How
does
this
difference
in grid
bias
affect
the
tuned
R.F.
and
I.F. stages?
The
explanation is
quite
simple.
The
diagram
illustrates a
commonly
employed
I.F.
stage
and
diode
second
detector,
wherein
the
grid bias is
obtained
from
an
A.V.C.
network.
Concentrate
on
the input
(grid)
side
of this
intermediate
frequency
amplifier.
The
input
circuit
consists
of the
grid of
the tube
and the
secondary
of the
interstage
I.F.
transformer,
the
transformer
return network and
the
semi-variable
tuning
condenser,
which
resonates the
I.F.
transformer
winding to
the
inter-
mediate
frequency.
For
the
sake of
illustration, we
shall
assume
this is
approximately
460
Kc,
one of
the common
I.F.'s. In
addition
to the
I.F.
trimmer
condenser,
there are
still
other
capacities
effectively
bridged across
the
I.F. trans-
former
secondary, and
hence, in
parallel
with
the
trimmer
condenser.
These
consist
of
stray
circuit
capacities,
which
are the
capacities
between
wiring as
well as to
the
chassis,
plus the
very finite
capacity
which
exists
between the
prongs
of the
tube
and between the
socket
contacts.
To
this we add
another important
factor, and
that
is
the
input
capacity of
the I.F.
amplifier tube
itself.
Just
upon
examination
of
the schematic
for the
tube, the
reader can
immediately
see that
the
control
grid
is
physically
located
between
the cathode
and screen
grid.
As we
all know,
when two or
more
pieces of
metal are
brought close to the
other,
we
have, in
effect, a
capacitor.
In this
particular
case,
the
capacitor
consists of
the
capacity of
grid
to
cathode and
grid to
screen
plus even the
additional
capacities of grid to
suppressor, and
grid to
plate.
These
latter two
capacities are,
for a
properly designed
screen
grid
tube circuit,
comparatively
small and almost
neg-
ligible.
However, the capacity of
the
grid to cathode and
screen alone
can be
quite
appreciable.
Normally
when we
think of a
capacitor,
we assume
that its
capacity is
depend-
ent
purely on
physical
dimensions,
namely
the area of the
plates,
the
spacing
of the
plates and
the type
and nature of
dielectric
or
insulation between
these
two
plates.
This is
partially
true in a vacuum
tube.
Hnvrever. the input capacity
ni a vacuum tuhe varies
with the voltage annlied to thfi
input
grid, and it is here
that our
difficulty arises.
When
the grid bias
of an
R.F. or I.F.
amplifier stage is
varied by
the A.V.C.
control
system,
the input
capacity of
the
controlled tube
changes. As
just
previously
mentioned,
this input
capacity is
part of the
tuned
circuit.
Accordingly,
the
resonant point of
this tuned
circuit
will shift as the
input
capacity of
the tube
in question
varies
with a change in
automatic
volume control voltage.
We can now
see how,
after
carefully adjusting
our receiver
(below the A.V.C.
threshold) , all
this
work can be
partially
nullified due to the
effects of the
same
A.V.C. system
which
had
temporarily
been
forgotten, or at
least caused not to
work.
In some
receivers this
shift in the
resonant
point of the
tuned circuits
may not be very
serious, but in
many
cases
this factor
can assume major
proportions.^The
higher the
I.F. frequency,
this factor
becomes all the more
important!)
Thy pnA r<»«iilta of
a
shift
in
the
resonant point
of the
A.V.C.
controlled
tuned stages are some
common
troubles
such as poor audio
quality and
distortion due to
favoritism
towards one of
the carrier side bands,
increased
noise level,
reduction in
sensitivity,
etc.
THE
A.V.C.
SUBSTITUTION
SYSTEM
This
Signal Generator
overcomes the
foregoing
alignment
defects,
by
supplying Its Own A.V.C.
Voltage,
directly
controllable at
the front of the
instrument
panel, from a
calibrated A.V.C.
voltage
substitution
network.
By supplying proper
A.V.C.
voltage to the
controlled
stages, it
is no
longer necessary to
adjust
the Signal Gen-
erator
for
minimum
output and set the receiver to
the ab-
normal
condition of
maximum sensitivity
with
concurrent
noisy
operation.
This voltage is
independently
adjusted to
coincide
with
the actual A.V.C.
developed by the r
eceiver
under normal
average conditions
of
reception^This
assures
more
efficient
tuned circuit adjustments as
compared to
a
receiver aligned
without the
use
of the
A.V.C.
Substitution
Method.
The
use of
the A.V.C.
substitution
method
is
purely op-
tional
though it is to be
recommended,
especially in
adjust-
ments on
superheterodyne
receivers incorporating either
single
or
dual
automatic volume
control circuits. If the
operator
does not desire to
employ it, the setting of the
"A.V.C.
Control" of the Series
E-200-C
in no way
affects
the
application
of this instrument to
other means of receiver
alignment.
7
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