Z87200 Datasheet, PDF (52/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

Z87200
Spread-Spectrum Transceiver
Zilog
THEORY OF OPERATION (Continued)
In both of the cases shown above, and especially the sec-
ond, the level of the distortion is low enough so that the
performance penalty would not be very great. And, of
course, in a spread-spectrum system the effective distor-
tion is reduced by the processing gain realized in de-
spreading the signal at the receiver. In both of these exam-
ples, however, the PN chip rate is a very modest 10% of
the frequency of the system clock; if the chip rate is in-
creased to 40% of f_RXIFCLK, then the situation is very dif-
ferent, as shown in Figure 20.
In Figure 20, both the chip rate and the carrier frequency
have been set at 40% of the clock frequency. As a result,
the baseband mainlobe straddles the Nyquist frequency,
and the first alias of the mainlobe overlaps the spectrum of
the baseband mainlobe, thereby creating very significant
aliasing distortion which cannot be eliminated by filtering.
This level of distortion would severely affect the perfor-
mance of the system and, in general, would be completely
unacceptable.
Figure 18. Spectrum of DDS Modulated at 0.4 x f_RXIFCLK
When Carrier Frequency is set to 0.4 x f_RXIFCLK
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DS96WRL0400