<s>
In	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
,	O
a	O
digital	O
down-converter	O
(	O
DDC	O
)	O
converts	O
a	O
digitized	O
,	O
band-limited	O
signal	O
to	O
a	O
lower	O
frequency	O
signal	O
at	O
a	O
lower	O
sampling	O
rate	O
in	O
order	O
to	O
simplify	O
the	O
subsequent	O
radio	O
stages	O
.	O
</s>
<s>
Often	O
the	O
DDC	O
converts	O
from	O
the	O
raw	O
radio	O
frequency	O
or	O
intermediate	O
frequency	O
down	O
to	O
a	O
complex	O
baseband	B-Algorithm
signal	I-Algorithm
.	O
</s>
<s>
A	O
DDC	O
consists	O
of	O
three	O
subcomponents	O
:	O
a	O
direct	O
digital	O
synthesizer	O
(	O
DDS	O
)	O
,	O
a	O
low-pass	B-Algorithm
filter	I-Algorithm
(	O
LPF	O
)	O
,	O
and	O
a	O
downsampler	B-Algorithm
(	O
which	O
may	O
be	O
integrated	O
into	O
the	O
low-pass	B-Algorithm
filter	I-Algorithm
)	O
.	O
</s>
<s>
The	O
lowpass	B-Algorithm
filters	I-Algorithm
pass	O
the	O
difference	O
(	O
i.e.	O
</s>
<s>
baseband	B-Algorithm
)	O
frequency	O
while	O
rejecting	O
the	O
sum	O
frequency	O
image	O
,	O
resulting	O
in	O
a	O
complex	O
baseband	B-Algorithm
representation	O
of	O
the	O
original	O
signal	O
.	O
</s>
<s>
Assuming	O
judicious	O
choice	O
of	O
IF	O
and	O
LPF	O
bandwidth	O
,	O
the	O
complex	O
baseband	B-Algorithm
signal	I-Algorithm
is	O
mathematically	O
equivalent	O
to	O
the	O
original	O
signal	O
.	O
</s>
<s>
In	O
its	O
new	O
form	O
,	O
it	O
can	O
readily	O
be	O
downsampled	B-Algorithm
and	O
is	O
more	O
convenient	O
to	O
many	O
DSP	O
algorithms	O
.	O
</s>
<s>
Any	O
suitable	O
low-pass	B-Algorithm
filter	I-Algorithm
can	O
be	O
used	O
including	O
FIR	O
,	O
IIR	O
and	O
CIC	O
filters	O
.	O
</s>
<s>
The	O
most	O
common	O
choice	O
is	O
a	O
FIR	O
filter	O
for	O
low	O
amounts	O
of	O
decimation	B-Algorithm
(	O
less	O
than	O
ten	O
)	O
or	O
a	O
CIC	O
filter	O
followed	O
by	O
a	O
FIR	O
filter	O
for	O
larger	O
downsampling	B-Algorithm
ratios	O
.	O
</s>
<s>
DDCs	O
are	O
most	O
commonly	O
implemented	O
in	O
logic	O
in	O
field-programmable	B-Architecture
gate	I-Architecture
arrays	I-Architecture
or	O
application-specific	O
integrated	O
circuits	O
.	O
</s>
<s>
While	O
software	O
implementations	O
are	O
also	O
possible	O
,	O
operations	O
in	O
the	O
DDS	O
,	O
multipliers	O
and	O
input	O
stages	O
of	O
the	O
lowpass	B-Algorithm
filters	I-Algorithm
all	O
run	O
at	O
the	O
sampling	O
rate	O
of	O
the	O
input	O
data	O
.	O
</s>
<s>
CORDICs	B-Algorithm
are	O
an	O
alternative	O
to	O
the	O
use	O
of	O
multipliers	O
in	O
the	O
implementation	O
of	O
digital	B-Algorithm
down	I-Algorithm
converters	I-Algorithm
.	O
</s>
