<s>
A	O
roofing	B-Algorithm
filter	I-Algorithm
is	O
a	O
type	O
of	O
filter	O
used	O
in	O
a	O
HF	B-Algorithm
radio	I-Algorithm
receiver	O
.	O
</s>
<s>
The	O
goal	O
of	O
a	O
roofing	B-Algorithm
filter	I-Algorithm
is	O
to	O
limit	O
the	O
passband	B-Algorithm
of	O
the	O
first	O
intermediate	O
frequency	O
(	O
IF	O
)	O
stage	O
.	O
</s>
<s>
For	O
general	O
purpose	O
HF	B-Algorithm
radio	I-Algorithm
reception	O
,	O
a	O
passband	B-Algorithm
of	O
about	O
6	O
–	O
20	O
kHz	O
is	O
acceptable	O
.	O
</s>
<s>
The	O
receiver	O
's	O
overall	O
bandwidth	B-Algorithm
is	O
not	O
determined	O
by	O
the	O
roofing	B-Algorithm
filter	I-Algorithm
but	O
by	O
a	O
following	O
crystal	B-Algorithm
filter	I-Algorithm
,	O
mechanical	O
filter	O
or	O
DSP	B-General_Concept
filter	O
.	O
</s>
<s>
These	O
allow	O
a	O
much	O
better	O
filtering	O
curve	O
than	O
a	O
roofing	B-Algorithm
filter	I-Algorithm
,	O
which	O
often	O
uses	O
a	O
high	O
first	O
IF	O
above	O
40	O
MHz	O
.	O
</s>
<s>
Roofing	B-Algorithm
filters	I-Algorithm
are	O
usually	O
crystal	O
or	O
ceramic	O
filter	O
types	O
.	O
</s>
<s>
For	O
more	O
demanding	O
uses	O
like	O
listening	O
to	O
weak	O
CW	O
or	O
SSB	O
signals	O
,	O
a	O
roofing	B-Algorithm
filter	I-Algorithm
is	O
required	O
that	O
gives	O
a	O
smaller	O
passband	B-Algorithm
appropriate	O
to	O
the	O
reception	O
mode	O
in	O
use	O
.	O
</s>
<s>
These	O
narrow	O
filters	O
require	O
that	O
the	O
receiver	O
uses	O
a	O
first	O
IF	O
well	O
below	O
VHF	B-Algorithm
range	O
,	O
perhaps	O
9	O
or	O
11MHz	O
.	O
</s>
