Z87200 datasheet(51/54 Pages) ZILOG

51 / 54 page

Z87200

Zilog

Spread-Spectrum Transceiver

DS96WRL0400

4-51

The example of Figure 18 demonstrates that aliasing dis-

tortion of the BPSK/QPSK modulator output will result if

significant energy of the baseband component’s spectrum

falls beyond the Nyquist frequency of fRXIFCLK/2. The first

alias will then shift that energy into the region below the

Nyquist frequency and potentially interfere with the desired

signal. In Figure 19 the second and third sidelobes of the

first alias fall within the mainlobe of the baseband compo-

nent, where the magnitude of this corrupting signal is ap-

proximately –13 dBc.

In Figure 20, by contrast, the level of distortion is consider-

ably reduced by programming an I.F. frequency that in-

creases the separation of the baseband mainlobe from the

alias mainlobe. Here, the carrier frequency has been re-

duced to 0.25 x fRXIFCLK, and now the fourth and fifth side-

lobes of the first alias lie in the same part of the spectrum

as the baseband mainlobe, reducing the distorting energy

to approximately –23 dBc at the peak of the fourth side-

lobe.

Figure 17. Spectrum of DDS modulated at 0.1 x fRXIFCLK

when carrier frequency is set to 0.25 x fRXIFCLK

Z87200 Datasheet(PDF) 51 Page - Zilog, Inc.