AD9879-EB datasheet(23/32 Pages) AD

23 / 32 page

AD9879

Rev. A | Page 23 of 32

FREQUENCY RELATIVE TO I/Q NYQUIST BW

1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

–1

–2

–3

–4

–5

–6

Figure 15. Cascaded Filter Pass-Band Detail (N = 4)

FREQUENCY RELATIVE TO I/Q NYQUIST BW

1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

–1

–2

–3

–4

–5

–6

Figure 16. Cascaded Filter Pass-Band Detail (N = 3)

To 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 representing it to the AD9879.

Without oversampling, the Nyquist bandwidth of the baseband

data corresponds to the fNYQ. Consequently, the upper end of the

data bandwidth suffers 6 dB or more of attenuation due to the

frequency response of the digital filters.

There is an additional concern if the baseband data applied to

the AD9879 has been pulse shaped. 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 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 the filters.

The maximum value of α that can be implemented is 0.45. This

is because the data bandwidth becomes:

()

NYQ

725

(16)

which puts the data bandwidth at the extreme edge of the flat

portion of the filter response.

If a particular application requires an α value between 0.45 and

1, then the user must oversample the baseband data by at least a

factor of four. The combined HBF1, HBF2, and CIC filter

introduces a worst-case droop of less than 0.2 dB over the

frequency range of the data to be transmitted.

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