -'
L evel Recorder 2307
o
Heterody ne Analyzer 2010
173580
Fig.3.3. Set-up for Determination of the Response to Random Noise
of a Test Circuit
If a narrow bandwidth is used for the analysis, averaging time considerations are import-
ant, and may lead to a long analysis time . The high stability of the 1405 is essential for
such an analysis. The
Heterodyne Analyzer Instruction Manual should be consulted for
further information on the analysis of random signals. Other similar applications for the
1405 in this mode (perhaps requiring different analyzers) are the determination of the
frequency response of tape recorders, including crqss talk; measurement of the frequency
response of microphones, allowing third octave sensitivities to be obtained; and the de-
termination of the effective noise bandwidth of filter systems.
3.3.2. Mode 2; White Noise, 20 kHz
8
The application given for this mode is that of reverberation measurement, for which only
the audio range of frequencies from 20 Hz to 20 kHz is required. The set-up is shown in
Fig .
3.4 .
Isotropic Sound
Source 4241
Noi se Generator
1405
Band P
ass Fi lter Set
1614/1615
Ha lf In ch Microphone
+ Pr eamplifier
I L
eve l Recorder 2305/2307
I I
L ______ L ___ ___ .L_
Remote Control Cable 173583
Fig. 3.4. Set-up for Reverberation . Measurements

The 1 405 controls should be set as follows :
POWER
COMPR. SPEED
MODE SWITCH
OUTPUT
"On"
"Compr. Off"
"White Noise 20 kHz"
"Cal."
The white noise signal is fed to a Band Pass Filter Set Type 1 614 or 1 61 5* for third oc-
tave
or octave filtering . The filtered signal is then fed to the Isotropic Sound Source
which contains its own Powe r. Amplifier . The noise signal is received by a Half-Inch Mi -
crophone and passed to an
Audio Frequency Spectrometer for amplification and third oc-
tave
filter ing to improve the signal to noise ratio of the test . Both sets of filters should be
switched to the same bandwidth and centre frequency . The resulting signal is finally
passed to a Level Recorder. The noise emitted by the Isotropic Sound Source is inter-
rupted by stopping the noise generator in the 1405, allowing the reverberation decay
curve to be traced
out by the Level Recorder.
The noise
output of the 1405 can be stopped manually by pressing the GEN. STOP but-
ton on the front panel or remotely using the REMOTE CONTROL socket on the rear
panel.
The REMOTE CONTROL socket can be used in two ways . First a simple switch con-
nected as
shown in Fig.3.5(a) can be used, contact closure stopping the generator; or a
DC voltage between 2,5 and 20V connected as shown in Fig .3.5(b). Opening the con-
tact
will stop the generator and closing the contact will start the generator . The genera-
tor returns to full output 10 ms after restarting.
1
2,5- 20V
(a ) (b)
173581
Fig.3.5. Connections to the REMOTE CONTROL socket for Auto-
matic Generator Stop (External View)
Shown in Fig. 3.5 are remote control cables connecting the Level Recorder, Spectrome-
ter , Band Pass Filter Set and 1405 . These cables enable automatic reverberation decay
curves to be
drawn out. The cables carry control signals which automatically switch the
1405 off (using the simple contact closure method) and simultaneously lower the Level
Recorder pen. After some time has been allowed for drawing the decay curve the Spec-
trometer and Band Pass Filter set are indexed to the next third octave band, the Level Re-
corder pen raised and
the 1405 generator switched on in readiness for the next meas-
urement. The necessary wiring of the remote control cables is shown in Fig.3.6.
* (not to a Third Octave Filter Set Type 1616 as it would cause overloading. See section 3. 3.4)
9

(a)
1615
Remote Control
(b)
1615
Remote Control
2 11 3
Remote Contro l
2113
Rem ote Control
2305
Cam-Switch
2307
Ca m -Sw itch
23 05
Remote Co nt rol
2307
R emote Co n tro l
Fig . 3.6 . Remote Control Cables for Reverberation Measurements
(a) wi th Level Recorder Type 2305
(b) with Level Recorder Type 2307
(All Sockets Viewed Externally)
1405
Re mot e Control
173585
1405
R emote Control
173586
For further information on this application, for instance Level Recorder settings, use of
paper loops to save paper and the reading of Reverberation Time from the decay curves,
the relevant Level Recorder Instruction - Manual should be consulted.
3.3.3 . Mode 3; Pink Noise , 50 kHz
10
The application given in this mode is the measurement of transmission loss through a
pa
rtition in an open plan office, or through an absorptive panel placed between two re-
verberant rooms. This application also makes use
of the compressor facility of the 1405 .
The set-up is shown in Fig.3.7.

Noise Generator
1405
Band P
ass Filter Set
1614/1615
Partition under test
H al f Inch Microphone
+ Pr eam plifi er
Isotropic Sound Source
4241 Remote
Half In ch Microphone
+ Preampl ifier
-\\-1r--;'-'-~ ,","",,_.n_
A
_C
Output
Control Cable -I
AC Output
Pr eamp. Input I
L_
Measuring Amplifier
2609 Level R
eco rder 2305/2307
Fig .3. 7. Set-up for the Measurement of Transmission Loss across Panels
The 1405 controls should be set as follows :
POWER
COMPR. SPEED
MODE SWITCH
OUTPU =r
"On"
As required, see below
"Pink Noise 50 kHz"
"Cal .."
173584
The output of the noise generator of the 1405 is fed internally to an AGC circuit, and
from there to the OUTPUT sockets on the front and rear panel. This signal is then third
octave filtered * and passed to the Isotropic Sound Source. Pink noise (falling off with in-
creasing frequency at 3 dB/Octave) is chosen for this test so that after it is third octave
filtered the Sound Source will receive a constant voltage signal regardless of the filter
centre frequency selected (as the bandwidth of consecutive filters increases with fre-
quency at 3 dB/Octave). The Sound Source is thus given the best possibility of produ-
cing a
flat frequency output spectrum.
Placed in front of the partition under test is a Half-Inch Microphone which detects the
sound pressure
falling on it . This signal is returned, via a Measuring Amplifier to the
COMPRESSOR INPUT socket on the 1405 . The compressor in the 1405 will maintain
the sound level impinging on the partition constant at a preset level (which can be read di-
rectly from the Measuring Amplifier) as the Band Pass Filter Set is indexed to consecu-
tive
third octave bands. Setting of the COMPR. SPEED and the COMPR. VOLTAGE knobs
is described in section
3.3 .6.
The
attenuated sound on the other side of the partition is received by a Half-Inch Micro-
phone and the signal passed to a Measuring Amplifier and then to a Level Recorder for
recording of the received sound spectrum. It may be necessary to include third octave fil-
tering on the receiving side to improve the signal to noise ratio of the test. By compar-
* (But not us ing a Third Octave Filter Type 1616 as this would cause overloading . See Sect io n 3. 3.4 .)
11

ison with the transmitted sound spectral level (held constant across the test frequency
range by the 1405 compressor) the received sound spectrum can be used to give the at-
tenuation across the partition in third octave bands.
The
test can be performed automatically using a remote control cable between the Level
Recorder and Band Pass Filter Set REMOTE CONTROL sockets. The necessary connec-
tions for this cable are given in the relevant Level Recorder Instruction Manual.
3.3.4. Mode 4; Pink Noise. -10 dB
12
The application given for this mode is again the measurement of reverberation decay
curves,
but with the emphasis placed on portable instrumentation. The set-up is shown
in Fig. 3.8 .
Noise Generator
-1405
Isotropic Sound Source 4241
Third Octave
Filter Set 1616
Pr ecision Sound
Level Meter 2203
Third Octave
Filter Set 1616
Tape Recorder
7004
173592
Fig. 3.8. Set-up for Reverberation Measurements using Portable Instruments
If required the 1405 can be battery powered in this application, two batteries of between
22 and 30 V being necessary which are connected via the REMOTE CONTROL socket on
the rear panel, as shown in Fig. 3.2.
The 1405 controls should be set as follows:-
POWER
COMPR. SPEED
MODE SWITCH
OUTPUT
"On"
"Compr . Off '
"Pink Noise -10 dB"
"Cal."
In this mode the pink noise output by the 1405 is lowered by 10 dB and is suitable for
direct connection to the Third Octave Filter Set Type 1616 . The "Pink Noise, 50 kHz" set-
ting of the MODE SWITCH should not be used as it will overload the 1616 . The third oc-
tave
filtered signal is then passed to the Isotropic Sound Source. The noise generator in
the 1405 is stopped by pressing the GEN. STOP button .
The reSUlting decay curve is received by a
Sound Level Meter (with associated Third Oc-
tave Filter Set) and fed to a Tape Recorder for recording and future analysis.

It should be stated that this set-up is not fully portable as the Power Amplifier in the Iso-
trop ic Sound Source must be connected to the mains supply .
3.3.5. Mode 5; Filter , -3 dB / Oct.
The application given for this mode is the measurement of power spectral density (PSD)
of any random signal in the frequency range 20 Hz to 50 kHz using a constant percen-
tage
bandwidth analyzer. The set-up is shown in Fig.3.9.
-
Signal fo r Co mpr .
Analysis Input '---"---..J
L eve l Recorder 2305/2307
Drive Cable UB 0014
F re quency Analyzer 2120
Fig. 3.9. Set-up for the Measurement of Power Spectral Dens i ty
The 1405 controls should be set as follows :
POWER
COMPR. SPEED
MODE SWITCH
"On"
"Compr. Off "
"-3 dB/Oct."
173591
The random signal for analysis is fed to one of the COMPRESSOR INPUT sockets and
output after frequency weighting at -3 dB/Octave to the GEN . OUTPUT socket on the
rear panel. The signal is then passed to a Frequency Analyzer Type 2120 which permits
a continuously swept analysis with 1%, 3%, 10% or third octave bandwidth . As the
bandwidth selected on the Analyzer increases at the rate of 3 dB/Octave, the signal fre -
quency weighting at -3 dB/Octave is equivalent to division by the bandwidth as re-
quired for a PSD plot.
The signal is finally passed to a Level Recorder for tracing out on suitable frequency cali-
brated paper (OP 1130 with the Frequency Analyzer 2120).
Calibration of Power Spectral Density Levels
1. Level calibration is simplified by inserting the Power Spectral Density scale SA
0055 into the meter of the Frequency Analyzer . The 50 mV reference of the Ana-
lyzer should then be used to adjust the Analyzer and Level Recorder to a deflection
of 6 dB below full scale.
The analysis can then be traced out on the frequency calibrated paper (OP 1130) fol-
lowing the instructions given in the Frequency Analyzer Type 2120 Instruction Man-
ual. At this stage there is no difference between this analysis and a normal spectral
analysis as
the dB scale is already a logarithmic power scale.
13

14
2. The row of small range indicator windows at the bottom of the PSD scale should
now be inspected and the power of ten corresponding to the illuminated window re-
corded. The
power multiplied by ten will be the level of the uppermost line of the Lev-
el Recorder paper in dB re 1 V2 (e.g. if the 10-
4
window is illuminated then the lev-
el
of the top line on the paper is -40 dB re 1 V2) .
3. Next
the reduction to unity bandwidth should be made. This correction is made by
subtracting a certain number of dB from the Level Recorder paper annotations just
calculated. The variation of this correction with the Effective Noise Bandwidth of the
filtering used for the analysis is given in Fig.3 . 10. It should be stressed that this
graph only applies to the -3 dB/Octave filter in the 1405 which has unity gain at
RI;RI-b
"0
!
.0
::>
(J)
'"
.0
B
CD
"0
3
,."
./
25 V
V
/"
./
V"T
0
./
I
./
2
,./
I
~
I
./
Third Octave Octave
~
F i lt ~ring
Filtering
5
/"
I
/"
I
/"
I
,./
I
0
./
I
./
I
,./
I
I
I
5 I
I
I I
I I
0 I
2 3 4 5 6 7 a
910 20 30 40 50 607Qa090100
Effective Noise Bandwidth % 173598
Fig. 3. 10. Correction Graph for Power Spectral Density
For the example already given the upper line of the Level Recorder paper corresponds
to -40 dB re 1 V2. If the analysis was made with third octave filtering (23% effec-
tive noise bandwidth) then the number of dB to be subtracted (from Fig.3.10) is 22 .
Thus the upper line of the Level Recorder paper corresponds to -62 dB re 1 V2 /Hz.
4. If
the original signal represented either sound or vibration, then a further correction
would have to be made to obtain the PSD in pressure or acceleration units . F'or ex-
ample if a microphone was used then the PSD is required in dB re 1 Pa
2/Hz. This
can be obtained by algebraically-
subtracting the microphone sensitivity in dB re
1 VI
Pa (exactly equivalent to dB re 1 V2/ Pa
2
in the dB system) from the level annota-
tion already calculated.
In
the example, the uppermost graduation on the Level Recorder paper corresponds
to -62 dB re 1 V2/Hz. If the original signal was obtained with a microphone of sen-
sitivity -36,6 dB re 1 V/ Pa, this value must be subtracted (algebraically) from the le -
vel
annotation of -62 dB re 1 V2/Hz, giving -25.4 dB re 1 Pa
2/
Hz as the PSD le-
vel
of the uppermost line on the Level Recorder paper.

3.3.6. Mode 6, Compressor only
The application given for this mode is the use of the 1405 compressor circuitry to con-
trol a vibration test , the excitation signal for which is supplied by a signal generator with
no compressor facility . The set-up is shown in Fig. 3.11 .
AC O ut put
Pream p. Inp ut
(via ZR 0024)
Meas uring Am p. 2609
Gen. Output
T est Signa I from
E xt . Generator.
Preamp. Inp
ut
(via ZR 0024)
AC Output
Response
A cce lerometer
Meas uring Amp. 2609
Cont rol
'---t:::m![r- Accelerometer
Vibr at ion E xc it er 4809 L
eve l Recorder 2305/2307
173590.
Fig.3. TT. Set - up for a Vibration Test using the T405 Compressor Cir -
cuit
The test signal from the generator is connected to the GEN. OUTPUT socket of the
1405 . The compressed test signal is fed to a Power Amplifier and finally to a Vibration
Exciter. The compressor control signal is obtained from a Control Accelerometer
mounted on the object fixture . The control signal is fed back to the 1405 via a Preampli-
fier and a Measuring Amplifier . The test level can be read from the Measuring Amplifier
meter .
A Response
Accelerometer detects the response of the test object, the signal being fed
to a Level Recorder via a Preamplifier and a Measuring Amplifier .
Setting up the Compressor Loop
The object of the compressor loop is to counteract any unevenness in the frequency re-
sponse
of the Exciter by regulating the excitation signal and thus maintaining a constant
level over the full test frequency range. Most Exciters generally show a similar frequency
characteristic, relatively low response at low frequencies increasing to a fairly constant
response in the middle frequency range and resonant peaks at the high frequency end.
At low frequencies the compressor loop must open to supply a high drive signal, but at
high
frequencies must close to supply a small drive signal as the Exciter passes through
resonances. As the compressor circuit of the 1405 has approximately 80 dB dynamic
range it is advisable to set the compression to about 40 dB in the flat response region at
medium frequencies. This will allow the compression to decrease towards zero at low
frequencies thus supplying a high drive signal and to increase towards 80 dB at the Exci-
ter resonances, supplying a low drive signal.
1. Before the instruments are switched on the Power Amplifier GAIN CONTROL should
be turned fully anticlockwise , that is to zero. This will protect the Exciter from tran-
sients and acci dental overloading. The generator can then be adjusted to the mid-
15

16
die frequency range (approximately 100 Hz to 1 kHz) with its output as high as possi-
ble
but not greater than 3 V RMS, the maximum acceptable level at the GEN. OUT-
PUT socket.
2. The Measuring Amplifier ATIENUATOR controls should be set so that the required
test level is on scale. Set
the Power Amplifier ATIENUATOR knob to "0 dB" .
3. The 1405 controls should be set as follows :-
POWER
COMPR. VOLTAGE
COMPR. SPEED
MODE SWITCH
"On"
"fully anticlockwise" (initially)
"30 dB/ s" (initially)
"Compr. Only"
4. With the Power Amplifier ATIENUATOR on "0 dB", slowly turn the GAIN knob clock-
wise . The Measuring Amplifier meter deflection should increase to a certain level
and
then remain constant. The 1405 COMPRESSION meter should also indicate in-
creasing compression. If these two indications do not occur, then there is a break in
the feedback circuit which must be rectified. If compression is obtained, turn the
GAIN knob on the Power Amplifier fully clockwise.
5. Carefully use
the COMPR. VOLTAGE knob on the 1405 to regulate the test level
(and hence the Measuring Amplifier meter deflection) to the required value. This ad-
justment should be done slowly so as to allow the compressor to keep up with the
change and not overshoot.
6. The
settings should now be checked by manually sweeping the generator slowly
across the test frequency range. The COMPRESSION meter deflection must always
stay
between 0 dB and 80 dB. If a large Exciter resonance gives greater than 80 dB
compression, then the Power Amplifier GAIN CONTROL must be turned anticlock-
wise until only 80 dB compression is required to maintain the test level at that fre-
quency .
7. The COMPR. SPEED should
finally be set to a compromise position taking into ac-
count two opposing influences. First, too high a setting will cause distortion (see
section 4.3), and
often instability at low frequency. Secondly, too Iowa setting will
not allow the compressor to follow sharp resonances in the system.
8. NEVER
switch the COMPR. SPEED to "Off' with the instruments set as described
as this will immediately apply full output to the Power Amplifier and Exciter which,
despite any built-in protection could cause damage. The Power Amplifier GAIN CON-
TROL knob should be
turned fully anticlockwise when the test is finished.

r--- -,
I I
I White I
Noi se
I Generator I
I I
L ___ ..J
O ven
Remote Con tr ol
4. DESCRIPTION
The method of operation of .the Noise Generator Type 1 405 is described with reference
to the block diagram shown in Fig.4.1 . This block diagr<:)m is a more detailed version of
that shown on the rear panel of the instrument . The block diagram on the rear panel is
useful as a ready reference for the connections to the instrument in its different modes.
I
Output
100kHz
Voltage Low P
ass
R egu l ator Fil ter
I
Gen erator 1-------1
20kHz
0)----; Stop ....L..Manual Control
..... --------1 L ow Pass 1-------.
Compr. Input
l
1 White No ise 100 kHz
2 White No
ise 20 kHz
3
Pin k Noise 50 kHz
4
Pin k N oise -10 dB
5 Filter -3 dB/Oct.
6 Compr. Only
Fi lter
/
1 /
30_' ____ ~
"0 • 1-3 dB/ oct.j
40 Filt er
.----------"" 5 06
I
Co mpr.
-----7
/
2 1/ /
/' Gen. Output
3
1
D---~O
5
O~--------------------~~----~
_ R T
L- Integr ator/ • __ AGC
Voltage ectl ler r-- ~ Reference h,.-- Ampl if ier
R
eg ulator
I
Fig.4. 1. Block Diagram
4.1. RANDOM NOISE OUTPUTS
I
omp ressor
Speed
I
~ ompreSSio ~
Meter I
Outpu
L..----40
173593
The heart of the 1405 is a White Noise Generator which consists of a matched pair of
zener diodes driven by constant current sources . The diodes provide an exceptionally flat
spectral density from 20 Hz to 100 kHz. The two zener diodes are housed in a tempera -
ture controlled oven which maintains the output of the diodes at 10 -4 V2/Hz over the
entire operating temperature range of the instrument . This feature allows the use of the
1405 as a high stability standard output random noise generator.
17