Z87200 데이터 시트보기 (PDF) - Zilog

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Z87200

SPREAD-SPECTRUM TRANSCEIVER

Zilog 

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 fRXIFCLK, 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 fRXIFCLK

When Carrier Frequency is set to 0.4 x fRXIFCLK

4-52

DS96WRL0400

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