18WuD2x

586 receiver front end • recombination rate
Low-noise, high-gain amplifiers are the rule.
Field-effect transistors are commonly used.
receiver IF chain In a wireless communications or
broadcast receiver, the series of radio-frequency
(RF) amplifier stages in which most of the gain
takes place. These stages are also where the best
possible RF selectivity is obtained. The intermediate
frequency (IF) on which the amplifiers works
is a constant frequency. This simplifies the design
of the amplifiers to produce optimum gain
and selectivity. Crystal-lattice filters or mechanical
filters are commonly used in these stages to
obtain the desired bandwidth and response.
receiver mixer In a superheterodyne wireless communications
or broadcast receiver, a stage that
converts the variable input signal frequency to a
constant intermediate frequency (IF), or a stage
that converts the first IF to a second IF usually
having a lower frequency. This type of circuit is
nonlinear, and combines the signal with a carrier
from a local oscillator (LO). The output is either
the sum or the difference of the signal frequency
and the LO frequency.
receiver muting See MUTING, 1.
receiver noise figure A quantitative measure of the
ability of a wireless receiver to respond to desired
signals while rejecting unwanted noise. This can
be quantified in various ways. In general, the
lower the noise figure, the better the sensitivity.
Gallium–arsenide field-effect transistors (GaAs-
FETs) are well known for the low levels of noise
they generate, even at quite high frequencies.
Other types of FETs can be used at lower frequencies.
Bipolar transistors tend to be noisy.
See also NOISE FACTOR, NOISE FIGURE.
receiver post-detector stages In a wireless communications
or broadcast receiver, one or more
stages of amplification and/or filtering employed
to refine the detector output to a form suitable for
feeding a speaker, headset, printer, fax machine,
slow-scan television picture tube, computer,
electromechanical device, or other peripheral
equipment.
receiver primaries See DISPLAY PRIMARIES.
receiver selectivity A quantitative measure of the
ability of a wireless receiver to respond to a desired
signal, but not to undesired ones. The frequency
window is established by a preselector in
the early RF amplification stages, and is honed to
precision by bandpass filters in later amplifier
stages. The preselector passes energy within a
range of about plus-or-minus 10 percent of the
signal frequency; other frequencies are attenuated.
This reduces the chance for strong, out-ofband
signals to impair the performance of
the receiver. The narrowband filter in the final
intermediate-frequency (IF) stage responds only
to energy within the actual signal band. This
minimizes adjacent-channel interference. In
some receivers, yet another bandpass filter is
used in the audio-amplifier stages.
receiver sensitivity A quantitative measure of the
ability of a wireless receiver to recover weak signals
and process them into readable data. The
most common expression is the number of signal
microvolts that must exist at the antenna terminals
to produce a certain signal-to-noise ratio
(S/N). Sometimes, the signal-plus-noise-to-noise
ratio (abbreviated S+N/N) is given. The front end,
or first RF amplifier stage, of a receiver is the
most important stage with regard to sensitivity.
Sensitivity is directly related to the gain of this
stage, but the amount of noise the stage generates
is even more significant. A good front end
should produce the best possible S/N or S+N/N
ratio at its output. All subsequent stages amplify
the front-end noise output as well as the frontend
signal output.
receiving set RADIO RECEIVER.
receiving station A station that ordinarily only receives
signals (i.e., it makes no type of transmission).
Compare TRANSMITTING STATION.
receptacle 1. See SOCKET. 2. The half of a connector
that is mounted on a support, such as a
panel, and that is therefore stationary.
recharge In certain cells and batteries, the restoration
of chemical energy following use so that the
device is ready to deliver its full rated electric current.
Also see RECHARGEABLE.
rechargeable Pertaining to a secondary cell or battery
that can accept a restoration of chemical energy
following use, and thus can be completely
charged and discharged numerous times. Examples:
nickel-metal-hydride (NiMH) battery and
lead-acid battery.
reciprocal impedances See INVERSE IMPED-
ANCES.
reciprocal ohm See SIEMENS and MHO.
reciprocation 1. The determination of a mathematical
reciprocal value from a given value.
2. The transmission of a message in response to
a received message.
reciprocity in antennas See RAYLEIGH-CARSON
THEOREM.
reciprocity theorem When a voltage E across
branch A of a network causes a current I to flow
in branch B of the network, the voltage can be applied
across branch B to cause the same value of
current to flow in branch A. Compare COM-
PENSATION THEOREM, MAXIMUM POWER
TRANSFER THEOREM, NORTON’S THEOREM,
SUPERPOSITION THEOREM, and THEVENIN’S
THEOREM.
recombination The refilling of holes by electrons
in a semiconductor.
recombination current In a transistor circuit,
base current resulting from recombination.
recombination rate In a semiconductor material,
the speed at which the electrons and holes recombine.
It can be expressed as the time
required for a certain proportion of charge carriers
to recombine.