Page 204 - Electrical Engineering Dictionary

ond moment of the time-averaged channel delta connection a three-phase power
impulse response. source or load in which the elements are con-
nected in series and are thus represented on
delay-line a transmission line of the ap- a schematic diagram as a triangular configu-
propriate length to result in a specific time ration.
delay. As an example, a line at 100 MHz that
is 90 degrees long (one-quarter wavelength) delta function in discrete-time, the func-
will exhibit a time delay of 2 ns. tion given by

0 n 6= 0
delay-locked loop (DLL) (1) a pseudo- δ[n]=
noise (PN) sequence tracking loop typically 1 n = 0.
used in receivers for spread spectrum signals. In continuous-time, the (Dirac) delta “func-
The tracking loop has two branches that cor- tion” is not a function at all and, although
relate the received signal with two shifts of a an abuse of mathematical rigor, is a tremen-
locally generated PN sequence: an advanced dously important concept throughout signal
and a retarded time shift of the phase of the and system theory. The Dirac delta “func-
signal being tracked. tion” is defined as
(2) a technique for symbol synchroniza-
0 t 6= 0
tionbasedontime-shiftedandreversedcorre- δ(t) =
lation functions of the desired symbol wave- ∞ t = 0
form, which results in a control function with such that
an s-shape (termed an s-curve). The control Z
function is used in a feedback loop similar to f(t)δ(t − t o )dt = f(t o )
a PLL to adjust the timing of receiver clock
used in sampling the received signal. DLLs
are used, e.g., in spread-spectrum receivers delta gun cathode ray tube (CRT) elec-
to maintain chip synchronization. tron gun structure has the red, green, and blue
electron gun components configured in the
delayed AGC See delayed automatic gain shape of an equilateral triangle; the structure
control. provides the smallest CRT neck size and has
the smallest deflection yoke diameter, but re-
delayed automatic gain control auto- quires color registration (color convergence)
matic gain control in which the control mech- correction in both the horizontal and the ver-
anism becomes effective only when the input tical CRT face. See cathode ray tube.
signal is larger than a predetermined level.
delta modulation a special case of differ-
delayed branch instruction a form of ential pulse code modulation (DPCM) where
conditional branch instruction in which one the digital code-out represents the change, or
(or executed irrespective of the outcome of slope, of the analog input signal, rather than
the branch). Then the branch takes effect. the absolute value of the analog input signal.
Used to reduce the branch penalty. A “1” indicates a rising slope of the input
signal. A “0” indicates a falling slope of the
delayed neutrons neutrons emitted by fis- input signal. The sampling rate is dependent
sion daughters after some time delay. on the derivative of the signal, since a rapidly
changing signal would require a rapid sam-
delectric resonator an unmetallized di- pling rate for acceptable performance.
electric object of high dielectric constant and
high quality factor that can function as an en- delta modulation control a pulse-time
ergy storage device. modulation method transplanted from the

c
2000 by CRC Press LLC