AD9857 Analog Devices, AD9857 Datasheet - Page 15

• Manufacturer Part Number: AD9857
• Description: CMOS 200 MSPS 14-Bit Quadrature Digital Upconverter
• Manufacturer: Analog Devices

The usable bandwidth of the filter chain puts a limit on the
maximum data rate that can be propagated through the AD9857.
A look at the pass band detail of the half-band filter response
indicates that in order to maintain an amplitude error of no
more than 1 dB, we are restricted to signals having a bandwidth
of no more than about 90% of f
bandwidth of the data in the flat portion of the filter pass band,
the user must oversample the baseband data by at least a factor
of two prior to presenting it to the AD9857. Note that without
oversampling, the Nyquist bandwidth of the baseband data cor-
responds to the f
bandwidth will suffer 6 dB or more of attenuation due to the
frequency response of the half-band filters. Furthermore, if the
baseband data applied to the AD9857 has been pulse shaped
there is an additional concern.
Typically, pulse shaping is applied to the baseband data via a
filter having a raised cosine response. In such cases, an
is used to modify the bandwidth of the data where the value of
is such that 0 <
to correspond to the Nyquist bandwidth. A value of 1 causes the
data bandwidth to be extended to twice the Nyquist bandwidth.
Thus, with 2 oversampling of the baseband data and
Nyquist bandwidth of the data will correspond with the I/Q
Nyquist bandwidth. As stated earlier, this results in problems
near the upper edge of the data bandwidth due to the roll-off
attenuation of the half-band filters. The following diagrams
illustrate the relationship between
cosine shaped pulses. The problem area is indicated by the shading
in the tail of the pulse with
region of the half-band filter.
The effect of raised cosine filtering on baseband pulse bandwidth,
and the relationship to the half-band filter response is shown in
Figure 24.
–0.002
–0.004
–0.006
–0.008
0.008
0.006
0.004
0.002
–0.01
0.01
0
0
NYQ
< 1. A value of 0 causes the data bandwidth
0.05
. Because of this, the upper end of the data
0.10
= 1, which extends into the roll-off
FREQUENCY
NYQ
. Thus, in order to keep the
and the bandwidth of raised
0.15
0.2
0.20
0.25
= 1, the
value
Programmable (2 –63 ) CIC Interpolating Filter
The Programmable Interpolator is implemented as a CIC
(Cascaded Integrator-Comb) filter. It is programmable by a
6-bit control word, giving a range of 2 to 63 interpolation.
This interpolator has a low-pass frequency characteristic that is
compensated by the Inverse CIC filter.
The Programmable Interpolator can be bypassed to yield a 1 (no
interpolation) configuration by setting the bit in the appropri-
ate control register, per each profile. Whenever the Programmable
Interpolator is bypassed (1 CIC rate) power to the stage is
removed. If the Programmable Interpolator is bypassed, the
Inverse CIC filter (see above) is automatically bypassed, since
its compensation is not needed in this case.
The output of the Programmable Interpolator is the data from
the 4 interpolator upsampled by an additional 2 to 63 , accord-
ing to the rate chosen by the user. This results in the input data
being upsampled by a factor of 8 to 252 .
BANDWIDTH
of I or Q
DATA
f
NYQ
f
f
NYQ
NYQ
= 0.5
= 0
= 1
2*f
2*f
2*f
NYQ
NYQ
NYQ
HALF-BAND FILTER RESPONSE
2
OVERSAMPLE RATE
AD9857
f
IQ
f
f
f