z87200 ZiLOG Semiconductor, z87200 Datasheet - Page 52

• Manufacturer Part Number: z87200
• Description: Spread-spectrum Transceiver
• Manufacturer: ZiLOG Semiconductor

THEORY OF OPERATION (Continued)
Z87200
Spread-Spectrum Transceiver
In general, when a DDS is used to generate an unmodu-
lated signal, the stepped sine wave generated by the DDS
has spectral components at integer multiples of the DDS
sampling clock. In other words, the Z87200’s BPSK/QPSK
modulator, when programmed to generate a signal at I.F.
frequency f
as well as at (n
negative integer. Because of these aliases, one generally
cannot program the NCO to provide an output frequency
f
the I.F. frequency f
tions to the output signal due to aliasing can result even
when f
In particular, the Z87200’s PN modulation results in a
transmit signal that has a power spectral density charac-
terizable as a sinc function (sin(x)/x) centered about the
I.F. frequency f
ger multiples of the PN chip rate, and the null-to-null signal
bandwidth of the Z87200’s transmit signal about f
twice the transmit chip rate. The presence of modulation
sidelobes and their interaction with aliases due to sam-
pling, however, will result in distortion of the mainlobe of
the baseband component centered at f
4-50
OUT
greater than the Nyquist frequency f
OUT
OUT
is less than f
, will produce spectral components at f
OUT
RXIFCLK
. Nulls of the sinc function occur at inte-
OUT
RXIFCLK/2
is modulated, however, degrada-
f
OUT
Figure 16. Spectrum of DDS modulated at 0.1 x f
), where n is a positive or
.
when carrier frequency is set to 0.4 x f
OUT
RXIFCLK/2
unless atten-
PS010202-0601
. When
OUT
OUT
is
tion is paid to the interaction of the chip rate, the I.F. fre-
quency f
In the example of Figure 18, the spectrum drawn in bold
represents a signal where f
be (0.4 x f
rate of (0.1 x f
quency version of this signal appears centered about (0.6
x f
seen, energy of the second and third modulation sidelobes
of the first alias is present within the mainlobe of the base-
band component, resulting in distortion. One would typical-
ly filter the digital-to-analog converted output of the
Z87200’s BPSK/QPSK modulator to remove the energy
outside the modulation mainlobe, but such filtering will not
affect any aliasing distortion within the mainlobe as de-
scribed here. Note that the nulls of the modulated signal
aliases in this example coincide here only due to the
choice of values for the I.F. frequency, sampling rate, and
PN chip rate; in general, the nulls will not coincide. Note
also that the filtering effect of sampling has been neglected
in this discussion — in general, the aliases will be sup-
pressed by a second sinc function, sin(f’)/(f’), where f’ =
baseband component and first alias.
f/f
RXIFCLK
RXIFCLK
OUT
RXIFCLK
) and is shown as the lighter curve. As can be
, but this effect is not very significant for the
, and the sampling rate fRXIFCLK.
RXIFCLK
RXIFCLK
) and has been PN-modulated at a chip
RXIFCLK
). The first alias of the negative fre-
OUT
has been programmed to
Zilog