GNSS Antennas and Front Ends - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 56

Ideal filter

0dB

Insertion loss

− 1dB

− 3dB

Less frequency selective

More frequency selective

f center

FIGURE 4.3. Comparison of bandpass filters.

sources that could enter the front end and saturate later-stage components. For
this reason, a bandpass filter typically is the first component following the an-
tenna. The other filters within Figure 4.2 serve specific roles, as will be discussed
in the following sections, but all are used to perform the primary role of a filter:
pass selective frequencies and attenuate others.
Filters can be characterized by their insertion loss , or the attenuation of the
desired frequency components. Ideally, there would be no insertion loss, but alas
that is not the case for practical components, and the lower this value the better.
Note that this filter insertion loss will result in a system noise figure penalty when
it is placed prior to the first amplifier. Yet this is still very often the case to mini-
mize any issues from adjacent frequency bands given the limited selectivity of the
antenna itself. If the receiver will be operated in an environment that will not have
high-power adjacent frequency band transmitters in close proximity, then this fil-
ter may not be necessary and the position of the first amplifier and filter can be
switched. Or the impact to the noise figure can be minimized by selecting a filter
with a low insertion loss.
The second filter parameter is the bandwidth . Again, since no filter is ideal,
typically a 3 dB bandwidth is specified. This indicates at which frequency(s) the
attenuation will be 3 dB (or 50% of the signal power). However, these two param-
eters cannot completely describe most filters but only provide some insight into
their performance, as shown in Figure 4.3.
A goal in filter design is to provide sharp transitions between the desired (pass-
band) and undesired (cutoff) frequencies while maintaining a minimal insertion
loss. Depending on the practical implementation of the filters [options include
cavity, surface acoustic wave (SAW), ceramic, or lumped elements (resistors, ca-
pacitors, and filters)], this can be done by increasing the number of sections/ele-
ments within the design.