Cathode-ray oscilloscope Oscilloscope types

between electron gun , screen 2 opposed pairs of metal plates called deflection plates. vertical amplifier generates potential difference across 1 pair of plates, giving rise vertical electric field through electron beam passes. when plate potentials same, beam not deflected. when top plate positive respect bottom plate, beam deflected upwards; when field reversed, beam deflected downwards. horizontal amplifier similar job other pair of deflection plates, causing beam move left or right. deflection system called electrostatic deflection, , different electromagnetic deflection system used in television tubes. in comparison magnetic deflection, electrostatic deflection can more readily follow random , fast changes in potential, limited small deflection angles.

common representations of deflection plates misleading. one, plates 1 deflection axis closer screen plates other. plates closer provide better sensitivity, need extend far enough along crt s axis obtain adequate sensitivity. (the longer time given electron spends in field, farther s deflected.) however, closely spaced long plates cause beam contact them before full amplitude deflection occurs, compromise shape has them relatively close toward cathode, , flared apart in shallow vee toward screen. not flat in quite-old crts!

the timebase electronic circuit generates ramp voltage. voltage changes continuously , linearly time. when reaches predefined value ramp reset , settles starting value. when trigger event recognized, provided reset process (holdoff) complete, ramp starts again. timebase voltage drives horizontal amplifier. effect sweep screen end of electron beam @ constant speed left right across screen, blank beam , return deflection voltages left, speak, in time begin next sweep. typical sweep circuits can take significant time reset; in cros, fast sweeps required more time retrace sweep.

meanwhile, vertical amplifier driven external voltage (the vertical input) taken circuit or experiment being measured. amplifier has high input impedance, typically 1 megohm, draws tiny current signal source. attenuator probes reduce current drawn more. amplifier drives vertical deflection plates voltage proportional vertical input. because electrons have been accelerated typically 2kv (roughly), amplifier has deliver hundred volts, , wide bandwidth. gain of vertical amplifier can adjusted suit amplitude of input voltage. positive input voltage bends electron beam upwards, , negative voltage bends downwards, vertical deflection @ part of trace shows value of input @ time.

the response of oscilloscope faster of mechanical measuring devices such multimeter, inertia of pointer (and perhaps damping) slows down response input.

observing high speed signals, non-repetitive signals, conventional cro difficult, due non-stable or changing triggering threshold makes hard freeze waveform on screen. requires room darkened or special viewing hood placed on face of display tube. aid in viewing such signals, special oscilloscopes have borrowed night vision technology, employing microchannel plate electron multiplier behind tube face amplify faint beam currents.

tektronix model c-5a oscilloscope camera polaroid instant film pack back.

although cro allows 1 view signal, in basic form has no means of recording signal on paper purpose of documentation. therefore, special oscilloscope cameras developed photograph screen directly. cameras used roll or plate film, while in 1970s polaroid instant cameras became popular. p11 crt phosphor (visually blue) effective in exposing film. cameras (sometimes using single sweeps) used capture faint traces.

the power supply important component of oscilloscope. provides low voltages power cathode heater in tube (isolated high voltage!), , vertical , horizontal amplifiers trigger , sweep circuits. higher voltages needed drive electrostatic deflection plates, means output stage of vertical deflection amplifier has develop large signal swings. these voltages must stable, , amplifier gain must correspondingly stable. significant variations cause errors in size of trace, making oscilloscope inaccurate.

later analog oscilloscopes added digital processing standard design. same basic architecture — cathode ray tube, vertical , horizontal amplifiers — retained, electron beam controlled digital circuitry display graphics , text mixed analog waveforms. display time interleaved — multiplexed — waveform display in same way dual/multitrace oscilloscope displays channels. features system provides include:

on-screen display of amplifier , timebase settings;
voltage cursors — adjustable horizontal lines voltage display;
time cursors — adjustable vertical lines time display;
on-screen menus trigger settings , other functions.
automatic measurement of voltage , frequency of displayed trace

dual-beam oscilloscope

a dual-beam oscilloscope type of oscilloscope once used compare 1 signal another. there 2 beams produced in special type of crt.

unlike ordinary dual-trace oscilloscope (which time-shared single electron beam, losing 50% of each signal), dual-beam oscilloscope simultaneously produced 2 separate electron beams, capturing entirety of both signals. 1 type (cossor, uk) had beam-splitter plate in crt, , single-ended vertical deflection following splitter. (there more type of oscilloscope near end of article.)

other dual-beam oscilloscopes had 2 complete electron guns, requiring tight control of axial (rotational) mechanical alignment in manufacturing crt. in latter type, 2 independent pairs of vertical plates deflect beams. vertical plates channel had no effect on channel b s beam. channel b, separate vertical plates existed deflected b beam only.

on dual-beam oscilloscopes time base, horizontal plates , horizontal amplifier common both beams (the beam-splitter crt worked way). more elaborate oscilloscopes tektronix 556 , 7844 employ 2 independent time bases , 2 sets of horizontal plates , horizontal amplifiers. 1 @ fast signal on 1 beam , slow signal on beam.

most multichannel oscilloscopes not have multiple electron beams. instead, display 1 trace @ time, switch later stages of vertical amplifier between 1 channel , other either on alternate sweeps (alt mode) or many times per sweep (chop mode). few true dual-beam oscilloscopes built.

with advent of digital signal capture, true dual-beam oscilloscopes became obsolete, possible display 2 simultaneous signals memory using either alt or chop display technique, or possibly raster display mode.