Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 119

FIGURE 6.8 Frequency aliasing for complex data collection.
FIGURE 6.9 I and Q down converter.
aliased to a lower frequency as shown in Figure 6.8. This phenomenon affects
the data conversion procedure discussed in Section 6.14.
There are two ways to build an I-Q down converter as shown in Figure 6.9.
In Figure 6.9a, a 90-degree phase shift is introduced in the input circuit. In
Figure 6.9b, the 90-degree phase shift is introduced in the oscillator circuit. Both
approaches are popularly used. If one wants the output frequency to be zero,
the local oscillator is often selected as the input signal or at 1575.42 MHz.
For a wideband receiver the I-Q approach can double the input bandwidth with
the same sampling frequency. Since the GPS receiver bandwidth is relatively
narrow, this approach is not needed to improve the bandwidth. This approach
uses more hardware because one additional channel is required. The amplitude
and phase of the two outputs are difficult to balance accurately. From the software
receiver point of view, there is no obvious advantage of using an I-Q channel
down converter.