Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 111

6.5 FIRST COMPONENT AFTER THE ANTENNA ( 6 )

The first component following the antenna can be either a filter or an amplifier. If
the antenna is integrated with an amplifier, the first component after the antenna
is the amplifier. Both arrangements have advantages and disadvantages, which
will be discussed in this section.
The noise figure of a receiver can be expressed as: ( 6 )

F 2 − 1 F 3 − 1

G 1 G 2 +···+

1
G 1 G 2 ···

F N −

F = F 1 + + ( 6 . 4 )

G 1 G N 1

where F i and G i ( i = 1 , 2 ,...N ) are the noise figure and gain of each individual

component in the RF chain.
If the amplifier is the first component, the noise figure of the receiver is low
and is approximately equal to the noise figure of the first amplifier, which can
be less than 2 dB. The overall noise figure of the receiver caused by the second
component, such as the filter, is reduced by the gain of the amplifier. The potential
problem with this approach is that strong signals in the bandwidth of the amplifier
may drive it into saturation and generate spurious frequencies.
If the first component is a filter, it can stop out-of-band signals from entering
the input of the amplifier. If the filter only passes the C/A band, the bandwidth
is around 2 MHz. A filter with 2 MHz bandwidth with a center frequency at
1575.42 MHz is considered high Q. Usually, the insertion loss of such a filter is
relatively high, about 2 - 3 dB, and the filter is bulky. The receiver noise figure
with the filter as the first component is about 2 - 3 dB higher than the previous
arrangement. Usually, a GPS receiver without special interfering signals in the
neighborhood uses an amplifier as the first component after the antenna to obtain
a low noise figure.

6.6 SELECTING SAMPLING FREQUENCY AS A FUNCTION OF THE C/A
CODE CHIP RATE

An important factor in selecting the sampling frequency is related to the C/A
code chip rate. The C/A code chip rate is 1.023 MHz and the sampling fre-
quency should not be a multiple number of the chip rate. In other words, the
sampling should not be synchronized with the C/A code rate. For example, using
a sampling frequency of 5.115 MHz ( 1 . 023 5 ) is not a good choice. With this

sampling rate the time between two adjacent samples is 195.5 ns (1/5.115 MHz).
This time resolution is used to measure the beginning of the C/A code. The
corresponding distance resolution is 58.65 m (195 . 5 × 10 − 9

×

× 3 × 10 8 m). This
distance resolution is too coarse to obtain the desired accuracy of the user posi-
tion. Finer distance resolution should be obtained from signal processing. With