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General Description and Method of Measurement
A
block diagram of the instru-
ment,
clearly
indicating
its three
main sections, is shown in Fig.11
.
The Generator Section
The generator section consists of
two sine generators with automatic
attenuators, a summing amplifier to
combine the signals, a manual at-
tenuator to set the desired signal
output level, and a power amplifier
to drive the external load e.g. loud-
speaker or small vibrator. The con-
trol logic circuit tunes the oscillator
to sweep the test range in 256 dis-
crete steps, each incrementing the
frequency by 1 ,8%.
By continuous reference to the
stored correction curve, the control
logic adjusts the automatic attenua-
tors at each step so that the output
signal is maintained constant at its
pre-selected
level.
Both
the
fre-
quency and the level of the output
signal are thus controlled automati-
cally.
Only one oscillator is used in the
"Read in", "Freq.
Resp
.
", and "Har-
monic" modes, but for intermodula-
tion
distortion
measurements
the
outputs
of
both
oscillators
are
summed, the amplitude of each sig-
nal being attenuated by 3 dB so that
the resulting level corresponds to
the
value
selected
on
the
front
panel
.
A maximum of 3 Watts can be de-
livered into an 8 Ohm load with
very
low distortion.
The Analyzer Section
The measurement transducer
is
connected to the input preamplifier
of the Audio Test Station, the sensi-
tivity of which can be adjusted over
a 20 dB range to accommodate var-
iations in the sensitivity of the mea-
Sinewave
Oscillator
Gl
Automatic
Attenuator
GENERATOR SECTION
TEST
CHAMBER
Manual
Attenuator
Adjustable
Amplifier
Sinewave
Oscillator
G2
Master
Clock
Automatic
Attenuator
~-
-..,
I Memory
I
L ___ J
Continue
"In"
Comp.
"Out"
L.ogic Control Circuit
0.
s
(/)
N
(!)
0.
0
ci)
c
a
Q)
a:
suring chain. After the adjustable
amplifier, the signal
is
passed to
the bandpass filter, which is pre-
ceded and followed by autoranging
amplifiers which automatically se-
lect
the
best
possible
dynamic
range. The filter is a six pole, 1 5%
Butterworth
type,
controlled
digi-
tally
from
the
logic
circuit.
The
100Hz
to
10kHz
measurement
range is covered by 64 frequency
bands, each filter step covering 4
oscillator
tuning
steps,
of which
there are 256
.
The filter may be by-
passed in the "Calibrate" mode.
The following
logarithmic Mean
Square Detector gives a DC output,
proportional
(in
dB) to the
RMS
value
of
the
filter
input
signal,
which is then combined with correc-
tion signals from the autoranging
amplifiers to give a DC signal pro-
proportional to the measured signal
level at the transducer
.
.!:
~
Q)
a:
ANALYZER SECTION
Input
Auto-
ranging
Amplifier
I
Output
Auto-
ranging
Amplifier
L _ _ _
_
IL--.....,......---'
--~
I
I I
I I
c
::J
a:
"up"
SPL
I I
t
t
I
L
I
L_
Logarithm.
Mean
Square
Detector
DC
Summing
Amplifier
Analogue
..------1 Compa-
rator
Dig./Analg.
Converter
"down"
Limit Switches
Rear Panel Switches
Front Panel Switches
780711
Fig.11.
Simplified block diagram of the Type 2116
6
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