Resolving
Signals
of
Equal
Amplitude
In
responding
to
a
continuous-wave
signal,
a
scanning
receiver
traces
out
the
shape
of
its
intermediate
frequency
(IF)
lter
.
As
we
change
the
lter
bandwidth,
we
change
the
width
of
the
displayed
response
.
If
a
wide
lter
is
used
and
two
equal-amplitude
input
signals
are
close
enough
in
frequency
,
then
the
two
signals
appear
as
one
.
Thus
,
signal
resolution
is
determined
by
the
IF
lters
inside
the
receiver
.
F
or
maximum
exibility
,
the
receiver
allows
you
to
select
from
the
CISPR
200
Hz,
9
kHz
and
120
khz
6
dB
measurement
IF
bandwidths
,
or
from
the
30
Hz
to
3
MHz
3
dB
diagnostic
IF
bandwidth
in
a
1,
3,
10
sequence
,
plus
5
MHz.
The
1
MHz5
MHz.
The
1
MHz
bandwidth
maybe
a
6
dB
bandwidth.
The
bandwidths
tell
us
how
close
together
equal
amplitude
signals
can
be
and
still
be
distinguished
from
each
other
.
The
IFBW
function
selects
the
IF
lter
setting.
Generally
,
to
resolve
two
signals
of
equal
amplitude
,
the
IF
bandwidth
must
be
less
than
or
equal
to
the
frequency
separation
of
the
two
signals
.
If
the
bandwidth
is
equal
to
the
separation,
a
dip
of
approximately
3
dB
is
seen
between
the
peaks
of
the
two
equal
signals
,
and
it
is
clear
that
more
than
one
signal
is
present.
See
Figure
6-2.
In
order
to
keep
the
receiver
calibrated,
sweep
time
is
automatically
set
to
a
value
that
is
inversely
proportional
to
the
square
of
the
IF
bandwidth.
So
,
if
the
IF
bandwidth
is
reduced
by
a
factor
of
10,
the
sweep
time
is
increased
by
a
factor
of
100
when
sweep
time
and
bandwidth
settings
are
coupled.
(Sweep
time
is
proportional
to
1/BW
2
.)
F
or
fastest
measurement
times
,
use
the
widest
IF
bandwidth
that
still
permits
discrimination
of
all
desired
signals
.
6-2
Making
EMI
Diagnostic
Measurements
