ADC31RF80 Datasheet, PDF (40/136 Pages) Texas Instruments – 3-GSPS Telecom Receiver and Feedback Device

ADC31RF80
SBAS860 – AUGUST 2017
Figure 96 shows a dual-band example with a divide-by-8 complex.
NCO 1,
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ADC ^{3 GSPS}
IQ 3 GSPS
Filter
8
IQ
375 MSPS
IQ Output
Band 1
IQ 3 GSPS
8
IQ
375 MSPS
IQ Output
Band 2
NCO 2,
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Filter
Band 2
fS/4
Band 1
Band 1
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fS/16
Band 2
fS/16
NCO 2
NCO 1
fS/2
Figure 96. Dual-Band Example
The decimation filter responses normalized to the ADC sampling clock are illustrated in Figure 96 to Figure 119
and can be interpreted as follows:
Figure 97 shows that each figure contains the filter pass-band, transition bands, and alias bands. The x-axis in
Figure 97 shows the offset frequency (after the NCO frequency shift) normalized to the ADC sampling clock
frequency.
For example, in the divide-by-4 complex, the output data rate is an f_S / 4 complex with a Nyquist zone of f_S / 8 or
0.125 × f_S. The transition band is centered around 0.125 × f_S and the alias transition band is centered at 0.375 ×
f_S. The alias bands that alias on top of the wanted signal band are centered at 0.25 × f_S and 0.5 × f_S (and are
colored in red).
The decimation filters of the ADC31RF80 provide greater than 90-dB attenuation for the alias bands.
Filter
Transition
Band
Bands That Aliases On
Top of Signal Band
Band That Folds Back On
Top of Transition Band
Figure 97. Interpretation of the Decimation Filter Plots
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