Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 118

FIGURE 6.7 Band aliasing for direct sampling of the L1 frequency at 5 MHz.

With today's technology, it is easier to build a down-converted approach, but
the direct digitization is attractive for its simplicity. There is another advantage
for direct digitization, which is to alias more than one desired signal into the
baseband. This approach will be discussed in Section 6.11.

6.10 IN-PHASE (I) AND QUADRANT-PHASE (Q) DOWN
CONVERSION ( 10 )

In many commercial GPS receivers, the input signal is down converted into I-Q
channels. The data collected through this approach are complex and the two sets
of data are often referred to as real and imaginary. Since there are two channels,
the Nyquist sampling is f s = f . A common practice is to choose f s > 1 . 25 f
to accommodate the skirt of the filter. The relation between the input and the
output frequencies is

f o = f i − nf s and f o <f s ( 6 . 7 )

where n is a positive integer. The relation between the input and output band is
shown in Figure 6.8. In the I-Q channel digitization, as long as f < f s there
is no spectrum overlapping in the output baseband.
A common frequency selection is shown in Figure 6.8. The center of the
output frequency is placed at zero. This approach usually can be achieved only
through a down-converted design, and the input frequency f i is usually set to
zero or to a multiple of the sampling frequency f s . In this arrangement, the
input frequency is divided into two equal bands. The lower input frequency is
aliased to a higher frequency in the baseband and the higher input frequency is