Z87200 Datasheet, PDF(45/54 Page) Zilog, Inc. – Spread-Spectrum Transceiver

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Z87200 Datasheet, PDF (45/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

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Zilog

Z87200

Spread-Spectrum Transceiver

Figure 13. G(Ï) = dom (Ïâ)/Ïâ, where Ïâ = (2ÏNf)/fSA

The choice of the I.F. frequency and sampling rate is cru-

cial so that the unwanted alias of the signal in the base-

band Nyquist region lies as far as possible from the de-

sired signal to permit maximum attenuation. The optimum

separation of the desired signal and the unwanted alias oc-

curs when the alias is centered at the bounds of the base-

band Nyquist region, | f | =1/2 fSA as shown in Figure 14.

In this case, the desired signal is equally spaced from the

unwanted aliases in both the positive and negative fre-

quency domains and f1 = 1/4 fSA. Consider, then, the worst

case appropriate for Direct I.F. Sampling Mode. If B<1/4

fSA as has been said to be appropriate for Direct I.F. Sam-

pling Mode, then N=fSA/B=4, the break frequency is

3 /8 fSA or greater, and the attenuation provided by the In-

tegrate and Dump filter is given by the curve of Figure 13

for values of (N f/fSA) greater than 3/2. Here, the attenua-

tion is at least ~21 dB, offering much better attenuation of

the unwanted alias than in the previous worst case exam-

ple. Further analysis shows that if the input SNR is 15 dB,

then the alias attenuated by 21 dB will reduce the SNR by

approximately 1 dB.

Figure 14. Optimum Condition for Bandpass Sampling

DS96WRL0400

4-45

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