Z87200 Datasheet, PDF (51/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

Zilog
Z87200
Spread-Spectrum Transceiver
The example of Figure 18 demonstrates that aliasing dis- In Figure 20, by contrast, the level of distortion is consider-
tortion of the BPSK/QPSK modulator output will result if ably reduced by programming an I.F. frequency that in-
significant energy of the baseband component’s spectrum
falls beyond the Nyquist frequency of f_RXIFCLK/2. The first
creases the separation of the baseband mainlobe from the
alias mainlobe. Here, the carrier frequency has been re-
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alias will then shift that energy into the region below the duced to 0.25 x f_RXIFCLK, and now the fourth and fifth side-
Nyquist frequency and potentially interfere with the desired lobes of the first alias lie in the same part of the spectrum
signal. In Figure 19 the second and third sidelobes of the as the baseband mainlobe, reducing the distorting energy
first alias fall within the mainlobe of the baseband compo- to approximately –23 dBc at the peak of the fourth side-
nent, where the magnitude of this corrupting signal is ap- lobe.
proximately –13 dBc.
Figure 17. Spectrum of DDS modulated at 0.1 x f_RXIFCLK
when carrier frequency is set to 0.25 x f_RXIFCLK
DS96WRL0400
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