AD9857AST Analog Devices Inc, AD9857AST Datasheet - Page 20

• Manufacturer Part Number: AD9857AST
• Description: IC QUADRATURE DGTL UPCONV 80LQFP
• Manufacturer: Analog Devices Inc
• Series: AD9857r

Specifications of AD9857AST

• Rohs Status: RoHS non-compliant
• Function: Upconverter
• Frequency: 5MHz ~ 200MHz
• Rf Type: HFC Cable Network
• Package / Case: 80-LQFP

AD9857
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) is 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 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
combined insertion 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 filters, 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
performance 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 does not exceed 0.002 dB. The stop
band extends from 120% to 400% of the baseband Nyquist
frequency (0.3 to 1.0 on the frequency scale) and offers a
minimum of 85 dB attenuation. Figure 24 and Figure 25 show
the composite response of the two half-band filters together.
–100
–120
–130
–140
–110
IQ
IQ
–10
–20
–30
–40
–50
–60
–70
–80
–90
, hereafter referred to as f
10
Figure 24. Half-Band 1 and 2 Frequency Response; Frequency
is not the same as the baseband of the user’s symbol rate
0
0
0.2
0.2
Relative to HB1 Output Sample Rate
0.3
0.4
0.6
0.8
FREQUENCY
NYQ
1.0
IQ
.
1.2
IQ
or 8 × f
represents the baseband
1.4
NYQ
IQ
. Because the
1.6
). Their
1.8
IQ
) puts a
–85
2.0
Rev. C | Page 20 of 40
IQ
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 (Figure 25) indicates that in order to maintain an
amplitude error of no more than 1 dB, signals are restricted to
having a bandwidth of no more than about 90% of f
keep the 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 corresponds to the f
upper end of the data bandwidth suffers 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 α value is
used to modify the bandwidth of the data where the value of α
is such that ≤ α ≤ 1. A value of 0 causes the data bandwidth 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 α = 1, the
Nyquist bandwidth of the data corresponds 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. Figure 26 illustrates the
relationship between α and the bandwidth of raised cosine
shaped pulses. The problem area is indicated by the shading in
the tail of the pulse with α = 1 which extends into the roll-off
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
are shown in Figure 26.
–0.002
–0.004
–0.006
–0.008
–0.010
0.010
0.008
0.006
0.004
0.002
0
Figure 25. Combined Half-Band 1 and 2 Pass Band Detail;
0
RELATIVE FREQUENCY (HB1 OUTPUT SAMPLE RATE = 1)
Frequency Relative to HB1 Output Sample Rate
0.05
0.10
NYQ
0.15
. Because of this, the
0.20
NYQ
. Thus, to
0.25