AD9857 Analog Devices, AD9857 Datasheet - Page 14

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

AD9857
Inverse CIC Filter
The Inverse CIC Filter precompensates the data in order to off-
set the slight attenuation gradient imposed by the CIC filter (see
the Programmable (2 –63 ) CIC Interpolating Filter section).
The I (or Q) data entering the first half-band filter occupies a
maximum bandwidth of one-half f
(where f
filter). This is shown graphically in Figure 21.
If the CIC filter is employed, the inband attenuation gradient
could pose a problem for those applications requiring an extremely
flat pass band. For example, if the spectrum of the data as supplied
to the AD9857 I or Q path occupies a significant portion of the
one-half f
will receive slightly more attenuation than the lower frequencies
(the worst-case overall droop from f = 0 to one-half f
< 0.8 dB). This may not be acceptable in certain applications. The
Inverse CIC filter has a response characteristic that is the inverse of
the CIC filter response over the one-half f
The net result is that the product of the two responses yields in
an extremely flat pass band, thereby eliminating the inband
attenuation gradient introduced by the CIC filter. The price to
be paid is a slight attenuation of the input signal of approximately
0.5 dB for a CIC interpolation rate of 2 dB and 0.8 dB for inter-
polation rates of 3 to 63.
The Inverse CIC Filter is implemented as a digital FIR filter
with a response characteristic that is the inverse of the Program-
mable CIC Interpolator. The product of the two responses yields a
nearly flat response over the baseband Nyquist bandwidth. The
Inverse CIC filter provides frequency compensation that yields a
response flatness of 0.05 dB over the baseband Nyquist band-
width, allowing the AD9857 to provide excellent SNR over its
performance range.
The Inverse CIC Filter can be bypassed by setting Control
Register 06h<0>. It is automatically bypassed if the CIC
interpolation rate is 1 . Whenever this stage is bypassed, power
to the stage is shut off, thereby reducing power dissipation.
Symbol
t
t
DS
DH
DATA
INBAND
ATTENUATION
GRADIENT
DATA
is the sample rate at the input of the first half-band
f
region, the higher frequencies of the data spectrum
Table I. Parallel Data Bus Timing
DATA
/2
f
DATA
Definition
Data Setup Time
Data Hold Time
CIC FILTER RESPONSE
DATA
as defined by Nyquist
DATA
region.
4f
DATA
Min
4 ns
0 ns
f
DATA
is
Fixed Interpolator (4 )
This block is a fixed 4 interpolator. It is implemented as two
half-band filters. The output of this stage is the original data
upsampled by 4 .
Before presenting a detailed description of the half-band filters,
recall that in the case of the Quadrature Modulation Mode the
input data stream is representative of complex data; i.e., two
input samples are required to produce one I/Q data pair. The
I/Q sample rate is one-half the input data rate. The I/Q sample
rate (the rate at which I or Q samples are presented to the input
of the first half-band filter) will be referred to as f
AD9857 is a quadrature modulator, f
of the internal I/Q sample pairs. It should be emphasized here
that f
data, which must be upsampled before presentation to the AD9857
(as will be explained later). The I/Q sample rate (f
limit on the minimum bandwidth necessary to transmit the f
spectrum. This is the familiar Nyquist limit and is equal to one-half
f
Together, the two half-band filters provide a factor-of-four increase
in the sampling rate (4 f
loss is 0.01 dB, so virtually no loss of signal level occurs through
the two half-band filters. Both half-band filters are linear phase fil-
ters, so that virtually no phase distortion is introduced within the
pass band of the filters. This is an important feature as phase
distortion is generally intolerable in a data transmission system.
The half-band filters are designed so that their composite perfor-
mance yields a usable pass band of 80% of the baseband Nyquist
frequency (0.2 on the frequency scale below). Within that pass
band the ripple will not exceed 0.002 dB. The stopband extends
from 120% to 400% of the baseband Nyquist frequency (0.3
to 1.0 on the frequency scale below) and offers a minimum of
85 dB attenuation. The composite response of the two half-
band filters together are shown in Figures 22 and 23.
IQ
, hereafter referred to as f
IQ
–100
–110
–120
–130
–140
–10
–20
–30
–40
–50
–60
–70
–80
–90
is not the same as the baseband of the user’s symbol rate
10
0
0
0.2
0.2
0.4
0.3
0.6
IQ
or 8 f
NYQ
0.8
FREQUENCY
.
1.0
NYQ
IQ
1.2
). Their combined insertion
represents the baseband
1.4
1.6
1.8
IQ
IQ
–85
. Since the
) puts a
2.0
IQ