GNSS Antennas and Front Ends - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 55

on the noise figure. Likewise, components that follow a high gain amplifier in the
cascade will have a minimal effect on the overall noise figure.
For example, consider working with a GNSS receiver in a laboratory environ-
ment. The optimal position for the GNSS antenna will be on the rooftop, clear of
any obstructions. In most cases, this will require a lengthy cable run to the GNSS
receiver with its self-contained front end. This RF cable from the antenna to the
front end will be the first component within the cascade of components. Since all
RF cables have some degree of attenuation, or noise figure, and no gain, the sys-
tem noise figure will be severely degraded. This can be improved if an amplifier
can be incorporated within the antenna itself prior to the long cable. This imple-
mentation is the norm in many GNSS antennas, and such a design is known as an
active antenna and is characterized by the gain of the amplifier.
This active antenna approach complicates things slightly as the antenna itself
is now considered an active element and requires power for the internal amplifier.
This is accomplished in most cases using a bias-tee. The bias-tee component has
three ports: RF, RF+DC, and DC. This component injects DC power onto the
antenna cable from the front end to power the amplifier within the antenna. Thus,
the antenna cable is utilized to pass the GNSS signal from the antenna to the bulk
of the analog signal conditioning and then also to provide a DC voltage from
the analog signal conditioning to the amplifier within the antenna. This is the
approach outlined in Figure 4.2.
A passive antenna is practical in those designs that have the antenna in close
proximity to the analog signal conditioning and, in particular, the first amplifier.
This is commonly the case in the handheld GNSS receivers or for configurations
employing expensive low-loss RF cables.
4.2.2 Filter
The first component within the RF path is a filter. A filter is a frequency selective
device that allows only certain frequencies to pass and attenuates others.
The treatment of the filter as well as the following individual components will
be kept terse. It is expected the reader has a basic background in signal processing;
this will allow the focus to be on the overall GNSS front-end design.
This first filter in Figure 4.2 is a bandpass filter, as opposed to a lowpass or
highpass filter, and its purpose is to provide additional frequency selectivity. Ide-
ally, the antenna would only induce voltages for precisely the frequency band of
interest. However, the antenna, like practical filters, is not ideal. The ideal compo-
nent would pass a range of frequencies and completely eliminate those frequency
components outside that range. Unfortunately, such a filter does not exist, and
the transition between those frequencies that are passed and removed is a grad-
ual transition. Further, even signals at frequencies within the passband typically
experience some level of attenuation.
Typical antennas have fairly poor frequency selectivity. When this is consid-
ered, along with the received signal power levels (and the amplification that will
be required), it is important to try to eliminate any high-power, out-of-band signal