Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 117

FIGURE 6.6 Arrangement for Direct Digitization.

RF gain. One filter is used after the first amplifier to limit out-of-band signal and
the second filter is placed in front of the ADC to limit the noise bandwidth. A
direct digitization arrangement is shown in Figure 6.6. In this arrangement the
second filter is very important. Without this filter the noise in the collected data
can be very high and it will affect signal detection.
In the direct sampling case, the frequency of the input signal is fixed; one
must find the correct sampling frequency f s to avoid band overlapping in the
output. In this approach there are two unknowns: f s and n , in Equation (6.6).
An exact solution is somewhat difficult to obtain. However, the problem can be
easily solved if the approximate sampling frequency is known.
Let us use an example to illustrate the operation. In this example, the input GPS
L1 signal is at 1575.42 MHz, and the sampling frequency is about 5 MHz. First
use Equation (6.5) with f o = 1 . 25 MHz, f i = 1575 . 42 MHz, and f s = 5MHz
to find n

= 629 . 66. Round off n = 630 and use f o = f s /4 in Equation (6.6). The

5 . 009 MHz and the center is aliased to 1.252 MHz.
In one arrangement a scope is used to collect digitized data because the per-
sonal computer - based ADC card cannot accommodate the frequency of the input
signal. The scope has a specified bandwidth of dc-1000 MHz, but it can digitize
a signal at 1600 MHz with less sensitivity. The scope can operate at 5 MHz,
but the sampling frequency cannot be fine-tuned. If 5 MHz is used to sample
the input frequency, the center frequency will be aliased to 420 kHz. Since the
bandwidth of the C/A code is 2 MHz, there is band overlapping with the center
frequency at 420 kHz as shown in Figure 6.7.
Actual GPS data were collected through this arrangement. Although there
is band overlapping, the data could still be processed, because the overlapping
range is close to the edge of the signal where the spectrum density is low. In this
arrangement, the overall amplification is reduced because the scope can digitize
weak signals.
In another arrangement, an experimental ADC built by TRW is used. The
ADC can sample only between approximately 80 to 120 MHz, limited by the
circuit around the ADC. In order to obtain digitized data at 5 MHz, the output
from the ADC is decimated. For example, if the ADC operates at 100 MHz and
one data point is kept out of every 20 data points, the equivalent sampling rate
is 5 MHz. The actual sampling frequency is selected to be 5.161 MHz; the input
signal is aliased to 1.315 MHz, which is close to the center of the output band
at about 1.29 (5.161/4) MHz.

result is f s =