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The
two
signals applied
to
the ratio circuit
are
Vf
derived
from
the
Level
AC/DC
converter and
-Vh from
the Distortion
AC/DC
converter.
-
Switches Sa
(Q107)
and Sb
(Q114)
are
reset
in the
open
condition
at
time
interval
T
(see
Fig.
3.4)
by
a clock triggered
flip-flop
QA105.
Thus
with
Sa open
the signals
Vf
and
-Vh
combine
to
charge
capacitor
C113.
When
the
output
of
QA103
reaches Vc,
comparator
QA104
actuates
fliprflop
QA105
to
close
Sa
leaving
signal
-Vh
to
charge
C113
in
the
reverse
direction for
time
T
-Tx
(Fig. 3.4).
The
flip-flop
change over time
Tx
is
therefore dependent
on
the relative
values
of
Vf
and -Vh.
Clock
QAI03
OAI05
OUTPUT
Fig.
3.4
Timing
sequence
of
ratio circuit
The
switching
action
of
Sa
is
repeated
by
Sb (Q114)
producing a
pulse, the
period
of
which
is
identi
cal to
T
-Tx,
integrated
by
Circuit
QA108/C126/R183
and
thus deriving
an
output voltage
Vo
directly
proportional
to
the
ratio of the
harmonic
content
of the
signal
and
its
total
value
(Vh/Vf).
Distortion
meter
(M2)
indicates the
value
Vo.
TF
2337A
1
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