GNSS Antennas and Front Ends - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 53

Active antenna
Gain 30 dB

Noise figure

≈

≈ 2 . 5dB

Analog-to-digital
converter
f sampling ≈

Amplifier(s)
Gain

Amplifier(s)
Gain

Rooftop cable
Loss ≈ 8.0 dB

≈ 50 dB 50 dB

Noise figure

≈ 38 . 192 MHz

4-bit samples

≈ . ≈ .

Noise figure 4 0dB 4 0dB

BPF BPF BPF ADC

Bandpass filter
f center = 1575 . 42 MHz
3dB BW

Bandpass filter
f center = 47 . 74 MHz
3dB BW

Bandpass filter
f center = 47 . 74 MHz
3dB BW ≈ 50 MHz ≈ 18 MHz ≈ 6 . 0MHz

Sampled signal
to be processed
IF = 9.548 MHz

÷ 40

TCXO

∼

PLL

Sampling
clock

f = 10 . 00 MHz

PLL output

f = 68 MHz

@7dBm

1527 .

FIGURE 4.2. GNSS L1 front end.

4.2 GNSS L1 Front-End Components

4.2.1 GNSS Antenna
The antenna is typically not considered part of the front-end design, but since it
is the first component in the signal path and dictates elements that follow, it is
important to summarize when describing the GNSS front end. There are numer-
ous texts on antenna theory and design, e.g., Balanis (1996), Straw (2003). Also
the trade publication GPS World , over the past four years, has published a GPS
Antenna Survey that lists all GPS antennas and their features. All of these are
excellent references for additional information on GNSS antennas.
As is the case with most of the components associated with analog signal con-
ditioning, there is an extensive set of parameters associated with an antenna that
describe its performance. Three fundamental parameters to be discussed here are
the frequency/bandwidth , polarization ,and gain pattern .
The antenna will be designed to induce a voltage from radio waves propagating
at the GNSS L1 frequency or 1575.42 MHz. In addition, the design should ac-
commodate the appropriate bandwidth of the desired signal. This is usually spec-
ified using two additional antenna parameters: the Voltage Standing Wave Ratio
(VSWR) and impedance . Practically all GNSS front-end components utilize an
impedance of 50

, which is typical for a majority of radio frequency design.
VSWR is a measure of impedance mismatch or the measure of how much of the
incident power will be absorbed and how much will be reflected. And, of course,
this is a function of frequency. The VSWR is typically on the order of 2.0:1, which
equates to 90% power absorption across the bandwidth of desired frequencies.
Polarization refers to the orientation of the electric field from the radio fre-
quency transmission. Received GNSS signals are right-hand circularly polarized
(RHCP), and the antenna should be designed as such. The decision to employ
RHCP for GNSS was definitely not arbitrary. One of the most difficult error