GNSS Antennas and Front Ends - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 57

4.2.3 Amplifier
Amplification is the process that increases the signal magnitude. Thus, an ampli-
fier is a component that does just that. Unlike most filters, an amplifier is an active
component and requires power to accomplish its function. Note that the ideal am-
plifier would only increase the amplitude of the signal. However, any commercial
amplifier will not only increase the amplitude but also add noise to the resulting
signal. The goal, of course, is to have a component that amplifies the signal and
adds minimal noise.
The fundamental parameters used to describe an amplifier are
1. gain , usually expressed in dB, and often assumed constant over a
2. specified frequency range ;anda
3. noise figure , again usually expressed in dB, and indicative of the amount of
noise that will be added to the signal being amplified.
Note that this discussion simplifies the practical amplifier. We are assuming the
amplifier is a packaged device, ignoring the actual fabrication. Further, parameters
such as the third-order intercept point, power requirements, and maximum power
handling are all additional factors that could be considered but are neglected to
simplify the discussion.
Also the amplification shown in Figure 4.2 shows a single amplifier capable
of 50 dB gain. It would be unusual to have a single amplifier capable of such
gain. What is represented as a single amplifier in Figure 4.2 can be constructed of
cascaded multistage amplifiers.
The goal of the amplifier is to raise the extremely weak incident signal to a
level practical for analog-to-digital conversion. Thus, the amount of amplification
is based on the specific ADC and will be discussed in that section. Further, there
is typically a distribution of amplification or gain across different frequencies for
reasons that will become obvious in the next subsection.
4.2.4 Mixer/Local Oscillator
The basic function of the mixer/local oscillator combination is to translate the
input 1575.42 MHz RF carrier to a lower intermediate frequency (IF) and preserve
the modulated signal structure. The most obvious reason for this is to bring the
frequency to usable ranges in which to operate on the signal, in particular perform
the analog-to-digital conversion. However, there are fewer obvious reasons for the
frequency translation, as is discussed within this subsection.
The design illustrated in Figure 4.2 utilizes a single stage of analog frequency
translation. However, it is possible to utilize multiple stages of analog frequency
translation in a single front-end design. The choice is a design trade-off based
on the components available and their individual specifications. The focus of this
section is to illustrate the functionality of the single-stage approach shown in Fig-
ure 4.2.