Digital Signal Processing Reference
In-Depth Information
12.2 Direct Digital Frequency Synthesizer
A DDFS is an integral component of high-performance communication systems [1]. In a linear
digital modulation system like QAM, the baseband signal is translated in frequency. This translation
is usually done in more than one stage. The first stage of mixing is performed in the digital domain
and a complex baseband signal s
b
[n] is translated to an intermediate frequency (IF) bymixing it with
an exponential of equivalent digital frequency
o
o
:
s
IF
½n¼s
b
½ne
jo
o
n
ð
12
:
1
Þ
This IF signal is converted to analog using aD/Aconverter and thenmixed using analogmixers for
translation to the desired frequency band.
At the receiver the similar complex mixing is performed. The mixer in the digital domain is best
implemented using a DDFS. Usually, due to differences in the crystals for clock and frequency
generation at the transmitter and receiver, the mixing leaves an offset:(12.2)
s
b
½ne
jo
o
n
e
jo
0
o
n
ð
12
:
3
Þ
The receiver needs to compute this frequency error, the offset being
Do¼o
o
o
0
o
. The
computation of correction is made in a frequency correction loop. The frequency adjustment again
requires generation of an exponential tomake this correction. ADDFS is used to generate the desired
exponential:
s
b
½ne
jo
o
n
e
jo
0
o
n
e
jDon
ð
12
:
3
Þ
A DDFS generates a spectrally pure sine and cosine for quadrature mixing and frequency and
phase correction in a digital receiver, as shown in Figure 12.1. At the front end, a DDFS mixes with
the digitized IF signal. The decimation stage down-converts the signal to baseband. The phase
detector computes the phase error and the output of the loop filter generates phase correction for
DDFS that generates the correction for quadrature mixing.
CORDIC
CORDIC
Slicer
IF
signal
I
I
I
Filter and
decimation
x
x
Q
Q
Q
DDFS
DDFS
Loop
filter
atan(Q\I)
x
*
CORDIC
Phase detector
Figure 12.1
Use of DDFS in a digital communication receiver
Search WWH ::
Custom Search