BRUEL & KJAER 2021 User guide
.•
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~
...
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..
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•·
•·
...
_ ·-..
...._
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,_-· "
Heterodyne Slave Filter
Type
2021
A constant bandwidth filter which is
tuned from the high-frequency
outputs of a B &K vibration exc!ter
control. Designed for use in a
selective feedback loop, it can also
be used for frequency analysis
of
the feedback signal. Phase
repeatability between filters is
better than 1
o , and a rejection output
can be used for distortion
measurements down to approx.
0.2%
.
B & K INSTRUMENTS:
ACOUSTICAL
....
Condenser
Microphones
Piezo-Electric
Microphones
Microphone
Preamplifiers
Microphone
Calibration Equip.
Sound Level Meters
(general
purpose-precision-
and impulse)
Standing Wave Apparatus
Tapping Machines
Noise
Limit
Indicators
ELECTROACOUSTICAL.
...
Artificial
Ears
Artificial
Mouths
Artificial
Mastoids
Hearing Aid Test Boxes
Telephone Measuring Equipment
Audiometer
Calibrators
Audio
Reproduction Test Equip.
STRAIN
....
Strain Gauge Apparatus
Multipoint
Panels
Automatic Selectors
Balancing Units
VIBRATION
..••
Accelerometers
Accelerometer
Preamplifiers
Accelerometer
Calibrators
Vibration Meters
Magnetic Transducers
Capacitive Transducers
Vibration Exciter
Controls
Vibration Programmers
Vibration Signal Selectors
Mini-Shakers
Complex Modulus Apparatus
Stroboscopes
GENERATING
....
Beat Frequency Oscillators
Random Noise Generators
Sine-Random Generators
MEASURING
....
Measuring Amplifiers
Voltmeters
Deviation Bridges
Megohmmeters
ANALYZING
....
Band-Pass
Filter
Sets
Frequency Spectrometers
Frequency Analyzers
Real-Time Analyzers
Slave Filters
Psophometer Filters
Statistical Analyzers
RECORDING•
•••
Level Recorders
(strip-chart and polar)
Frequency Response Tracers
Tape Recorders
BRilEL&K~A:.R
DK-2850
Nrerum, Denmark. Teleph.:
{01)
80
05 00
. Cable: BRUKJA, Copenhagen. Telex:
5316
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•••
!
...
~
...
.
~
..
.
,.
-.
•
..
~
.........
,,
•·
,_
,
....
Heterodyne Slave Filter
Type
2021
A constant bandwidth filter which is
tuned from the high-frequency
outputs of a 8 &K vibration exc!ter
control. Designed for use in a
selective feedback loop, it can also
be used for frequency analysis
of
the feedback signal. Phase
repeatability between filters is
better than 1
o,
and a rejection output
can be used for distortion
measurements down to approx.
0.2%
.
HETERODYNE SLAVE FILTER TYPE
2021
September 1971
CONTENTS
1.
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.
1.
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
h2.
Arrangement
of
This Manual . . . . . . . . . . . . . . . . . . . 8
2.
CONTROLS
.........................................
10
2.1.
Front
Panel
2.2. Rear
Panel
.................................
10
14
3. DESCRIPTION
.......................................
18
3.
1.
Introduction
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2. Reference Oscillator
..............
.
...........
20
3.3.
Input
Amplifier
&
Attenuator
..................
20
3.4. 15 kHz
Cut-Off
Filter
.....................
.
...
20
3.5. Heterodyne
Slave
Filter
.......................
20
Principle
of
Operation
........................
20
Filter
Tuning Frequencies
......................
22
Input
Mixer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
60
kHz
Band-Pass
Filter
.......................
23
DC
Mixer
..................................
24
DC
Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Summation
Amplifier
.........................
26
60
kHz
Amplifier
and
Filter
....................
26
Output
Modulator . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
15 kHz
Output
Fi
Iter
.........................
27
Rejection
Amplifier
..........................
27
30
kHz
Low
Pass
Filter
........................
28
Unity
Gain
Amplifier
.........................
28
Calibration.
of
the Filter
.......................
29
3.6. Bandwidth Compensation . . . . . . . . . . . . . . . . . . . . . . 29
3.
7.
Bandwidth Programming . . . . . . . . . . . . . . . . . . . . . . 29
3.8. Meter
and
Recorder
Amplifier
..................
30
3.9. Ouasi-RMS Detector
..........................
31
4.
OPERATION WITH
AUTOMATIC
VIBRATION
EXCITER
CONTROL TYPE 1025
................................
33
4.
1.
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
4.2. High Frequency Tuning Connections
to
2021
34
Connection
of
a single 2021
to
1025
.............
34
Connection
of
more than one 2021
to
1025
........
35
Phase
Locking
two
2021 s
......................
35
4.3. Remote Bandwidth Control Connections
..........
36
Operation
without
3000 Hz Cross-over . . . . . . . . . . . . 36
Rewiring
RELAY
CONTROL
B socket
to
provide
3000
Hz Cross-over . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Separate Connecting Lead
for
3000
Hz
Cross-over
...
38
4.4. Calibration
.................................
38
Frequency Calibration
of
the Exciter Control
......
39
Modulator
Adjustment
of
the 2021
..............
40
Rejection Adjustment
of
the 2021
........
__
.....
41
Adjustment
of
2021 Meter Sensitivity . . . . . . . . . . . .
41
4.5. Applications
................................
42
Frequency Analysis
.........
: .
...............
42
Distortion
Measurements
......................
45
Use
in a simple Feedback Loop
.................
45
Feedback Loop
with
Vibration
Programmer
Type 4411
..................................
47
Use
of
multiple
2021 s
with
Control Signal Selector
Type4410
.................................
49
Use
of
both 4410
and
4411 in Selective
Feedback Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
4.6. Cables
used
with
1025/2021 Combinations
........
55
5.
OPERATION WITH SINE-RANDOM GENERATOR TYPE
1042.
58
5.
1. General
...................................
.
5.2. High Frequency Tuning Connections
to
2021
Connection
of
a single 2021
to
1042
............
.
Connection
of
more than one 2021
to
1042
.......
.
58
59
59
60
5.3. Remote Bandwidth Control Connections
..........
62
Operation
without
3000
Hz
Cross-over
............
63
Rewiring
RELAY
CONTROL
socket
to
provide
3000Hz
Cross-over
...........................
64
Separate Connecting Lead
for
3000
Hz
Cross-over
...
65
5.4. Calibration
.................................
65
60
kHz Signal
Adjustment
.....................
65
Frequency Calibration
of
the Sine-Random Generator
67
Modulator
Adjustment
of
the 2021
...
.
..........
67
Rejection
Adjustment
of
2021
..................
68
Adjustment
of
2021 Meter Sensitivity
............
69
5.5. Applications
................................
70
Frequency Analysis
..........................
70
Distortion
Measurements . . . . . . . . . . . . . . . . . . . . . .
71
Use
in a simple Feedback Loop
.................
72
Feedback Loop
with
Vibration
Programmer
Type
4411
..................................
74
Use
of
multiple
2021 s
with
Control Signal Selector
Type4410
.................................
76
Use
'Of
both
4410
and
4411 in Selective
Feedback Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.6. Cable
used
with
1042/2021 Combinations
........
82
6.
OPERATION WITH EXCITER CONTROL TYPE 1026
........
85
6.
1.
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
85
6.2. High Frequency Tuning Connections
to
2021
85
6.3. Remote Bandwidth Control Connections
..........
86
6.4. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Modulator
Adjustment
of
the 2021
..............
86
Rejection
Adjustment
of
the 2021
...............
87
Adjustment
of
2021 Meter Sensitivity
............
88
6.5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
Frequency Analysis
..........................
89
Distortion
Measurements
......................
90
7.
OPERATION WITH AUTOMATIC
VIBRATION
EXCITER
CONTROL TYPES 1008 & 1019
.........................
91
7.1. General
....................................
91
7.2. High Frequency Tuning Connections
to
2021
92
Connections
of
a single 2021
to
1008
or
1019
......
92
Connections
of
more than one 2021
to
1008/1019
..
93
Phase
Locking
two
2021 s
......................
93
7.3. Remote Bandwidth Control Connections
..........
94
Operation
without
3000
Hz
Cross-over . . . . . . . . . . . .
94
Re-wiring
RELAY
CONTROL B socket
to
provide
3000
Hz
Cross-over . . . . . . . . . . . . . . . . . . . . . . . . . . .
95
Separate Connecting Lead
for
3000Hz
Cross-over
...
97
7.4. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
Frequency Calibration
of
the Exciter Control
......
97
Modulator Adjustment
of
the 2021
..............
98
Rejection Adjustment
of
the 2021
...............
99
Adjustment
of
2021 Meter Sensitivity
............
99
7.5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Frequency Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 100
Distortion Measurements . . . . . . . . . . . . . . . . . . . . . 102
Use
in a simple Feedback Loop . . . . . . . . . . . . . . . . 102
Feedback Loop
with
Vibration
Programmer
Type 4411 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Use
of
multiple
2021
s
with
Control Signal Selector
Type
4410
................................
107
Use
of
both
4410
and
4411 in Selective
Feedback Loops
............................
109
7.6. Cables
used
for
1008
and
1019 Combinations
with
2021
.................................
113
8.
OPERATION WITH SINE-RANDOM GENERATOR
TYPE 1040
AND
1041
................................
116
8.1. General
...................................
116
8.2. High Frequency Tuning Connections
to
2021 117
Connections
of
asingle 2021
to
1040
or
1041 . . . . . 117
Connections
of
more than one 2021
to
1040/1041 . 118
8.3. Remote Bandwidth Control Connections
to
1040
..
120
Operation
without
3000Hz
Cross-over
...........
121
Re-wiring
RELAY
CONTROL
socket
to
provide
3000Hz
Cross-over
..........................
121
Separate Connecting Lead
for
3000Hz
Cross-over . . 123
8.4. Remote Bandwidth Control Connections
to
1041
..
123
Operation
without
3000Hz
Cross-over
...........
123
Connection
of
RELAY
CONTROL
socket
to
provide
3000Hz
Cross-over
..........................
124
8.5. Calibration
................................
124
60kHz
Signal Adjustment
....................
125
Frequency Calibration
of
the Sine-Random Generator 126
Modulator Adjustment
of
the 2021
.............
126
Rejection Adjustment
of
the 2021 ·
..............
127
Adjustment
of
2021 Meter Sensitivity
...........
128
8.6. Applications
...............................
129
Frequency Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 129
Distortion Measurements . . . . . . . . . . . . . . . . . . . . . 130
Use
in asimple Feedback Loop . . . . . . . . . . . . . . . .
131
Feedback Loop
with
Vibration
Programmer
Type 4411
.................................
134
Use
of
multiple 2021 s
with
Control Signal Selector
Type4410
................................
137
Use
of
both 4410
and
4411 in Selective
Feedback Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . .
141
8.
7.
Cables
used
for
1040
and
1041 Combinations
with
2021
.................................
146
9.
APPENDIX
.........................................
151
9.1. Error due
to
rate
of
Sweep in Rejection
Measurements
..............................
151
9.2. Random Signal Averaging
.....................
155
9.3. Filter Definitions
...........................
158
Ideal
Filter
................................
159
Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
10. SPECIFICATIONS
...................................
160
1.
INTRODUCTION
1.1. GENERAL
The
Heterodyne
Slave
Filter
Type
2021
is
a
constant
bandwidth
filter
whose
centre
frequency
is
tuned
from
the
high
frequency
outputs
of
a
B & K
Vibration
Exciter
Control.
Its main
applications
will
therefore
be
found
in
the
Vibration
Test
and
associated
fields,
where
the
Slave
Filter
is
ideal
for
use as a
narrow
band
frequency
analyzer
as well as a
means
of
isolating
the
compressor
loop
control
signal
from
wider
band
noise. Special
attention
has
been
given in
the
design stage
to
the
repeatability
of
phase
characteristics
between
different
filters.
Two
Type
2021 s
therefore
provide
convenient
means
of
making
frequency-selective
mechanical
impedance
measurements,
or
for
measuring
the
cross spectral densities.
Ability
to
reject
the
tuned-frequency
fundamental
gives also
the
possibility
of
distortion
measurements
down
to
about
-55
dB,
or
to
approx.
0.2%
distortion.
The
2021
incorporates
a measuring
amplifier
to
provide
suitable
input
signal processing
and
output
characteristics
to
match
most
types
of
record·
ing
equipment.
It has a
meter
rectifier
of
particularly
wide
dynamic
range,
with
averaging
times
selectable
from
0.3
sec
to
100
sec. By
an
internal pro-
gramme
using also
the
remote
filter
bandwidth
selection
possibilities,
the
averaging
time
of
the
rectifier
can
be
switched
to
provide
for
Gaussian noise
input
an
effective averaging
error
less
than
a selected value
between
3%
and
30% (see section
9.2
"Random
Signal Averaging").
Five
filter
bandwidths
are
provided
in
the
2021;
3.16Hz,
10Hz,
31.6
Hz,
100Hz,
and
316Hz.
These
are
the
-3
dB
bandwidths
of
the
4
pole
Butterworth
filters,
for
which
the
Effective
Bandwidth
is
2.5% higher.
A
1/JB
factor
(see
section
3.6)
can
be
applied
to
the
filter
output
signal so
that
with
a
suitable
meter
scale
measurement
of
power
spectral
density
can
be
performed
in a single
operation.
Suitably
calibrated
meter
scales are
provided
which
convert
directly
to
power
spectral
density.
Unity
gain
outputs
provide
filtered
and
rejection
components
of
the
input
signal
at
unchanged
levels
from
the
input
.
This
leaves
the
measuring
7
section
of
the
2021 available for measurements
on
the
input
signal
without
affecting
the
gain
in
the
servo loop.
Separate
recorder
outputs
are available
for graphic recording
of
either
the
AC
output
from
the
filter
or
the
fully
averaged
DC
output
from
the
meter
rectifier.
For
phase monitoring, a
60kHz
output
is
provided. This has
the
same phase characteristics as
the
input
signal
at
the
selected
tuning
frequency. All
outputs
may
be used
simultaneously.
1.2.
ARRANGEMENT
OF
THIS MANUAL
This manual describes
the
operation
of
the
2021
Heterodyne
Slave Filter
with
the
B & K Vibration Exciter
Controls
which are
in
most
common
use
at
the
present
date.
The
Slave Filter can
in
principle be used
with
any
signal
generator which
produces
its
output
by
mixing sinusoidal signals
of
60kHz
and
50-
60kHz
or
30kHz
and
25-
30kHz.*)
Output
level
of
the
high
frequency
signals should be as follows:
30kHz
25-30kHz
60kHz
61
kHz
50-60kHz
2.8
Volts±
15%
7
Volts±
15%
1.5 mV
minimum
125
mV ± 15%
5
Volts±
15%
The
load applied
by
the
2021
is
1
MQ//180
pF.
For
obvious reasons,
however,
the
descriptions
in
the
manual are confined
to
particular genera-
tors. Each
of
the
Operation
chapters
is
self-contained
with
all
information
necessary for
operation
of
the
2021 with
that
generator.
The
generators with which
the
2021 will
most
commonly
be used are
the
B & K
Types
1025,
1026, 1042,
while
the
1008,
1019,
1040,
and
1041
may
also be used
for
the
full range
of
applications.
The
1026
will
not
normally
employ
the
2021 as a selective filter in
the
feedback
loop since this genera-
tor
has built-in slave filters
of
6% relative
bandwidth.
However,
the
1026
can be used
to
tune
the
2021 for
frequency
analysis, and mechanical imped-
ance
or
distortion
measurements,
and
these
applications are described
in
the
appropriate
chapter. If it
is
desired
to
use B & K generators
other
than
those
*)
The
Heterodyne
Slave
Filter
Type
2020
is
designed
for
use
with
the
B & K
Audio
Frequency Oscillators
1022
and 1024. These have high
frequency
tuning
signals
of
120
kHz
and 100
to
120
kHz.
8
mentioned, direct correspondence
with
the 8ruel & Kjmr
factory
on the
particular application
is
invited.
Inevitably
with
applications involving many instrument types,
several
special cables
are
required
for
the various measurement setups. Such cables
are
not
generally available
from
8 & K stock, although in many
cases
the
8 & K agents
will
be
. able
to
assist.
Full details
of
the wiring
of
these special
cables
will
be
found
in
the
final section
of
each
operation chapter. The
numbering system employed
for
these cables
is
consistent
throughout
this
book
and
is
also consistent
with
the special cable numbering
used
in the
instruction manual
for
8 & K Types 2502, 4410, 4411
and
the manuals
for
the Exciter Controls.
9
2.
CONTROLS
2.1. FRONT PANEL
Meter and Recorder E
ffective
Averagi
ng
Gain Control Uncal. P
ower
Meter Scale
Outpu
ts Time
Input
Filter
Sens.
Ref.
Input
Attenuator
METER
SCALE
POWER
GAIN
CONTROL
10
Preadjust
ment
Fil
ter
B
andwidth
~~~!!J!!~-l-
Cross Over
Frequency
Beat
Adjustmen
t
Overload
Meter and
Recorder
Attenuator
Fig.2.
1.
Front
Panel
of
2021
Release
the clip (top centre) by a light
downward push.
Tilt
the
glass
panel for-
ward
to
slide
the
scale
out.
On/Off
switch for power.
For continuous control of the input am-
plifier gain. When
in
position
"Cal.",
the
gain
is
fixed at 0 dB
re.
the
INPUT
ATTENUATOR
pos1t1on.
Maximum
attenuation from
"Cal."
position
is
10-
11
dB.
The
GAIN
CONTROL
operates for
all
selected input
signals.
UNCAL
INPUT
FILTER
SENS.
REF.
INPUT
ATTENUATOR
Warning light
is
lit
when
the
GAIN
CONTROL
is
in
any
other
position
than
"Cal.".
Three
pushbuttons
which select:
"Direct"
input
from
rear panel
socket.
"Ref.
100
mV"
internal reference signal
for
calibration.
100
mV,
1 kHz sine
wave signal.
"15
kHz
Cut
Off"
limits
the
input
signal
components
to
below
15kHz
(-3
dB
point).
Only
one
of
the
buttons
"Direct"
and
"Ref.
100
mV"
can
be selected
at
any
time.
Three
pushbuttons,
"Linear"
no filter included
"Int.
Filter"
selects
the
internal hetero-
dyne
slave filter
"Ext.
Filter"
selects an
external
filter
which can be
connected
at
the
rear
panel sockets.
Only
one
of
the
buttons
"Int.
Filter"
and
"Linear"
can
be
selected.
An
external
filter
is
placed in series
with
the
other
weighting selected.
Screwdriver
adjusted
potentiometer
which changes
the
gain
of
the
input
am-
plifier
for
calibration purposes. Range
is
approx.
± 1.5 dB.
Warning light
is
lit
when
pushbutton
"Ref.
100
mV"
is
selected.
Controls
the
overall gain .
of
the
input
amplifier
in
10
dB steps.
The
voltage
indications
around
the
knob
indicate
the
nominal
input
voltage
for
full scale
meter
deflection
with
the
METER AND
RECORDER
ATTENUATOR
in
its
"X
1"
position.
For
the
switch
positions
11
OVERLOAD
MODULATOR BAL
PR EADJUSTMENT
B.W.
COMPENSATION
BEAT
ADJUSTMENT
Fl
L
TER
OUTPUT
REJECTION BAL
12
connected
by
the
"Unity
Gain
Output"
line, voltage level
of
the
output
from
the
rear panel UNITY GAIN
OUTPUT
socket
is
unchanged
from
its
input
level.
Warning light flashes
when
the
input
amplifier
is
overloaded
by
too
high a sig-
nal level. When
the
light
rs
lit,
the
meter
reading
is
inaccurate
and
less
than
the
correct
value.
Two
finger-adjusted
potentiometers
for
tuning
the
filter
for
maximum
dynamiG
range.
To
be used
when
the
PRE-
ADJUSTMENT
"Modulator"
push-
button
is
depressed.
Two
pushbuttons
which
must
be
held
depr"essed
when
adjusting
the
balance
potentiometers
"Modulation"
and
"Re-
jection".
Two
pushbuttons
which
select
either
normal
operation
with
"Off"
depressed,
or
division
of
the
output
signal
by
the
square
root
of
the
selected filter effec-
tive
bandwidth
for
power
spectral densi-
ty
measurements
with
"1/.J""B" selected.
Only
one
of
the
buttons
can
be
selected.
For
fine
adjustment
of
the
Slave
Filter's
60
kHz
tuning
signal
when
a
61
kHz
tuning
input
is
employed.
One
of
two
pushbuttons
can be selected
to
give
either
0°
or
+
90°
phase
shift
at
the
selected
centre
frequency.
With
+
90°
phase selected,
there
is
no RE-
JECTION
mode.
Two
finger-adjusted
potentiometers
by
which
the
rejection
of
the
filter can
be
tuned
to
a
maximum
when
the
PRE-
METER
AND
RECORDER
ATTENUATOR
OVERLOAD
Fl LTER
BANDWIDTH
and
CROSS-OVER FREQUENCY
METER
AND
RECORDER
OUTPUTS
ADJUSTMENT
"Rejection"
push-
button
is
depressed.
Controls the overall· gain
of
the
output
amplifier in 10 dB
steps.
The
multiply-
ing factors marked around the knob
should
be
applied
to
the voltage indica-
tion
on the INPUT
ATTENUATOR
position
to
obtain the meter
full
scale
deflection. This control
has
no influence
on the
UNITY
GAIN
OUTPUT level.
Warning light which flashes when the
output
amplifier
is
overloaded
by
too
high a signal level. When the light
is
lit,
the meter reading
is
inaccurate
and
is
less
than the correct values.
Two
rows
of
pushbuttons which select
the internal
filter
bandwidth
of
the
2021. When
"Auto"
is
selected,
and
the
rear panel connections
to
a signal gener-
ator
are
made, the
filter
bandwidth
is
switched automatically at the selected
CROSS-OVER FREQUENCY. The
lights above the
FILTER
BANDWIDTH
buttons indicate which
filter
is
selected.
If
more than one
filter
is
selected, the
light
lit
(and
filter
actually in circuit)
will
be
the lowest bandwidth selected.
If
one CROSS-OVER FREQUENCY
is
selected also, the
filter
selected
will
be
the lowest selected bandwidth below
the cross-over, and the second lowest
selected bandwidth
filter
above
that
cross-over frequency.
Two
pushbuttons
with
which either the
filtered signal
("Filter")
or the input
sig-
13
nal
minus
the
filtered signal ("Rejec-
tion")
can be selected
for
display on
the
meter
of
the
2021
and
for
output
at
the
rear panel RECORDER
OUTPUT
sock-
ets.
EFFECTIVE AVERAGING TIME This switch
controls
the
effective aver-
aging
time
of
the
RMS
meter
rectifier in
steps
from
0.3
sec
to
100
sec. When one
of
the
three
positions
from
the
extreme
anticlockwise position
is
selected,
the
averaging
time
is
switched
at
the
same
time
as
the
filter
bandwidth
to
give a
mean statistical
error
for
the
RMS level
of
Gaussian noise less
than
the
"Error
%"value
selected.
2.2.
REAR
PANEL
Signal Reference Fixed Osc.
Frequency
Power
Meter
Sens.
Fig.2.2. Rear
Panel
of
2021
Fixed Osc.
Level
Adj
.
Input
F
ixed
Osc.
FIXED OSC. FREQUENCY 4-position screwdriver
operated
switch
on
which
the
frequency
of
the
fixed
oscillator
of
the
generator
tuning
the
Slave Filter should be selected.
14
SIGNAL
REFERENCE
POWER
REMOTE BANDWIDTH
CONTROL
Short
circuit
link
between
two
termi-
nals. With the
link
connected between
the terminals, signal ground
is
connect-
ed
to
the
chassis.
With the
link
removed,
signal ground
is
isolated
from
the
chas-
sis.
Input
connections
for
the power cord
provided. For selection
of
the appropri-
ate
supply voltage, remove the fuse in
the centre
of
the voltage selector and
turn
the white marker
to
indicate the
correct supply voltage.
Seven
pin
input
socket
for
the signals
which select the CROSS-OVER FRE-
QUENCY relays. Accepts Cannon plug
No.
WK-
A7-
21C
(B
& K stock num-
ber
JP
0700). Connection
of
the 2021
is
shown in Fig.2.3.
30
Hz
~1
Not
Connected
300
Hz s 4 3 1
00
Hz
3000
Hz
7 6 1000
Hz
Chassis 271153
Fig.2.3. Internal connections
of
REMOTE
BANDWIDTH
CONTROL
socket
RECORDER OUTPUTS
Two
BNC sockets
from
which the out-
put
signal
from
the
Slave
Filter
can
be
taken
to
an
external recorder. The
"AC"
signal
from
the
output
of
the out-
put
amplifier
is
10 Volts
RMS
for
full
scale
meter deflection
with
minimum
load impedance 15
krU
/1
nF. Alterna-
tively the
"DC"
output
of
the meter
rectifier
can
be
used
for
which approx.
15
UNITY GAIN OUTPUTS
60kHz
OUTPUT
EXT. FILTER
PHASE LOCK
VARIABLE OSC.
FIXED OSC.
"61
R:Hz"
16
+
5.5
Volts corresponds
to
full scale de-
flection.
Output
resistance
is
1
k.Q.
Two
BNC sockets
on
which either
the
"Filter"
or
"Rejection"·. signal appears
at
the
same level as
the
input
signal.
For
use in a feedback circuit where
the
in-
put
signal
is
also required for control
of
a signal generator
output
level. Maxi-
mum
output
is
15 Volts Peak, minimum
load impedance
10
k.Q//1 nF.
BNC
socket
which provides a
60
kHz
signal with
the
same
amplitude
and
phaSe characteristics as
the
fundamental
tuned
signal.
The
phase difference be-
tween
any
two
2021 s
at
this
output
is
less
than
1°.
Two
BNC
sockets
which
permit
connec-
tion
of
an
external filter.
Output
level
of
the
EXT. Fl LTER
"Input"
socket
is
a maximum
of
9 Volts
peak
to
peak.
Minimum load resistance
500
n.
Maxi-
mum
input
level
to
the
EXT.
FILTER
"Output"
socket
is
9 Volts
peak
to
peak.
BNC socket carrying a
50-60kHz
out-
put
signal suitable
for
driving a second
2021 when an accurate phase relation
is
required between
the
two
filters.
BNC
socket
for
input
of
the
Variable
Oscillator frequency from a signal gener-
ator. Either
the
25
-
30
kHz, or
the
50-60kHz
of
a 8 & K
Generator
will
normally be
connected
here.
BNC
socket
for
connection
of
the
61
kHz signal from a 8 & K Sine-Ran-
"30kHz-60kHz"
INPUT
FIXED
OSC.
LEVEL
ADJ.
METER SENS.
dom Generator when
such
agenerator
is
to
be
used
with
the 2021 .
Triaxial Belling-Lee socket
for
connec-
tion
of
the 30 kHz
of
a B & K
sine
gen-
erator or
for
the connection
and
(when
required) return
of
the 60 kHz
from
a
B & K Sine-Random Generator.
BNC socket
for
the input signal
to
the
Filter.
Input
impedance
is
1
MiU/180
pF.
Screwdriver operated potentiometer
for
adjustment
of
the level
of
the 60 kHz
return signal
from
the 2021 when a
61
kHz
input
is
used.
Screwdriver operated potentiometer
which adjusts the gain
of
the meter cir-
cuit
for
calibration purposes.
17
3.
DESCRIPTION
3.1. INTRODUCTION
The
Heterodyne Slave Filter
Type
2021 contains
the
input
and
output
amplifiers necessary for. signal processing bef!Jre
and
after
the
filtering.
The
only
essential item
of
additional
equipment
required
is
a
beat
frequency
oscillator with high frequency
outputs
of either
60
kHz
or
30kHz
fixed and
either
50
kHz
to
60kHz
or 25 kHz
to
30kHz
variable.
The
overall block diagram
of
the
instrument,
considerably simplified,
is
shown
in
Fig.J.1, while a more detailed
block
diagram
is
in
Fig.3.2. In
the
following sections details
of
the
block diagram are discussed
in
the
order
of
the
signal path through
the
instrument.
50-60
kHz or
25-30
kHz
18
External
Generator
Signals
External Filter
Input
Input
Amplifier
~--~
~--~
and
Attenuator
Filter
Unity
Gain
Outputs
1/JB
Attenuator
Low
Pass
Filter
15 kHz
t----.....,
Meter and Recorder
Recorder
Outputs
271309
Fig.
3.
1.
Simplified
block
diagram
of
2021
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