AD9856/PCB Analog Devices Inc, AD9856/PCB Datasheet - Page 20

• Manufacturer Part Number: AD9856/PCB
• Description: BOARD EVAL FOR AD9856
• Manufacturer: Analog Devices Inc
• Type: DDS Modulatorsr

Specifications of AD9856/PCB

• Rohs Status: RoHS non-compliant
• For Use With/related Products: AD9856
• Lead Free Status / RoHS Status: Not Compliant

AD9856
HALF-BAND FILTERS (HBFS)
Before presenting a detailed description of the HBFs, recall that
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
AD9856 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
AD9856 (as is explained later). The I/Q sample rate (f
a limit on the minimum bandwidth necessary to transmit the
f
one half f
HBF 1 is a 47-tap filter that provides a factor-of-two increase in
the sampling rate. HBF 2 is a 15-tap filter offering an additional
factor-of-two increase in the sampling rate. Together, HBF 1
and HBF 2 provide a factor-of-four increase in the sampling
rate (4 × f
0.01 dB, so virtually no loss of signal level occurs through the
first two HBFs. HBF 3 is an 11-tap filter and, if selected,
increases the sampling rate by an additional factor of two. Thus,
the output sample rate of HBF 3 is 8 × f
exhibits 0.03 dB of signal-level loss. As such, the loss in signal
level through all three HBFs is only 0.04 dB and may be ignored
for all practical purposes.
In relation to phase response, all three HBFs are linear phase
filters. As such, 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.
In addition to knowledge of the insertion loss and phase
response of the HBFs, some knowledge of the frequency
response of the HBFs is useful as well. The combined frequency
response of HBF 1 and 2 is shown in Figure 31 and Figure 32.
The usable bandwidth of the filter chain puts a limit on the
maximum data rate that can be propagated through the device.
A look at the pass-band detail of the HBF 1 and HFB 2 response
indicates that to maintain an amplitude error of no more than
1 dB, users are restricted to signals having a bandwidth of no
more than about 90% of f
in the flat portion of the filter pass band, users must oversample
the baseband data by at least a factor of two prior to presenting
it to the AD9856. Without over-sampling, the Nyquist band-
width of the baseband data corresponds to the f
the upper end of the data bandwidth suffers 6 dB or more of
attenuation due to the frequency response of HBF 1 and HBF 2.
Furthermore, if the baseband data applied to the AD9856 has
been pulse shaped, there is an additional concern. Typically,
pulse shaping is applied to the baseband data via a filter having
IQ
spectrum. This is the familiar Nyquist limit and is equal to
IQ
is not the same as the baseband of the user’s symbol rate
IQ
IQ
, which is referred to as f
or 8 × f
NYQ
). Their combined insertion loss is a mere
NYQ
. To keep the bandwidth of the data
NYQ
IQ
.
represents the baseband
IQ
or 16 × f
IQ
. Because the
NYQ
NYQ
. As such,
. HBF 3
IQ
) puts
Rev. C | Page 20 of 36
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 0 ≤ 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 frequency
response of HBF 1 and 2.
To reiterate, the user must oversample the baseband data by at
least a factor of two (2). In addition, there is a further restriction
on pulse shaping—the maximum value of α that can be imple-
mented is 0.8. This is because the data bandwidth becomes
1/2(1 + α) f
extreme edge of the flat portion of the filter response. If a
particular application requires an α value between 0.8 and 1,
then the user must oversample the baseband data by at least a
factor of four (4).
Figure 32. Pass-Band Detail: Combined Frequency Response of HBF 1 and 2
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
–1
–2
–3
–4
–5
–6
0
1
0
0
0
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW
NYQ
Figure 31. Half-Band 1 and 2 Frequency Response
0.1
0.5
= 0.9 f
0.2
1.0
0.3
NYQ
, which puts the data bandwidth at the
1.5
0.4
0.5
2.0
0.6
2.5
0.7
3.0
0.8
3.5
0.9
4.0
1.0