Brüel & Kjær 2425 User guide

Electronic Voltmeter • 0.5 Hz - 0.5 MHz
0 dB -60 dB
1 M!l 47 pF
dB 60
Typ e 2425
Briiel & Kjier
Copenhagen
40 30 20
Electronic Voltmeter
Type 2425
A mains or battery operated precision
instrument, measuring true RMS, Average
+Peak, - Peak
or Max Peak voltage
levels from 100
fl V to 300 V over the
0.5 Hz to 500 kHz range. Included
are a Peak Hold mode and time
constants giving Fast and Slow
meter damping characteristics . The
the Average Fast mode are in
accordance with the ANSI Standard
C16. 5-
1961 for volume ("vu")
measurements.

ELECTRONIC VOLTMETER TYPE 2425
August 1972

CONTENTS
1. INTRODUCTION ...................................... 5
2. CONTROLS .......................................... 6
2.1.
Front Panel ....... ~ .......................... 6
2.2.
Rear Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. OPERATION ........................................ 12
3.1. Preliminary ... ... . .... . . .................... 12
Mounting
....... ...... ..................... 12
Mains Voltage and
Fuse selection ................ 12
Meter
Scale Installation ....................... 13
Grounding Precautions
........................ 14
3.2. Measurement Terms and their Applications
........ 16
RMS ...................................... 16
Average
.... . ............................... 18
Peak ............. . ........................ 20
"dBV" and "dBm" ........................... 21
"Volume" ..... . ............................ 22
3.3. Voltage Measurements
........................ 23
3.4.
dBV and dBm Measurements ................... 24
3.5. Volume Measurements
........................ 26
3.6. AC OUTPUT Socket . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.7.
Use of DIN socket ........................... 28
DC Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Connection
of External TIME CONSTANTS ....... 29
Remote Meter
Reset . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Battery operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4. DESCRIPTION ....................................... 32
4.1.
4.2.
AC Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Input Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Output Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Meter Circuit ............................... 34
Input Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
RMS Rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Peak Rectifier
Average Rectifier
........................... .
Meter
Amplifier ............................ .
4.3. Power Supply
.............................. .
38
40
41
42
42
42
43
4.4. Overall Characteristics ....................... .
Dynamic
Range ............................ .
Frequency and
Phase Characteristics ............ .
Influence of Phase Characteristics on
Measurement Accuracy
........................ 44
5. APPLICATIONS ...................................... 47
5.1. Recording .................................. 47
AC Recording
............................... 47
DC Recording
............................... 50
5.2. Use as a Compressor Amplifier .................. 52
6. ACCESSORIES ....................................... 56
7. SPECIFICATIONS .................................... 59

1. INTRODUCTION
The Electronic Voltmeter Type 2425 is a precision instrument for true
RMS, Average, +Peak, -Peak and Max. Peak measurements on signals with
voltage levels from
100 J.J.V to 300 V in the 0.5 Hz to 500 kHz frequency
range.
It is basically a wide range measuring amplifier with calibrated gains of
-50 to + 60 dB, which are selectable in 10 dB steps to give twelve voltage
measurement ranges
of 1 mV to 300 V full scale. The gains are factory
preset, eliminating
the need of further calibration by the user and therefore
make the 2425 an extremely simple instrument to operate.
For R
MS measurements the meter circuit of the 2425 includes a true
RMS rectifier with a measurement accuracy of ±0.5 dB for signals with
crest factors
up to 5 ( 13 dB above FSD). Rectifiers are also included for
Average, and for +Peak, -Peak and Max. Peak measurements. The Peak
rectifier has two selectable rise times enabling signals with peak durations as
short as
50 J.).sec to be measured. For measurements on non-repetitive signals
a Peak Hold mode
is also provided.
To facilitate accurate reading of the display meter the rectifiers contain
electrical time constants giving "Fast" or "Slow" meter damping character-
istics which
in the RMS mode correspond to averaging times of 0.27 and 3.0
seconds respectively and in the Peak modes correspond to decay times of
2.7 and 30 seconds respectively. Facilities are also provided for connection
of external time constants permitting the choice of further averaging and
q~cay times. The time constant in the Average Fast mode gives meter damp-
ing characteristics in accordance with the ANSI Standard C 16.5-1961 for
volume ("vu") measurements, enabling the 2425 to be used as a volume
indicator for measurements on audio frequency transmission or telephone
lines.
For use
of the 2425 as a calibrated amplifier for recording purposes, or as
a compressor amplifier for regulating
the output of measurement systems,
both AC and DC outputs are available from the instrument. For field or
laboratory measurements the 2425 may be operated from a mains or bat-
tery supply.
5

2. CONTROLS
2.1. FRONT PANEL
Range 1---+-·•
1 Mn 41 pF
Meter
Function
Input--+-
METER:
6
Fig.2. 1. Front Panel of the 2425
A moving coil indicating meter with mirrored
back
to avoid parallax errors. The meter has
three interchangeable meter scales. These are:
SA 0162. This scale is provided with the in-
strument and has two voltage ranges and one
dB range. The voltage ranges are calibrated
from 0 to 10 V and 0 to 31.6 V respectively,
where 10V and 31.6V represent full scale.
The dB range is calibrated from -20 to 0 dB ,
where 0
dB represents full scale.

POWER ON:
RANGE 1:
RANGE II :
SA 0163. This scale
is available on separate
order. It
is similar to SA 0162, but also in-
cludes a range for dB m measurements (see sec-
tion 3.2.4). The range is calibrated from -20
to+ 2 dBm re 0.775 V which corresponds to a
power level
of 1 mW in a 600 n line.
SA 0168. This scale is also available on sepa-
rate order. It has two ranges, one calibrated in
"vu" for volume measurements (see sec-
tion 3.2.5) the other calibrated in percentage
modulation for broadcasting purposes. The vu
range is calibrated from -20 to+ 3 vu and the
percentage modulation range from 0 to 100%.
To change the meter scales refer to sec-
tion 3.1.3.
Push-button On/Off switch for power. When
set
to "On" (i.e. inner position) and an AC
mains supply is connected to the POWER
INPUT
socket at the rear of the instrument,
the meter scale lamps should light.
Two push-button switches actuating the input
attenuator of the 2425. They are used in con-
junction with the RANGE II push-button for
the selection of the V and mV voltages ranges
of the instrument. The two RANGE SELEC-
TOR I
push-buttons are mechanically inter-
linked so
that only one range may be selected
at a time.
Six push-button switches actuating a 0 to
50 dB attenuator for the selection of six dif-
ferent V or mV voltage ranges in conjunction
with the RANGE I push-buttons. The values
marked
on the push-button refer to the input
voltage (V or mV) for full scale deflection on
the meter. The dB value marked above each
push-button are dB re 1 V. The RANGE
SELECTOR II push-buttons are all mechan-
ically interlinked so
that only one range may
be selected
at a time.
7

INPUT:
METER FUNCTION:
METER DAMPING:
8
BNC input socket for application of the signal
to be measured. The input impedance is 1 MQ
in parallel with 47 pF. The socket accepts the
BNC plug (B & K Type JP 0035) provided.
Four push-button switches for selection of
"RMS", "Avg." (Average),"+ Peak"," -Peak"
and "Max. Peak" meter characteristic. The
required characteristic
is selected by pressing
the appropriate push-button so that it locks
into its inner position.
For "Max Peak" push-
in the "+Peak" and "-Peak" push-buttons
simultaneously so
that both are locked in
their inner positions. The Peak push-button
modes are used
in conjunction with the PEAK
HOLD switch of the instrument.
Two position switch for
the selection of time
constants giving "Fast" and "Slow" meter
damping characteristics for "RMS", "Aver-
age" and "Peak" measurements. The Fast
mode should
only be used for measurements
on signals with frequency components above
20 Hz, whilst the "Slow" mode can be used
for measurements
on frequency components
down to 0.5 Hz.
For the "RMS" mode of the METER FUNC-
TION push-buttons,
the time constants in the
Fast and Slow modes give averaging times of
0.27 and 3.0 seconds respectively.
For
the "Peak" modes of the METER FU NC-
TION push-buttons the time constants in the
Fast and Slow modes give decay times of 2.7
and
30 seconds respectively. The rise time for
"Peak" measurements is less than 50 psec and
500 psec respectively for the Fast and Slow
modes.
For "vu" measurements using the "Avg"
METER FUNCTION mode, the damping char-
acteristics of the meter with the "Fast"

PEAK HOLD:
METER DAMPING mode selected is in ac-
cordance with the ANSI Standard
c 16.5-1961.
Two position switch with the following func-
tions:
"Hold". When this mode is used in conjunc-
tion with one of the "Peak" modes of the
METER FUNCTION push-buttons the meter
will hold the peak value of the signal meas-
ured.
At 25°C, the peak value held in the
"Hold" mode will decay at less than
0.05 dB/sec in the "Fast" METER DAMPING
mode and at less than 0.005 dB/sec in the
"Slow" METER DAMPING mode.
"Reset". This mode is used to reset the meter
indication after a Peak Hold measurement has
been made, It
is also used for Peak measure-
ments
on continuous or repetitive signals
where the Hold facility is not required. The
decay time constant in this mode is dependent
on the METER DAMPING mode which when
set
to "Fast" and "Slow" modes is 2.7 and 30
seconds respectively.
Note: With
the RMS or "Avg" METER
FUNCTION modes selected the PEAK HOLD
switch is automatically disconnected from the
circuit and therefore has no influence on the
damping characteristics of the meter for RMS
or Average measurements.
9

2.2. REAR PANEL
DIN Socket
AC Output
DIN SOCKET:
AC OUTPUT:
10
Fig.2.2. Rear Panel of the 2425
Fuse and
Voltage
Selector
Power
Input
A 7-pin Dl N socket accepting the Dl N plug
(B & K Type JP 0703) provided for connec-
tion of the DC output from the meter circuit
of the 2425 to an external monitoring meter,
Level Recorder, etc.
The socket may also be
used for connection
of a battery supply as
well as for connection of external time con-
stants to give the choice of averaging and
decay times for R
MS and Peak measurements
respectively.
The necessary connections to the
socket are given in section 3.7.
Output socket for AC recording. The socket
accepts
the BNC coaxial plug (B & K Type
JP 0035) provided and has an output imped-
ance
of 100 n. The minimum AC load imped-
ance
is 2 kQ//200 pF. The maximum output
voltage is 4.46 V peak, whilst the output volt-
age corresponding to full scale deflection on
the meter is 1 V RMS.

FUSE AND
VOLTAGE SELECTOR:
POWER INPUT:
Transformer
shield
and chassis
Voltage selector for selection of the correct
mains voltage. On delivery,
the central knob
of the selector is fitted with a 1 00 rnA fuse
for operation with a
220 or 240 V mains sup-
ply. For operation with a 100, 115, 127,
or
150 V supply a 200 rnA fuse is provided.
To change the fuse, unscrew the central knob
and pull it straight
out.
Behind the fuse is a black selector bush con-
taining a window in which the mains voltage is
indicated. To adjust the selector, pull the bush
straight
out and set the pin behind the bush
into
the socket nearest the correct voltage
indication on the selector, which should then
appear in the window.
Note: Verify
the mains voltage selection and
the type of mains fuse before each use.
Input socket accepting European or American
power cables
(B & K Types AN 0005 and
AN 0006 respectively) for connection of an
AC mains supply. The connections to this
socket are shown in Fig.2.3.
AC
Supply
172008
Fig.23. POWER INPUT socket of the 2425
11

3. OPERATION
3.1. PRELIMINARY
3.1.1. Mounting
The 2425 can be used free standing (cabinet Type A) using the metal
stand provided to slightly raise the front of the instrument for better view-
ing (see Fig.3.1). It may also be used in the Bri.iel & Kjrer KK Rack Mount-
ing System which is described in the KA, KK, KS Assembly Instructions.
Voltmeter/Ampllfiet • 0.5 H1 - 0.6 MH•
Type 2426 ifi@ii
Bniel & Kjll!f
~n I"'S · Hip
10 JO 0
300 1 00 30 -10 3 1
Fig.3.1. Use of the metal stand to raise the front of the 2425 for better
viewing
3.1.2. Mains Voltage and Fuse Selection
Before each use of the 2425, check the VOLTAGE SELECTOR and the
mains fuse it contains corresponds to the mains supply voltage to be used.
12

On delivery the selector is fitted with a 100 rnA fuse, which is suitable for
use with a 220 or 240 V mains supply. For operation with a 100, 115, 127,
or 150 V mains supply a 200 rnA fuse is provided. The fuse is contained in
the central knob of the selector which may be unscrewed and pulled straight
out in order to change the fuse. Behind the fuse is a black selector bush
containing a window
in which the mains voltage is indicated. To set the
selector to the correct mains voltage, pull the bush straight out and set the
pin behind the bush into the socket nearest the correct mains voltage indica-
tion, which should now appear
in the window.
3.1.3. Meter Scale Installation
Three meter scales are available for use with the 2425. One is scale
SA 0162 which is provided with the instrument and is already mounted on
delivery, whilst the others are scale SA 0163 and scale SA 0168 which are
available on separate order.
Scale SA 0162 is calibrated for voltage measurements and has two volt-
age ranges and one dB range. The voltage ranges are calibrated from 0 to
10 V and 0 to 3.16 V respectively where 10 and 3.16 V represent full scale
deflection. The dB range is calibrated from -20 to 0 dB where 0 dB repre-
sents full scale.
Scale SA 0168 has
two ranges. One is calibrated from -20 to + 3 vu re
0.775 V RMS for volume measurements (see section 3.5) and the other
472073
Fig.3.2. Rear view of indicating meter showing insertion of a meterscale
13

which is used in conjunction with the vu range indicates percentage modula-
tion for broadcasting purposes, and is calibrated from 0 to 100%.
Scale SA
0163 is similar to SA 0162, but also includes a range for dBm
(measurements see section 3.4). The range
is calibrated from -20 to
+ 2 dB m re. 0. 775 V which corresponds to a power level of 1 mW in a
600 n line.
In order
to change the scales, the top panel of the instrument must be
removed. This can be
done by removing the fastening screw for the top
panel which is on the rear panel of the 2425, and sliding the top panel
towards the rear of the instrument. The meter scale may then be removed
by unscrewing
the two knurled nuts shown in Fig.3.2 to release the meter
scale lamps, enabling the meter scale to be lifted straight out.
3.1.4. Grounding Precautions
In common with most mains operated equipment the 2425 has a small
capacitance path to ground which may result in noise pick-up. This coulrJ
influence
the accuracy of measurements, but may be considerably reduced
by making sure
that the measurement system is adequately grounded. Usual-
ly,
this is best carried out by connecting the chassis of the 2425 to the
chassis of the equipment to be investigated and to ground the system at a
point
on the equipments chassis which is nearest the measurement point.
This may be done using coaxial cable as shown in Fig.3.3.
The cable's screen
will not only serve to connect the two chassis, but will also help to prevent
Equipment
being investigated
· -~ ~ ~
i
~ .... ~ ..... ~ ,. .. !~!~!~ ...... ~ .. . .. ~ . _, !1!1!!1 .••, !1111 . II
Screened Cab I~ .... • ... "" iiii
I .: ' ~~ . ....
Ge----------------~~~ ~ --------
1 I
..L
...
-
Electronic Voltmeter
2425
472075
Fig.3.3. Connection of the 2425 to the equipment to be investigated show-
ing the correct grounding arrangement
14

noise pick-up through radiation when the system is situated in strong electri-
cal fields.
If it
is not possible to ground the system at a point on the equipments
chassis which is near to the measurement point, then it may be grounded
using the CHASSIS GROUND socket or the chassis and transformer shield
connections of the POWER INPUT socket (see Fig.2.3) on the rear panel of
the 2425. However, which ever point is used it is important to remember
that it should be the only point in the system which is grounded. If more
than one point is grounded then a ground loop will be formed which will
also give rise
to noise pick-up.
It is also important to note that the cabinet of the 2425 is connected to
its chassis and although the instrument is mounted on four insulating feet, it
is possible to inadvertently introduce an extra ground path if the cabinet is
touching other instruments or metal surfaces.
This latter fact is not only important from a measurement point of view,
but also as regards safety when checking the voltage across two terminals
which are both at some DC potential with respect to ground. In this case a
c
c Electronic Voltmeter
. 2425
, Chassis
Ground
Socket
Fig.3.4. Measuring arrangement for checking the voltage across two termi-
nals, which
are both at some DC potential with respect to ground.
The capacitors in series with the measurement line are used to
isolate the
2425 from the measurement point
15

capacitance should be connected in series with both the screen and the core
of the coaxial cable used to connect the INPUT of the 2425 to measure-
ment point as shown in Fig.3.4. The system may then be grounded using the
CHASSIS GROUND socket or the chassis transformer shield connections of
the POWER INPUT socket (see Fig.2.3) of the 2425.
3.2. MEASUREMENT TERMS AND THEIR APPLICATIONS
3.2.1. RMS
The RMS (root mean square) mode of the 2425 will probably be found
the most useful of the three measurement modes provided as it is this value
which is most commonly used to characterize the magnitude of alternating
signals. It derives its significance from the fact
that it is directly proportion-
al to the amount of power dissipated in linear systems.
Mathematically the R MS value can be expressed as:
-(3.1
where VRMS is the RMS value of a signal V (t) which is averaged over a
timeT= t2 -t1, as shown in Fig.3.5.
Since it
is the RMS value which is usually required it has become com-
mon practice to calibrate instruments in terms of RMS even though they
actually measure average or peak values (see sections 3.2.2 and 3.2.3 respec-
tively for an explanation
of average and peak). This is justified for specific
V + Peak
V - Peak
472068
Fig.3.5. RMS, Average,+Peak and -Peak values of an alternating signal
16

types of signal which have fixed relationships between RMS, Average and
Peak values. These relationships are
the crest factor which is defined as:
and
the form factor:
= VPeak
Fe
VRMS
VRMS
V Average
-(3.2
-(3.3
However, the values given by these relationships change for different
types
of signal and therefore in order to obtain the RMS value of a wide
variety of signals a measurement circuit which measures true RMS values is
required. For this reason the 2425 has been given a RMS rectifier circuit
which measures
true RMS values for signals with crest factors up to 5 as
shown
in Fig.3.6.
Max. Input
Voltage +
380 V RMS
3.16
3.0
(/)
~
~ 2.5
0
>
c:
· ~ 2.0
(J
'0
~
Q)
13 1.5
(/)
~
Q)
:::?!
1.0
0.5
10
9
8
7
6
5
4
3
2
0 2
Crest factor li mitation
for all mV and V ranges
except the 300 V range
due
to max. output voltage
limitation of 4.46 V peak
Crest
factor
limitation of
300 Volt range
due
to max. input
Voltage lim itation of
380 V RMS and 600 V peak
3 4 5
Crest factor
VPeak!VRMS
472079
Fig.3.6. Crest factor limitation of the RMS measurement mode of the 2425
due to the maximum input and output voltage ratings of the instru-
ment
17

With the exception of the 300 V range of the instrument the crest factor
is limited by the maximum output from the AC Amplifier which is 4.46 V
peak. Therefore
at full scale deflection on the indicating meter the maxi-
mum crest
factor is limited to 4.46, whilst for deflections less than -1 dB
on the meter
scale it is limited to 5. For the 300 V measurement range the
crest
factor is limited by the maximum input voltage which in the "V"
modes is 380 V RMS or 600 V peak. In the "mV" modes the maximum
input voltage is 80 V RMS and therefore does not restrict the crest factor.
However care should
be taken to ensure that these maximum input voltage
ratings are not exceeded as otherwise the instrument could be damaged.
In extracting the RMS value the RMS rectifier of the 2425 employs two
averaging time constants which may be selected using the METER DAMP-
ING switch. In the "Slow" meter damping mode the averaging time con-
stant
is approximately 3.0 seconds which is suffuciently long to average
signals with repetition frequencies as low as 0.5 Hz the lower limiting fre-
quency*
of the "Slow" damping mode. In the "Fast" meter damping mode
which
has a lower limiting frequency of 20 Hz, an averaging time constant
of 0.27 is used. If averaging time constants other than those provided are
required
then external time constants may be connected to the R MS circuit
using the Dl N socket connections given in section 3.6.2.
3.2.2. Average
As
with the RMS value the average or rectified average value is a time
averaged quantity. Mathematically it is expressed as:
1 t2 I I
VAvg =T 1 V(t)
t1
dt
-(3.4
where VAvg_ is the average value of a signal V(t) which has been averaged
over a
timeT= t2 -t1 as shown in Fig.3.5.
The Average mode
of the 2425 gives the rectified average value and will
be found useful for vu (Volume Unit) measurements in which the dynamic
magnitude of complex audio frequency electrical waves are characterized
(see section 3.2.5). It will also prove useful in investigations dealing with
mechanical forces which are of an electromagnetic origin.
*
Unlike the cut-off frequency, the limiting frequency is an arbitrary limit placed on a
system
to define its frequency response and may be defined as required. In the case
of the 2425, the limiting frequency is the frequency at which the response falls by
0.5dB.
18