BRUEL & KJAER 4712 User guide

Frequency Response Tracer
Type 4712
The Frequency Response Tracer is designed
especially
for qua li ty control of electro-acoustic
devices. T oge ther w ith • B & K B ea t Frequency
Osci ll ato r Type 1022 it makes possib le fully auto·
matic, eas ily operated test systems, su itable for
checking the frequency response of microphones,
loudspeakers, hearing aids, tape recorders, audio-
frequency filters etc. in the frequency range
20-20000 Hz.
BRilEL& K~J.ER
NA;.RUM , DENMAR K. T ele ph .: 800500 . Cabl e: 8RUKJA. Telex: 5316. COP ENHAGEN
BB 4712.03000 09

Frequency Response Tracer
Type 4712
January 1968

Contents Page
O. Introduction ....... . .... .. . .. .. .. ... ... . . .. . .. . ... .. .. .. .. . . . .. . .. 3
1. Control Knobs, Terminals, etc. .. ... .. .. . ... . .. . ....... .. . .. .. . ...... 4
2. Operation ..... . ................... ..... .. ... . ... . ..... .. .. . ... . . . 7
Setting up .............. ...... .. . ...... . ...... . ... . ... . ... . . .. .. .. 8
Test Procedure . . . . . .. . . . . . .. . . . . . . .. . . . . . . .. . . . . .. . . . . . . .. . . . . .
.. 9
3. Technical Description . . . ... . .. . ... .. .. .. ..................... .... . 11
Potentiometer and Attenuator . .. . ... . . . .. ... . .. .. .. .. .. . .. . .. . . .. .. 11
Linear Amplifier . ..... .. . ............. . ...... . ............. . .... . .. 11
Logarithmic Amplifier ..... .. .. .. .. . ..... ... .... .. ...... .. .. . . .. .. . . 12
Frequency Sensitive Circuit ......... . ...... . . . ...... ... .......... . . 14
DC Amplifiers . ... ............. .. .. ...... .. .. . . .. . ... .... ... .. . . . .. 16
Diferentiator . ... . . ... ... .. .... . . . ....... .. . .. . . .. ... . ... . .. .. . .. . . 17
Sweep Limit Control . ............ .. . . .... . . . .. .. ............. . ... . . 17
4. Applications ...... .. ... ... ........ .. ... . . .......... . ..... .. ... .. .. 18
Measuring Voltage and Frequency .. .. .. .. .. .. . . . . .. ... ........ ... .. 18
Frequency Response of Amplifiers .. ..... . .............. . ..... ..... . 19
Frequency Characteristic of Tape Recorder .... . . . . . .. .. . .. . ... . . .. . 19
Frequency Characteristics of Hearing Aids ... ............ .. . .... . . .. 20
Frequency Response of Mechanical Components .... .. .. . . .. .... .... 22
Frequency Characteristic of Microphones . .............. .. ....... . . . 22
Frequency Characteristic of Earphones ..... . ... .. . . .. .. .. . .. . ...... 23
Automatic Testing of Telephone Sets . ... ......... . .... . . .. .. ... . .. .. 24
Frequency Characteristic of Transmission Lines . . .. .. .. ... . ....... ... 24
Frequency Response of Narrow Band Filters and Carrier Frequency
Equipment
..................... .. ... .. .. .. ......... .. . . ......... . 24
Appendix A . .......... . . ... . .. . .. .. ... ... . ................ . ..... .. .. 27
Appendix B ..... .. . ... . . .... .. ...... . . ....... . . .. . . ..... . . .. .... .... 30
Appendix C ..... . ..... .. ..... .. . ........ .. . .. ..... .. .. ... .... ... .... 31
Specifications . .. ... . .. .. .... .. ... .. .. ........................... . . .. 34

O. Introduction
The Frequency Response Tracer Type 4712 is designed especially for quality
control of electro-acoustic devices. Together with one of the B & K Beat Fre-
quency Oscillators it makes possible fully automatic, easily operated test
systems, suitable
for checking the frequency response of microphones, loud-
speakers, hearing aids, tape recorders, audio-frequency filters etc. in the
frequency range 20-
20000 Hz. This range can be extended to 200 kHz by
means of a mixer stage
or frequency dividers, see page 26 and 33 .
The frequency response
of mechanical constructions may also be investigated,
if transducers are available
for converting mechanical vibration into electrical
signals.
The frequency response of the test object is displayed for visual inspection
on a large calibrated screen with a
logarithmic frequency scale and selectable
logarithmic or linear amplitude scales.
Apart from typical production control applications the instrument will prove
useful for many laboratory purposes where components are investigated for
amplitude/frequency response. The Tracer can not, however, be used on its
own as a frequency analyzer since
it has no frequency selective properties.
It indicates the amplitude of the input signal as a function of the frequency.
The mechanical sweep
of the B & K oscillators is achieved with a small
electrical motor, supplied with the
Tracer . This motor is installed inside the
oscillator and obtains its power from the Tracer .
3

1. Control Knobs, Terminals etc.
Front Plate.
Dynamic Range
Inpu t Potentiometer
Input Attenuator
Signal Input
Ground -----'"
GRADUATED SCALE
DYNAMIC RANGE
4712
/6711 4
Fig. 1. 1. Front plate of the Tracer.
Vertical Speed
Intensity
Scale Illumination
Sweep Control
Ext. Fl1Iquency
Plug- in Unit
Interchangeable plexiglas scales are supplied, with
frequency graduation according to the plug-in unit
for frequency range. Standard ranges: 200- 5000 Hz
and 20-
20000 Hz.
Four-position
switch selecting the dynamic range
of the instrument. Logarithmic 0-5 dB re 100 mV,
0- 25 dB and 0- 50 dB re 10 mV, or linear 0-1 V.
The sensitivities
indicated are for zero attenuation
of the input signal.
INPUT POTENTIOMETER For fine adjustment of the input voltage to the
amplifiers. Covers a range
of approximately 10 dB .
4

INPUT ATTENUATOR
INPUT SOCKET
VERTICAL SPEED
INTENSITY
SCALE ILLUMINATION
AND POWER SWITCH
SWEEP CONTROL
EXT. FREQ. INPUT
PLUG-IN UNIT
Back Plate Controls.
BLANKING LEVEL
For adjustment of input voltage. Selectable in 10 dB
steps from 0 to 60 dB.
For the input signal. Input voltage should not ex-
ceed 200 V RMS. The socket fits the B & K screened
plug type JP 0018. Input impedance 100 kohm.
Controls the speed with which the vertical deflec-
tion will
follow a sudden change in input ampli-
tude. "
Slow" or "Fast" corresponding to rise times
of 60 or 6 msec respectively.
Potentiometer controlling
the intensity of the trace
on the screen.
Potentiometer
controlling the current through four
bulbs
for scale illumination. Also main power switch
for the instrument. Power is off when the switch
is turned fully
counterclockwise .
For
controlling the sweep motor in the Beat Fre-
quency Oscillator. Single shot
or continuous sweep-
ing.
For external frequency input when the input signal
is noisy
or when it is necessary to control the
x-deflection independently. A microswitch in the
socket is activated when the input plug (Type JP
0018) is inserted in the socket and switches off the
internal frequency input to the frequency sensitive
circuit. Input impedance 5 kohm.
Determines the frequency range
of the instrument.
Type ZS 0120
logarithmic 20-20000 Hz and Type
ZS 0121
logarithmic 200- 5000 Hz, are standard
ranges supplied with the instrument, Type ZS 0124
linear 150-4150 Hz is delivered on special order.
Continuously variable
screwdriver operated poten-
tiometer
controlling the level below which the trace
is extinguished. Covers at least the lower half of
the scale.
5

BlanklngL-1
SWMPSpeed
Blanking Level
,.-......
H~h
Sweep Speed
Motor DrI.,.
Rem*Sto"
Motor Drive Remote
Start
R etu mMod e
_01
Norma l
Up"",
Return Mode
Fast. • Fast
S-pUmiti
Sweep Limits
R etum Tra ce
On Off La-
Return Trace
law High
flf"
Fig. 1. 2. Control panel at the back of the Tracer.
MOTOR DRIVE
RETURN MODE
RETURN TRACE
SWEEP SPEED
REMOTE
START
SWEEP LIMITS
6
6-pin socket for control of the sweep motor in the
Beat Frequency Oscillator. Control cable with plugs
supplied with the instrument.
Determines the return sweep from the upper to the
lower frequency limit. The return may be clock-
wise or anticlockwise, fast or normal, depending
on the setting of this switch.
On-Off switch for the intensity of the trace during
backward scanning.
Continuously variable potentiometer controlling the
speed
of the sweep motor in the B.F.O. The speed
is variable from about 1/3 to 3 octaves per second.
For remote control of the sweep motor. Short-cir-
cuiting the two active pins will start the motor.
Continuously variable potentiometers determining
the upper and lower sweep limits. Both potentio-
meters cover the whole frequency range so that any
part can be selected for automatic scanning.

2. Operation
Principle of Operation.
The Frequency Response Tracer requires a sinusoidal signal generator for
its operation. One
of the Briiel & Kjrer Beat Frequency Oscillators Types 1013,
1017
or 1022 is eminently suited for this, since a sweep drive motor can be
built into the Oscillator and automatically controlled from the Tracer for quick
testing
of components on a production line.
The 1013 covers the frequency range 200- 200000 Hz, the 1017, the low fre-
quency range 2-
2000 Hz and the 1022 covers the whole audio frequency
range 20--20000
Hz each of them in one sweep, which is very convenient for
most applications of the Tracer.*)
The Beat Frequency Oscillator supplies a variable frequency signal to the
test object. The output from this is fed to the input socket of the Frequency
Response
Tracer which gives an x-deflection proportional to the frequency
of the signal and a y-deflection
proportional to amplitude. Then as the fre-
quency range is swept through, the
frequency response of the test object is
shown on the
14" screen of a cathode ray tube, the large screen giving ex-
cellent resolution of details in the curve obtained.
Beat Freq . Oscillotor
Freq . Re s ponse Tracer
Ex t. Freq. Input /6J,8
88
Fig. 2.1. Sketch of a typical test set-up for automatic frequency response
tracing.
*) With the introduction of the Frequency Re spo nse Tracer and the associat ed sweep motor,
it h as been found ne cessary to use a stronger construction of the sweep mechanism of
the Beat Frequency
Oscillator. If the Tracer is intended for production line purposes
with continuous operation, customers are advised to ord er a new Oscillator even
though they may already possess one. The modifications have been incorporated in all
the B & K Oscillators as from April 1964 .
7

Before automatic operation can be realized it is necessary to install the drive
motor for the Beat Frequency Oscillator . Instructions are given in Appendix A
on page
27 . Also make sure that the blind plug inserted in the BFO's REMOTE
CONTROL
socket is pulled out, otherwise the output from the BFO is shorted
during part
of the sweep which may result in undesired switching of the
sweep directions.
Setting up.
The setting up of the instrumentation depends entirely upon the kind of
test to be performed, so
that only a general outline can be given here. See
the chapter on applicat ions for some examples on different practical set-ups.
The sketch given in Fig. 2.1 indicates the most common components of a
test set-up,
but all of these may not be necessary in any particular case.
The purpose
of the various instruments and connections in this set-up are
as follows:
Signal Generator.
Supplies the input signal for the test obj e ct as well as an ex-
ternal frequency signal to the
Tracer when the output signal from
the test
object is noisy or badly distorted.
The applied generator is
the B & K Beat Frequency Oscillator
Type 1022 equipped with a sweep motor for automatic operation.
Frequency range 20-
20000 Hz.
Power Amplifier.
Used
for vibration tests but seldom necessary for electro-acoustic
devices as
the output impedance of the Beat Frequency Oscilla-
tor can be selected according to the input impedance of the test
object, and
it can supply up to about 2.5 watts power.
Compressor Amplifier.
Preamplifier
The compressor
amplifier is used in connection with the com-
pressor
circuit of the BFO for keeping the input voltage to the
test
object constant over the fr equency range.
This
amplifier may be required when the output from the test
object can not be loaded with an impedance as low as 100 kohm
(input impedance
of the Tracer), or when the signal is too weak.
Otherwise the
output can be ta !< en straight to the Tracer input
socket.
Motor Control Cable.
8
This cable , which is delivered with the Frequency Response
Tracer is necessary for the power supply to, and the sweep
control of,
the motor installed in the BFO.

Test Procedure.
1. Set up the test instrumentation as required. (See the chapter on
Applica-
tions for typical examples.)
2. Before
switching on make sure that the voltage selector at the back of
the instruments are set to the
corr e ct mains voltage.
3.
Adjust for a suitable output voltage from the oscillator, set DYNAMIC
RANGE as required, and sw ee p through the frequency range manually
to see
that the whole of the frequency response characteristic is shown
on the
Tracer . If there is no trace , check that the BLANKING LEVEL
potentiome
ter at the b ack of the instrument is turned fully down and
turn the INTENSITY knob
clockwise until the spot is just visible on the
screen when sta tion ary. Then adjust the INPUT ATTENUATOR and INPUT
POTENTIOMETER until a suitable
amplificat ion is obtained.
4. Set the SWEEP CONTROL
switch on the Tracer to "Cont. Sweep" and
the AUTOMATIC SCANNING
switch on the Beat Frequency Oscillator
to " Off" . The upper and lower sweep limits are now set with the SWEEP
LIMIT
potentiometers at the back of the Tracer, checking by turning the
frequency dial on the BFO manually that the sweep motor changes
direction at the correct positions. Set AUTOMATIC SCANI~ING to "On "
again.
5. Set the RETURN MODE
switch to the position required for the actual
test.
6.
Adjust the BLANKING LEVEL potentiometer to bl ank off any undesired
part of the lower half of the Tracer screen.
7.
Switch RETURN TRACE to the required position, "On" or "Off ".
8. Adjust the SWEEP SPEED potentiometer , starting with low speed and
increa sing until the curve is displaced in the sweep direction as much
as the measuring
accuracy allows for the particular test to be carried out.
9. Set VERTICAL SPEED to the desired position. "Fast " if only frequencies
above 200 Hz are of interest. At 200 Hz a vertical ripple of approx. 1
dB
is present in this position . At higher frequencies the ripple decreases
rapidly , and at lower frequencies it increases rapidly up to 20--30 dB at
20 Hz (when 20 Hz is constantly applied to the input a vertical line of a
length
of 20-30 dB shows up). If the signal strength to the vertical
amplifier (of a frequency above 200 Hz) is changed suddenly (e.g. by
means of the INPUT ATTENUATOR). the
spot will jump vertically and
indi
cate the change after 6 milliseconds . This time delay will always be the
same whatever the change to the vertical
amplifier . An " overshoot" of
appro ximately 10 per cent of the applied sudden change is normal.
The
position " Slow " should be used when also frequencies below 200 Hz
are
of interest to achieve sufficient readability. In this position the ripple
will be appro ximately 1 dB at 20 Hz and the above mentioned delay will
be 60 milliseconds approximately.
9

It is obvious that the delay will have an effect on the frequency response
curve to be indicated if the sweep speed (motor speed in the BFO) is
too high. When the spot moves from left to right, the curve will be to the
right of where it should be, and when it moves from right to left it will
be to the left.
It can be checked whether the motor speed chosen is too high by com-
paring the curve drawn from left to right with the one drawn from right
to left. If they are not sufficiently close to each other for the specific
case, the sweep speed (motor speed) should be reduced.
The instruments are now ready for automatic operation and all that is required
from the operator is to place the sample to be tested in the test position, and
use the sweep control switch on the Tracer.
For testing of mass produced items it may be helpful to draw in tolerance
curves on the Tracer screen for easy detection of undesirable deviations
from specifications. For this purpose 5 blank scales are supplied with each
instrument. They are easily placed in front of the calibrated scale behind the
two truncated cone nuts, but may be difficult to remove without the use of
the special suction
grip supplied.
10

3. Technical Description
A block diagram of the Frequency Response Tracer is given in Fig. 3.1 . A
description of its various functional blocks follows.
Signal
In put
Ext. Frequenc y
Input
I-_---'t~ y-Deflection
C oits
1-_---1~ ~~~:fl ection
'----'-.---_...J
Voltage for
Automat ic Br i ghtne ss
r--------j
_______ ~ Motor :
I in B. F.O . I
'- _______ 1 16 ~ 88 ~
Fig. 3.1. Block diagram of the Frequency Response Tracer.
Potentiometer and Attenuator.
The input potentiometer is continuously variable over about
10 dB, while
the attenuator is
switchable in 10 dB steps, so that continuous amplitude
control of the input voltage is obtained.*)
Input voltage may range from 0 to
200 V RMS, higher voltages will dissipate too much energy in the relatively
low resistance input potenti ometer and should therefore be avoided.
Linear Amplifier.
The linear
amplifier is a conventional design with a cathode follower in the
input, voltage
amplifier and phase inverter in the output. The output signals,
in opposite phase, are fed to a balanced rectifier, which in connection with
the succeeding
filter provides a balanced output voltage proportional to the
*) Note. If signa ls above 50 kHz are measured, the potentiometer should be set to ma xi mum.
Otherwise the frequency response will be affected .
11

arithmetic average of the input signal, but the screen is calibrated to give
correct RMS indication
for sinusoidal signals.
The linear scale is
accurate within 1 % from 1/ 10 to full scale. Non-linearity
at the
lower end is caused by diode characteristics .
Logarithmic Amplifier.
The Logarithmic Amplifier is designed to give an output which is proportional
to the logarithm of
the input voltage, in the graph Fig. 3.2 this is shown as
a
straight line. Also the output from a linear amplifier is indicated on the
graph. To convert the curved line into the straight line a number
of linear
amplitude limited amplifiers are employed in a cascade coupling. By means
of the amplitude limitation the
characteristic of each stage is changed into
the curves
J" JII , •• •• J
V1
in the graph, with horizontal spacing equal to the
amplification
per stage. To obtain the straight line the outputs are summed
by connecting the
output from each stage to one common load through
resistors which are large compared to the load resistor. Choosing the
correct
amount of limitation and amplification per stage it is possible to obtain a sum
curve J1 + J11 • • • • + JV1 with very small deviation from a straight line, and
thus the output is closely proportional to the logarithm
of the input signal.
Lin.
Vout
or lout
J
Logarithmic
Amplifier
1+ II--·.]1
o 6 12 18 24 30 36 42 48
Vin dB
Vin
2Olog Vref
f65f29
Fig. 3. 2. Curves showing the characteristic of the amplitude limited amplifier
stages in the logarithmic amplifier, as well as a curve indicating the characteri-
stic of a linear amplifier and a straight line indicating the sum curve of the
logaritmic amplifier .
12

The curves shown in Fig. 3.2 are valid for the 50 dB range of the Tracer, the
spacing between the curves
corresponding to 10 dB amplification per stage.
The
25 dB range is obtained by reducing the amplification to 5 dB per stage.
The curves
J" J" .... Jv, then have 5 dB spacing and the sum curve (straight
line) will have twice the slope shown. When changing from 10 to 5 dB ampli-
fication per stage 25 dB is lost in the total amplification. To keep the same
zero level,
25 dB amplif i cation is provided by the linear amplifier.
As in some cases
it is convenient to be able to observe small variations in
the
frequency response, the Tracer has been provided with a 5 dB range. This
has been achieved by
further reducing the amplification per stage to 1 dB,
but as no
more amplification is available, the additional loss of 20 dB causes
the zero level to increase
10 times: from 10 mV to 100 mV.
Special provisions have been made to make the amplifier suitable for low
frequency application . The limitation of the output from each stage is per-
formed
by saturating the stage. When a stage is saturated, the DC level at
the
collector is not allowed to change. This would cause charging of the
coupling capacitor to the following stage and consequently a large delay for
a sudden decrease of input signal would result, since the time constants of
the interstage
couplings have to be high. This effect has been reduced to
practically zero by proper setting of the working point resulting in sym-
metrical clipping of the signal. Since the peak output voltage from the stage
is
directly proportional to the battery voltage, a reference voltage is provided
for stabilization of the latter. The stability with temperature is measured to be
better than 0.2 % from 20° C to
50 ° C.
R
~ '"tp,t1
-1l1'
1 RL
output 2
165123
Fig. 3.3. Block diagram of the logarithmic amplifier.
Two diodes after each stage output provide symmetrical DC output in con-
junction with a symmetrical common load as shown in Fig. 3.3. As each stage
contains
only one transistor, the output from one of the stages is in opposite
phase
of that from the previous stage. When the stages are saturated as the
input signal increases, starting at the end stage, their contribution to ripple
13

is minimized, because their output waveforms will be almost square. Due to
the phase inversion in the stages the rectified signal ripple frequency will be
mainly of twice the signal frequency.
Frequency Sensitive Circuit.
The frequency sensitive circuit provides an output voltage which is pro-
portional to the logarithm of the input frequency. This is achieved by means
of interchangeable filter networks designed as plug-in units covering various
frequency ranges. As
an exception the Plug-in Unit ZS 0124 has a linear fre-
quency response for special measuring purposes (see Applications page 24) .
To make the output voltage independent of the input signal amplitude, a
bistable multivibrator has been employed in the circuit. It switches
for a very
small voltage, i.e. practically at the zero crossings of the signal. An important
requirement to the x-deflection is, that there must be no ripple, since a
combination
of X and Y ripple results in poor quality indication at the lower
frequencies. This requirement comes in conflict with the demand for quick
indication, which makes it impossible to use a conventional rectifier and
smoothing
filter causing a large delay at low frequencies.
To meet the above mentioned requirements a controlled signal rectifier
has been developed, and the complete frequency sensitive circuit operates
as described below: (see Fig. 3.4).
The square wave signal (a) from the multivibrator is differentiated (b), and
used as inputs for two monostable multivibrators
of opposite polarity. This
results in the signal (c) and (d) where LIt < T/2
for the highest frequency in
question. (T = 1/f where f is the input frequency). Now (c) and (d) are
differentiated, (e), and fed to a bistable multivibrator. As the multivibrator
switches over only when the incoming pulse is
of opposite polarity to the
previous pulse, we obtain (f) with a delay LIt relative to (a) and the pulses
(c) and (d) placed in time at the end of their particular half period, in-
dependent
of signal frequency. Finally, (f) passes through a suitable high-
pass filter whereby (g) is obtained, whose final instantaneous value increases
with increasing frequency. The sampling of the final instantaneous value is
taken care of by a gate which is controlled by the pulses (c) and (d) so that
the symmetrical DC output voltages represent the final instantaneous value of
the two half periods.
Between the high-pass filter and the controlled rectifier are inserted a prase
inverter and an emitter follower. The phase inverter is used for subtractiof'
of a square wave signal, (h) that is derived from (f) whereby (i) is obtained.
It is seen that the final value V, has one polarity, and the initial value the
opposite, both of about the same size. The signal (i) represents the lowest
frequency in the frequency range to be indicated. With increasing frequency
the 1/2 period is shortened to the left. Considering V, on signal (i) and
assuming the period shortened, V, will move down the curve, pass through
zero and finally at high frequencies end up with the opposite polarity but
of
14

Signal in I f= t
/-
T
~--
'" C90
f---+--+
C 91
t
t. fL
fm
t. fl
V2
fh
165130
Fig. 3.4. Block diagram of the frequency sensitive circuit with the
corresponding waveforms.
a
b
c
d
e
f
g
h
k
15

about the same size. The same happens to V, . This means that V, and V,
are opposite but of the same size as they were at the lowest frequency. This
is just what is needed
for the X-deflection coils since the neutral point of a
cathode ray
tube is in the middle.
The signal (k) in Fig. 3.4 represents the signal input to the gate at medium
frequency fm(scale midpoint)
with expanded time scale. Signal (I) expresses
the highest
frequency fh to be indicated on a time scale expanded by the
same
factor as the frequency with respect to (i).
The
input to the frequency sensitive circuit is obtained from the logarithmic
amplifier, or alternatively from an external frequency input when it is
desired to control the
x-deflection independently of the input signal. The plug-
in unit determines the frequency range of the instrument which may be
20-20000 Hz, 200- 5000 Hz or any other range down to one decade within
10-20000 Hz depending on the components contained in the plug-in unit.
The
two ranges mentioned are "standard" and the corresponding plug-in
units and calibrated scales are supplied with the instrument.*) The speed with
which
the x-deflection can follow sudden changes in input signal frequency
is fixed at about
30 mm per period of the signal.
DC Amplifiers.
Symmetrical DC
amplifiers provide the current for both the amplitude and
frequency
deflection yokes. Due to current feedback in the output , the current
is independent of
the deflection coil resistance and thereby temperature
sym. input
DC +
>
Bias
Fig. 3.5. The principle of the V-DC amplifier.
165126
*) A guide to plug-in unit design for other frequency ranges is given in Appendix C.
16

stable. To obtain zero signal level at the bottom of the screen the Y-DC
amplifier input is provided with a bias from a stable, floating DC source (see
Fig. 3.
5) . The power supply for the DC amplifier consists of two independent
voltage sources, the
current from each source flowing through the total out-
put load in opposite directions . This means that when the input signal is of
the same size as
the bias, but with the opposite polarity , the amplifier is in
balance and the current flow ing in the output load (deflection yoke and feed-
back resistors) is zero. When the input signal deviates from this value the
current will start flowing in a direction depending on whether the signal in-
creases
or decreases. The voltage developed across the feedback resistors
follows the input signal closely up to saturation of the output stage, which
occurs when the current through one output transistor is zero. At the same
time the current through the other output transistor will be twice the value
of the
current flowing through each output transistor when the amplifier is
in
balance . This means total drive current through the deflection yoke at
maximum input voltage, and thus a
high efficiency is obtained in c0mparison
to a conventional DC amplifier.
Differentiator.
The di fferentiator gives an output signal which is proportional to the speed
of the horizontal deflection and this is used for automat ic brightness control
of
the trace . When increasing the sweep speed the intensity will increase
within a limited range, and thus the trace brightness will appear reasonably
independent of sweep speed. The automatic brightness regulation circuit can
be set so that the trace is visible in the left to right direction only, or in both
sweep directions.
Screwdriver control for this adjustment is placed at the
back of the instrument.
Sweep Limit Control.
For production line testing it is not always necessary to sweep through the
whole
frequency range of the instrument. When a B & K Beat Frequency
Oscillator is used, with a sweep motor built in, the upper and lower limits of
the
frequency sweep can be set so that any part of the frequency range is
scanned automatically and repeatedly. This is done with a biassed relay
circuit from the x-deflection voltage. The sweep limits are set with two screw-
driver operated potentiometers at the back of the Tracer . A screwdriver
operated switch can be set to give fast return to the lower frequency limit ,
in
either a clockwise or anticlockwise sweep direction .
It should be noted
that the Tracer must be provided with a minimum signal
6
dB below scale zero on SIGNAL INPUT, or 0.1 V on EXT. FREQUENCY
INPUT
for the SWEEP LIMIT control to function .
17

4. AlJpl ications
The possible applications of the Frequency Response Tracer are numerous
since the
majority of audio-frequency devices may be tested for performance
with this instrument. To facilitate the setting up of instrumentation for practical
investigations a few typical examples are given here.
Measuring Voltage and Frequency.
In the frequency range 20- 20000 Hz the Frequency Response Tracer will
display not only the magnitude of the signal applied to its input terminals but
also the frequency. In the range 20-200 kHz the magnitude can still be
measured by supplying an external frequency signal of, say, 50 Hz to the
EXT. FREQUENCY INPUT.*) The
x-deflection will then correspond to the fre-
quency of
this signal, while the y-deflection will indicate the magnitude of
the signal applied to the SIGNAL INPUT socket. ** )
If it is desired to measure the frequency also in the range 20-200 kHz, a
frequency
divider can be employed instead of the 50 Hz signal. See passage
"Frequency
Scale Multiplication" Appendix C.
To find the magnitude of the input signal observe the following:
a) 50, 25 and 5 dB logarithmic amplifiers .
Turn the INPUT POTENTIOMETER fully clockwise (10) and read the
deflection on the screen. The input voltage is then: (The number of dB
read on the screen) + (the number of dB indicated by INPUT ATTENU-
ATOR) 50 and 25 dB ranges re 0.01 V,S dB range re 0.1 V .*** )
Example: Y-deflection 32 dB.
INPUT ATTENUATOR on 30,
Input voltage = 32 + 30 = 62 dB re 0. 01 volt
= 12.59 volt RMS.
b) Linear
amplifier.
Turn the INPUT POTENTIOMETER fully clockwise (10) and read the
deflection on the screen (0 to 1 volt). The input voltage is then: (The
voltage read on the screen) X (conversion
factor from the table below).
*) A proper input plug, B & K Type JP 0018 , must be used in order to activate the micro-
switch in the socket, switching off the internal frequency signal.
**) It shouId be noted that the Tracer employs an arithmetic average type rectifier cali-
brated to give correct RMS readings for sinusoidal signals . If the input waveshape
deviates
appreciably from sinusoidal there may be an error in the measured results.
*** ) To convert number of dB into a ratio divide by 20 and take antilog , or use conversion
table in the Appendix of this book.
18

INPUT ATTENUATOR setting o I
10 I 20 I 30 1 40 50 1 60
Conversion factor 1 3.16 1 10 I 31.6 1
100 316 11000
Example: Y-deflection 0.45 volt,
INPUT ATTENUATOR on 30 dB.
Input voltage = 0.45 X 31.6 = 14.22 volt RMS.
It should be noted that the vertical scale,
divided by fifty equidistant hori-
zontal lines, is the same for the 4 dynamic ranges.
Frequency Response of Amplifiers.
The Frequency Response Tracer is very well suited for checking the fre-
quency response
of amplifiers in the audio frequency range. A suitable set-up
is shown in Fig.
4.1.
The amplifier under test is fed from a beat frequency oscillator and the
output signal is taken to the SIGNAL INPUT terminals of the Tracer , which
shows the frequency response on the screen. The
functioning of tone controls
etc. is easily investigated with this instrumentation.
Beat Freq Osci llater
I
I
I
I
AF
Amplifier
Freq. Resp . Tracer
•
' •• " i' -' . : ••
'[email protected].~ - ~ G:i~ "
I 4712
L ____________________ ..J
Ext. Frequency Input 166 Z01t
Fig. 4.1. Set-up for finding frequency response of A.F. amplifiers.
Frequency Characteristic of Tape Recorder.
The frequency characteristic of a
tape recorder may be found simply by
recording the frequency sweep from a Beat Frequency
Oscillator on the tape
and then playing it back into the Frequency Response
Tracer as shown in
Fig. 4.2. If the signal from the BFO is constant in
amplitude the trace on the
screen will be the frequency
characteristic of the tape recorder.
19