Page 340 - Electrical Engineering Dictionary

R
+∞ jωt 1 jωt 2
e e dω = 0,t 1 6= t 2 . This technique is used to reduce the size of
−∞
page tables.
harmonic tuning the process of tuning an
hashing the act of translating a search key
amplifier circuit to a frequency that is an in-
intoatableindexusingamany–onemapping.
tegral multiple of the fundamental frequency
See also hash table.
at which the circuit would normally operate.
Hausdorffdistance animportantdistance
harmonically pumped mixer mixer
measure, used in fractal geometry, among
where the intermediate frequency (IF) sig-
other places. Given a distance function d
nal is at a frequency which is the sum or
defined on a Euclidean space E, one derives
difference of the RF and an integer multi-
from it the Hausdorff distance H d on the fam-
ple (usually two) of the LO (local oscillator)
ily of all compact (i.e., bounded and topo-
frequency.
logically closed) subsets of E; for any two
compact subsets K, L of E, H d (K, L) is the
Hartleyoscillator aparticularcaseofLC-
least r ≥ 0 such that each one of K, L is con-
oscillators when X 1 + X 2 + 2X m is realized
tained in the other’s dilation by a closed ball
as a single tapped coil, and X 3 is a capacitor.
of radius r, that is:
Well suited for variable-frequency operation
by varying a single capacitor. [ [
K ⊆ B r (p) and L ⊆ B r (p),
Hartley oscillators are usually not used at
p∈L p∈K
VHFs of higher frequencies. Similarly, the
circuit is avoided at very low audio frequen- where
cies. It is important to distinguish the Hartley
oscillator from the Armstrong topology. In B r (p) ={q ∈ E | d(p, q) ≤ r}.
the Armstrong oscillator, no ohmic connec-
tion exists between the two inductors. In-
stead, coupling is entirely magnetic. Hayes-compatible modem refers to a
modem when it is capable of responding at
Harvard architecture a computer design the commands of modems made by Hayes
feature where there are two separate mem- Microcomputer Products. The Hayes set of
ory units: one for instructions and the other commands represents a sort of standard for
for data only. The name originates from an modems.
early computer development project at Har-
vard University in the late 1940s. Compare haystack response bandpass frequency
with Princeton architecture. response characterized by flat midband re-
sponse with sloping sides.
hash table a table storing a mapping func-
tion whose domain is sparselyused andthat is hazard a momentary output error that oc-
accessed by indices that are computed from curs in a logic circuit because of input signal
the search field (“key”) using a many–one propagation along different delay paths in the
mapping (called a hash function). Hash ta- circuit.
bles are used for many memory and name
mapping functions, such as symbol tables in hazardous location a classification sys-
assemblers and compilers. tem used to define locations that are suscep-
tible to fire and explosion hazards associated
hashed page table a page table where with normal electrical arcing. A class I haz-
the translation of each virtual page number ardous location contains a flammable con-
is stored in a position determined by a hash centration of flammable vapors. A class II
function applied to the virtual page number. hazardous location contains a combustible

c
2000 by CRC Press LLC