Page 303 - Electrical Engineering Dictionary

portional to the frequency resolution; the un- frequency selective filter a filter that
certainty principle places a lower bound on passes signals undistorted in one or a set
the time-frequency resolution product. of frequency bands and attenuate or totally
eliminate signals in the remaining frequency
frequency response consider a system bands.
with transfer function given by
Y(s) frequency selective surface (FSS) fil-
H(s) = . ter made of two-dimensional periodic arrays
F(s)
of apertures or metallic patches of various
The term frequency response is used to de- shapes. Several layers can be used to obtain
note H(jω), (commonly written H(ω)). The a structure with a set of desired spectral prop-
frequency response is often used in describ- erties.
ing the steady-state (stable) system response
to a sinusoidal input. For example, in the frequency shift keying (FSK) (1) an en-
case of a stable continuous-time system with coding method where different bits of infor-
input signal f(t) = A cos(ωt), the steady-
mation are represented by various frequen-
state output signal is given by
cies; used for spread-spectrum signal encod-
ing for security and reduced interchannel in-
y ss (t) = A|H(jω) cos(ωt + H(jω) .
6
terference.
The term |H(jω)|, −∞ <ω < ∞,is (2) a digital modulation technique in
called the magnitude response. The term which each group of successive source bits
6 H(jω), −∞ <ω < ∞, is called the determines the frequency of a transmitted si-
phase response. The magnitude and phase nusoid.
responses are typically represented in the
form of a Bode plot. See also Bode plot,
frequency space the transformed space
transfer function.
of the Fourier transform.
frequency reuse a way to increase the ef-
fective bandwidth of a satellite system when frequency synchronization the process
available spectrum is limited. Dual polariza- of adjusting the frequency of one source so
tionsandmultiplebeamspointingtodifferent that it exactly matches that of another source:
earth regions may utilize the same frequen- more specifically, so that n periods of one
cies as long as, for example, the gain of one frequency are exactly equal to m periods of
beam or polarization in the directions of the the other frequency, for integral n and m. See
other beams or polarization (and vice versa) also phase-locked loop.
is low enough. Isolations of 27–35 dB are
typical for reuse systems. frequency synthesizer an oscillator that
produces sinusoidal wave with arbitrary fre-
frequency reuse cluster a group of quency. In common cases, generated fre-
cells in a cellular communications network quencies are allocated with a frequency spac-
wherein each frequency channel allocated to ing called the channel step.
thenetworkisusedpreciselyonce(i.e., inone
cell of the group). The size of the frequency
frequency variation a change in the elec-
reuse cluster is one of the major factors that
tric supply frequency.
determines the spectral efficiency and ulti-
mately the capacity of a cellular network. For
a given channel bandwidth, the smaller the frequency-modulation recording See
cluster size, the higher the network capacity. magnetic recording code.

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2000 by CRC Press LLC