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TYP/CAL SS8 FILTER CURVE
I--
RESULTAIIT
I
Rrsp~
OCUBIC
COMMUNICATIONS
S""IIIII
DIVISION
ceiving system,
the more readily a signalof a
given strength can be rec-
eived and understood.
This phenomenum occurs because what we really hear is
the signal to noise ratio, rather than the absolute signal strength, and the
narrower the received bandwidth,
the less noise is received; hence a better
signal to noise ratio and a more
understandable recovered signal.
The lim-
iting factor,
of course,
is the bandwidth required for intelligibility of
the desired signal.
This is one of the
chief advantages of single sideband
over other
communications methods.
Since
the necessary bandwidth
is Iess
than half that necessary for AM,
for instance,
it will result in more than
twice the
signal to noise ratio than an AM signal of comparable power, due
solely to the reduction in received noise.
Studies show that
the minimum
bandwidth required for intelligibIe tr-
ansmission of the human voice is about 2.5 kHz, although one can "get
by"
on
a bandwidth
of as low
as 2.0 kHz,
if a bit more degredation of the voice
quality can be tolerated.
In actual fact, however ,
these are average
fig~
ures,
and individual voice requirements vary widely.
It has been the prac-
tice (and still is, in many cases),
to use a fixed bandwidth,
set
by
a
high quality crystal filter
in the
IF of the receiver, to improve the
signal to noise ratio.
The res-
ponse of a typical filter is shown
to the right.
Since individual
voices
vary
in their requirement for bandwidth,
however,
i t would
be advantageous
in
many instances,
to be able to
vary the bandwidth to lower limits.
By being able to cut off the top or
the
bottom of
the passband,
thus
reducing the bandwidth,
it is possible to
reduce the
effects of close-by
interfering signaIs, often at acceptable levels of degredation of the desir-
ed signal quality.
Cr,ystal filters, however, do not lend themselves to such
operator adjustment.
Quite recently, a scheme
for
acco~plishing
this has been developed; it
is called
"Passband Tuning".
In this scheme, the basic bandwidth is set
by
a high-quality crystal filter in the IF as before.
But,
af
ter that filter,
a circuit
is included to
translate that IF frequency to another frequency,
usually higher.
The signal is then passed through another
crystal filter,
of similar bandwidth as the first, and then translated back to the first IF
frequency.
If the frequency of the
oscillator used to
translate
the
signal to
and back from the second
IF is such that the passband of the
two filters line
u1>,
no change in
the bandwidth is noticed;
however,
if
this
oscillator frequency
is
made variabIe, the 1>assbands of the
two filters no longer lines up, and
either the upper or the lower end
of the passband of the first filter
is cut off
by
the second,
as shown
in the accompanying diagram.
Since
the frequency of this passband tun-
7-9
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