Page 83 - The Illustrated Dictionary of Electronics

5059F-pB_56-92 4/9/01 4:45 PM Page 68

68 beam-positioning magnet • beat marker

beam-positioning magnet In a three-gun color tween the half-power points in the horizontal
television picture tube, a permanent magnet that plane. Occasionally, it is measured in the verti-
is used to position one of the electron beams cor- cal plane.
rectly, with respect to the other two. bearing The direction of an object or point ex-
beam power tube A tetrode or pentode vacuum pressed in degrees within a 360° horizontal clock-
tube, in which special deflector plates concentrate wise boundary, with the center of the circle
the electrons into beams in their passage from serving as the observation point.
cathode to plate. The beam action greatly increases bearing resolution In radar operations, the mini-
plate current at a given plate voltage. It is used in mum horizontal separation of two targets, in de-
some radio-frequency (RF) power amplifiers. grees, that permits the individual targets to be
beam-rider control system A missile-guidance displayed as two echoes, rather than one.
system in which a control station sends a radio beat Any one of the series of pulsations constitut-
beam to a missile. The beam is moved in such a ing a beat note, which results from heterodyning
way that as the missile stays within the beam, it one signal against another.
hits the target. beat frequency Either of two frequencies f C1 and
beam-rider guidance 1. An aircraft landing guid- f C2 resulting from the mixing of two signals of dif-
ance system, in which the aircraft follows a radio ferent frequencies f A and f B. Frequency f C1 is the
beam in its glide path. 2. The circuitry in a guided sum of the two input frequencies; f C1 = f A + f B.
missile using a beam-rider control system. Frequency f C2 is the difference; f C2 = f A – f B when
beam splitter A device used to divide a light beam f A is the higher of the two input frequencies.
(as by a transparent mirror) into two compo- beat-frequency oscillator Abbreviation, BFO. An
nents, one transmitted and the other reflected; oscillator used to set up audible beat frequen-
hence, a BEAM-SPLITTING MIRROR. cies with an incoming received signal and in-
beam splitting In radar, a method of calculating stalled in the intermediate-frequency (IF) stages
the mean azimuth of a target from the azimuth at of a superheterodyne communications receiver.
which the target is first revealed by one scan, and For single-sideband (SSB) reception, the BFO is
the azimuth at which the target information set at the frequency of the received suppressed
ceases. carrier. In continuous-wave (CW) Morse code re-
beam-splitting mirror In an oscilloscope-camera ception, the BFO is set at a frequency that dif-
system, a tilted, transparent mirror that allows fers from that of the incoming signal by about
rays to pass horizontally from the oscilloscope 400 to 1000 Hz. The resulting tone has an audio
screen to the camera and to be reflected vertically frequency equal to the difference between the
to the viewer’s eye. BFO frequency and the received signal carrier
beamwidth of antenna The angular width of the frequency. For reception of frequency-shift-
main lobe of the pattern of radiation from a di- keyed (FSK) signals, the BFO is set to such a fre-
rectional antenna. Generally, it is measured be- quency that the resulting audio beat notes are
appropriate for the mark and space inputs of a
Half-power terminal unit or modem.
width

IF AF
amp. Mixer amp.

From
previous BFO Audio
IF stage output

beat-frequency oscillator
beating 1. Also called heterodyning. The combina-
tion of signals of different frequencies resulting in
sum and difference frequencies. 2. The fluttering
noise heard when two audio tones, very close in
frequency and very similar in amplitude, are
emitted at the same time.
beat marker In the visual (oscilloscopic) alignment
of a tuned circuit, a marker pip that results from
the beat note between the sweep-generator signal
beamwidth of antenna and the signal from a marker oscillator.