Z87200 Datasheet, PDF (46/54 Pages) Zilog, Inc. – Spread-Spectrum Transceiver

Z87200
Spread-Spectrum Transceiver
THEORY OF OPERATION (Continued)
The optimum choice of I.F. frequency discussed above
can be extended beyond the primary Nyquist region. Since
an I.F frequency of n f_SA + f1 produces exactly the same
result for any value of n, the general condition for optimum
separation of the desired signal and the unwanted alias is:
f1 = n f_SA + 1 /4 f_SA and B< 1 /2 f_SA
for positive integer n and positive B and f₁.
And, if care is taken to handle the effect of high side con-
version, the following I.F. frequencies also fulfill the opti-
mum condition:
f1= n f_SA1/4 f_SA and B< 1/2 f_SA
for positive integer n and positive B and f₁.
Zilog
Using the Z87200 with Two ADCs in Quadra-
ture Sampling Mode
Quadrature Sampling Mode requires that quadrature I and
Q channel I.F. inputs are sampled by two ADCs and input
to the Z87200’s Downconverter. All four multipliers of the
Downconverter’s complex multiplier are then used to per-
form true single sideband downconversion to baseband.
Quadrature inputs imply that the input signal is complex,
and the input signal spectrum shown in line 1 of Figure 15
is thus only single-sided with no mirror image spectral
component. As a result, the image alias within the primary
Nyquist region associated with Direct I.F. Sampling Mode
does not appear and does not have to be attenuated by the
Integrate and Dump filter. As in the prior discussion, this
analysis holds as long as B < 1/2 f_SA, 1/2 B < f1, and the
input spectrum is only defined for frequencies within a sin-
gle Nyquist region; that is, non-zero over frequencies f
such that:
(n–1/2)f_SA <|f|<(n+1/2)f_SA
for positive integer n.
INPUT SPECTRUM
1
BANDWIDTH: B
–f_{1 0} f₁
SPECTRUM OF
2
SAMPLING PROCESS
–f_SA
0
f_SA
SPECTRUM
3
AFTER A/D
–f_SA
–f_{1 0} f₁
f_SA
QUADRATURE
4
NCO SPECTRUM
–f_SA
–f₁ 0
f_SA
5
SPECTRUM AFTER MIXER
2 f₁
f_SA –2 f₁
–f_SA
0
f_SA
6
SPECTRUM AFTER IDEAL DIGITAL LOW PASS FILTER
–f_SA
0
f_SA
FREQ.
FREQ.
FREQ.
FREQ.
FREQ.
FREQ.
Figure 15. Spectra of Signals in Quadrature Sampling Mode
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DS96WRL0400