Advertisement
Figure
8-7.
Simplified
Phase
Error
Detector
Schematic.
capacitors
Cadj
and
Cstrav.
Cadj
is
adjusted
to
be
equal
to
Cstray
so that
1
2
of the
output
of
A3U2
(a
voltage
= Vf)
is
dropped
across each. Since the current necessary
to
drive
the
stray
capacitance
(Cstray)
is
supplied
by
the
neutralizer
circuit,
the
output
of
the
notch
filter
is
not
loaded.
8-35.
Amplitude
Error Detector.
8-36.
The
purpose
of the
Amplitude
Error Detector
is
to
dfcgulate
the
amount
of
"feed-forward"
signal
required
to
^^timize
the
depth
of the
notch
filter.
Figure
8-6
shows
a
simplified
schematic
of the
amplitude
error detector
circuit.
The
input
signal to the
notch
filter
(from
input
buffer amplifier
A
3
U
101)
is
used
as the
reference signal
for
amplitude
detector
A4U2. The
output
of
the
notch
amplifier
(A3U3)
is
amplified by
A4U
I
and
is
used
as
the
control
signal to
A4U2.
Amplifier
A4U
1
supplies a
gain
of 200,
which
is
necessary
to
achieve
"notch depths"
in
excess of -100 dB.
The
output
of
amplitude
detector
A4U2
is
the
product
of
the
two
input
signals.
Mathematically,
the
output
of
A4U2
(Vo)
is
equal
to
the
reference signal
(Al
Cos
wt) times the control signal
(A2
Cos wt
+
0),
or
Vo
=
Al
A2
[(Cos
wt
+
0)].
By
trig
identity,
this
expression
is
equal
to:
Vo
=
1/2
A
i
A2
[Cos
(2
wt
+
0)
+
Cos
0],
The
differential
output
of
A4U2
is
converted
to
a single
ended output
by
A4U3A
and
applied
to
the integrator.
The
integrator
(A4U3B)
acts as a
low-pass
Filter
to
the
^vput
signal
from
the
amplitude
detector
and
responds
only
to
the
low
frequency
component
of the
signal.
The
error
signal
is.
therefore, effectively
equal
to:
Vo
=
1
2
AI.A2
Cos
o times
a
constant "K".
Since
the
notch
Filter
is
tuned
to
the
fundamental
frequency
of
the
input
signal, the
phase
difference
term
of
the error signal
(Cos
0)
is
equal
to
1
(Cos
0°
=
1).
The
error
signal as
seen
by
the integrator,
is
therefore a
dc
voltage equal
to:
K
(Al
A2),
2
Since
the
amplitude
of the reference signal (Al)
is
held
constant,
any changes
in
the error signal are
caused by
the
amplitude changes
of the control
signal
(
A2).
The
error
signal to the integrator
can
therefore be
expressed
as:
Vo
=
A2
(K.AI)
.
2
The
output
of the integrator
is
applied
to
a voltage-to-
current converter
(A4U3C
and
A3Q1)
which
drives
amplitude
control
module A3E2.
Control
module
A3E2
adjusts the gain of
notch
amplifier
A3U3
to
provide
the
proper
amount
of
feed-forward
signal
necessary
to
cancel
the
fundamental
frequency
at
the
output
of
the
notch
amplifier
and
therefore
reduce
the error signal
to zero.
8-37.
Phase
Error Detector.
8-38.
The
purpose
of
the
Phase Error Detector
circuit
is
to
"fine
tune"
the
notch
filter
to
the
fundamental
frequency
of
the input
signal.
The
circuit
shown
in
Figure
8-7
is
a simpliFied
schematic
of the
phase
detector
circuit.
8-6
Advertisement
