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Response
(dB)
Response
(dB)
0
0
100
Fundamental
Test tone
t,
200
2nd
lntermod.
3rd
lntermod.
distortion
distortion
product
product
, ,
' t t' , ,
100
200
300
Test
Tone 1
t,
300
2nd
Harmonic
Distortion
400
Test
Tone 2
f2
400
3rd
Harmonic
Distortion
500
600
Frequency
(Hz)
500
600
Frequency (Hz)
780712
Fig.12.
Harmonic distortion (upper) and intermodulation distortion (lower)
In the Analogue Comparator this
is compared with a signal defining
the current pen position, the differ-
ence signal being fed to the pen re-
corder Y
-axis
drive.
Three
Measurements are Avail-
able:
Frequency Response:
With the
"Freq.
Resp
."
parameter
selected, the output signal can be
adjusted in steps of 5 dB using the
switches
"Up",
"Down",
the
chosen level being
indicated
by an
LED
.
Depressing
RUN
m1t1ates
a
measurement, and the pen marks
the card with the measurement par-
ameter and the output level chosen.
When the pen reaches 1 00 Hz the
output signal is switched on and
the sweep continues automatically
to
10kHz.
On completion, the pen
is returned to the standby position
ready for a further measurement.
Harmonic Distortion
Harmonic distortion
is
the signal
which a single pure tone produces
in a non-linear circuit at frequen-
cies which are integral multiples of
the frequency of the original tone.
The distortion order is the same as
the harmonic number, as shown in
the upper figure of Fig
.
12
.
In the HARMONIC mode, either
2nd or 3rd order distortion measure-
ment can be selected, with the out-
put signal level preset as required.
The analyzer filter tunes in to the
selected harmonic, and the distor-
tion amplitude is then recorded on
the chart, using the fundamental,
i.e
.
the frequency
of the source
tone, as a reference along the fre-
quency axis.
Measurement
begins
when
the
pen indicates 100Hz and stops au-
tomatically when the filter reaches
10kHz
.
lntermodulation Distortion
lntermodulation distortion occurs
when two tones at different frequen-
cies are input to a circuit with non-
linearities
.
The resulting signal at the output
consists of the primary tones and a
number of distortion products with
different
frequencies
and
ampli-
tudes given by
Ap,q
Sin
21r
(±
pf2
±
qf
1
)t
where p and q are integer numbers
and f
1
and f
2
are the test tone fre-
quencies
.
The distortion order is the
result of adding p and q
.
The distor-
tion components at a higher fre-
quency
than
the
primary
tones,
soon fall outside the operating fre-
quency range of audio equipment
and are rapidly attenuated as
its
fre-
quency response falls off. Of more
interest are the distortion products
lower
than
the
primary
tones,
which
lie
within
the
frequency
range of audio equipment and there-
fore give a more realistic measure
of the influence on audio quality of
intermodulation
distortion
.
As
shown
in
the
lower
figure
of
Fig
.
1 2, the measured 2nd order in-
termodultion component occurs at a
frequency of
f2
- f
1
and the 3rd or-
der component occurs at 2f
1-b.
In
the
"lntermod"
mode either
the 2nd or 3rd intermodulation dis-
tortion product can be selected. The
output signal level, in this case the
sum of two pure tones, is predeter-
mined
using the
"SPL"
selector
.
The frequency relationship between
the 2 primary tones is pre-deter-
mined at a fixed ratio of
·
1:1 ,4,
which
ensures that the distortion
products are within the frequency
range. The 2nd order distortion pro-
duct is thus measured at 0,4 f
1,
and the 3rd order product at 0,6f
1
where f
1
is the frequency of the
lowest primary tone.
Pen Recorder
Both pen and carriage drives are
the stepping motor type, under the
control of
the
logic circuit.
For the Y
-axis
deflection this sup-
plies digital information about the
current pen position to a Digital to
Ana Iogue converter. The converter
output, which is proportional to the
pen position,
is
compared with the
analyzer DC signal, the difference
signal controlling the pen.
For the X -axis deflection the carri-
age drive is controlled by the
logic
circuit by continuous reference to
the number of generator frequency
step increments
.
The
chart
is visible throughout
the measurement. Results are re-
corded on a pre-printed chart of stiff
white card in A5 format ( 148 by
2 1 0
m m
:
5, 8 by 8, 3 in).
The
axes
are
chosen
so
that
20 dB
on
the
response
(vertical)
axis corresponds to one octave on
the frequency (horizontal) axis,
in
ac-
cordance with international agree-
ments
for
audiometry
.
When
a
measurement is started, the level
and the parameter chosen are re-
corded automatically on tt:f chart
before the measurement proceeds
.
There
is
no need for the operator to
intervene
at any stage or make rou-
tine notes about the measurement
conditions. Space is provided for de-
tails
of the test object, and the
other side is pre-printed with the
single
value
parameters
recom-
7
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