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ADVANCED FILTER TECHNOLOGY

When sampling with an ADC, the system designer needs Many RF components exhibit weakly non-linear memoryless
to select which Nyquist zone to digitise. The first Nyquist zone behavior. This means they can be approximated by a low order
ranges from DC to f S/2 (where fS is the sample rate of the ADC). polynomial. For example, a wideband frequency amplifier could
The second Nyquist zone is from f S/2 to f S and so forth. Anti- be modeled by the odd-order polynomial that includes only the
aliasing filters are used to reject interferer signals in Nyquist first-order and third-order terms:
zones adjacent to the desired Nyquist zone. Interferers at this
location in the signal chain can come from various sources, such
as the MxN spurs generated in the mixer, the down converted When there are two incident signals present at the input of the
signals adjacent to the desired signals, or from harmonics amplifier, within the operating frequency range, as might be the
generated in the IF signal chain. Any unwanted signals that case with a desired signal, ω 1, and a blocker signal, ω 2, the input
are input to the ADC will alias into the first Nyquist zone when signal can be described as:
performing digitisation. A frequency spectrum example of the
unwanted aliasing signals is shown in Figure 5.
Substituting the input equation into the odd-order polynomial
Blocker signals results in an output of:
In RF communications systems, a blocker is a received and
unwanted input signal that degrades the gain and signal-
to-noise-and-distortion (SINAD) ratio of the desired signals
of interest. A blocker can be a signal that masks the desired When the amplitude of the desired signal is much less than the
signal directly or creates spurious products that mask the blocker signal, A << B, then the polynomial in Equation 3 further
desired signal. These unwanted signals could be the result of reduces to:
unintentional or intentional interference. In the former case, it
comes from another RF communications system operating in
the adjacent frequency spectrum. In the latter case, it comes
from nefarious electronic warfare (EW) systems designed to Given the simplification in Equation 4, the desired signal
intentionally disrupt RF communication or radar systems. A amplitude is now a strong function of the blocker signal
frequency spectrum example of a blocker signal and a desired amplitude, B. Since most RF components of interest are
signal is shown in Figure 6. compressive, the alpha coefficients must be of opposite sign ,
1
such that α 1α 3 < 0. The result of the two statements mentioned
previously is consequential, in that the gain of the desired signal
goes to zero for large blocker signal amplitudes.

Filter definitions
To solve the problem of unwanted signals in RF communications
systems, engineers have relied upon filters to reduce these
signals and preserve the desired signals of interest. In simple
terms, a filter is a component that allows the transmission of
frequencies within a pass band and rejection of frequencies in a
band-stop. 2
Usually, the insertion loss (dB) of a filter can be described
as either low-pass, high-pass, band-pass, or band-stop (notch).
Figure 6: Desired and blocker signals. This nomenclature refers to the allowable pass-band frequency

Figure 5: Aliasing in the ADC can cause interfering signals to show up in a band if there is insufficient rejection.

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