Receiver Hardware Considerations - Fundamentals of Global Positioning System Receivers: A Software Approach - page 110

It is assumed that the system has a characteristic impedance of 50 .Asimple
way to estimate the gain of the amplifier chain is to amplify the noise floor to
this level, thus, a net gain of about 101 dB ( 111 ) is needed. Since in

the RF chain there are filters, mixer, and cable loss, the insertion loss of these
components must be compensated with additional gain. The net gain must be
very close to the desired value ( 10 ) of 101 dB. Too low a gain value will not
activate all the possible levels of the ADC. Too high a gain will saturate some
components or the ADC and create an adverse effect.

− 10 +

6.4 TWO POSSIBLE ARRANGEMENTS OF DIGITIZATION
BY FREQUENCY PLANS ( 8,9 )

Although many possible arrangements can be used to collect digitized GPS sig-
nal data, there are two basic approaches according to the frequency plan. One
approach is to digitize the input signal at the L1 frequency directly, which can
be referred to as direct digitization. The other one is to down-convert the input
signal to a lower frequency, called the intermediate frequency (IF), and digitize
it. This approach can be referred to as the down-converted approach.
The direct digitization approach has a major advantage; that is, in this design
the mixer and local oscillator are not needed. A mixer is a nonlinear device,
although in receiver designs it is often treated as a linear device. A mixer usually
generates spurious (unwanted) frequencies, which can contaminate the output. A
local oscillator can be expensive and any frequency error or impurity produced by
the local oscillator will appear in the digitized signal. However, this arrangement
does not eliminate the oscillator (or clock) used for the ADC.
The major disadvantage of direct digitization is that the amplifiers used in this
approach must operate at high frequency and they can be expensive. The ADC
must have an input bandwidth to accommodate the high input frequency. In gen-
eral, ADC operating at high frequency is difficult to build and has fewer effective
bits. The number of effective bits can be considered as the useful bits, which
are fewer than the designed number of bits. Usually, the number of effective
bits decreases at higher input frequency. In this approach the sampling frequency
must be very accurate, which will be discussed in Section 6.15. Another prob-
lem is that it is difficult to build a narrow-band filter at a higher frequency, and
usually this kind of filter has relatively high insertion loss.
In the down-converted approach the input frequency is converted to an IF,
which is usually much lower than the input frequency. It is easy to build a
narrow-band filter with low insertion loss and amplifiers at a lower frequency are
less expensive. The mixer and the local oscillator must be used and they can be
expensive and cause frequency errors.
Both approaches will be discussed in the following sections. Some consider-
ations are common to both designs and these will be discussed first.