APPLICATIONS: TELEVISIONS

Transcription

1 CATHODE RAY TUBE A cathde ray tube (CRT) is a specialized vacuum tube in which images are prduced when an electrn beam strikes a phsphrescent surface. Mst desktp cmputer displays make usef CRTs. The CRT in a cmputer display is similar t the "picture tube" in a televisin receiver. A cathde ray tube cnsists f several basic cmpnents, as illustrated belw. The electrn gun generates a narrw beam f electrns. The andes accelerate the electrns. Deflecting cils prduce an extremely lw frequency electrmagnetic field that allws fr cnstant adjustment f the directin f the electrn beam. There are tw sets f deflecting cils: hrizntal and vertical. (In the illustratin, nly ne set f cils is shwn fr simplicity.) The intensity f the beam can be varied. The electrn beam prduces a tiny, bright visible spt when it strikes the phsphrcated screen. T prduce an image n the screen, cmplex signals are applied t the deflecting cils, and als t the apparatus that cntrls the intensity f the electrn beam. This causes the spt t

2 race acrss the screen frm right t left, and frm tp t bttm, in a sequence f hrizntal lines called the raster. As viewed frm the frnt f the CRT, the spt mves in a pattern similar t the way yur eyes mve when yu read a single-clumn page f text. But the scanning takes place at such a rapid rate that yur eye sees a cnstant image ver the entire screen. The illustratin shws nly ne electrn gun. This is typical f a mnchrme, r singleclr, CRT. Hwever, virtually all CRTs tday render clr images. These devices have three electrn guns, ne fr the primary clr red, ne fr the primary clr green, and ne fr the primary clr blue. The CRT thus prduces three verlapping images: ne in red (R), ne in green (G), and ne in blue (B). This is the s-called RGB clr mdel. In cmputer systems, there are several display mdes, r sets f specificatins accrding t which the CRT perates. The mst cmmn specificatin fr CRT displays is knwn as SVGA (Super Vide Graphics Array). Ntebk cmputers typically use liquid crystal display. The technlgy fr these displays is much different than that fr CRTs. APPLICATIONS: TELEVISIONS Befre LCD r Plasma televisin, the CRT was used t create a mving image. It used the same principle as a CRT, and fr Black and White televisins, that wrked fine. B&W TVs were essentially the same thing as a CRT, as all that's needed is the cntrl f the brightness f the beam.

3 A CRT TV wrks by having the electrn beam "scan" the screen at an rate faster than ur eyes can perceive. This means that it shts acrss the screen like a machine gun, and the images we see are actually made frm many flurescent dts. The flurescence caused by the beam striking the screen lasts a bit lnger s that the next scan can be made withut the previus image disappearing. It scans twice each time, first filling in the dd "hles" then the even nes. Each scan is abut 1/50 f a secnd. Clur CRT TVs had 3 electrn guns rather than a single ne, a shadw mask, and a mdified flurescent screen. The 3 electrn guns were needed as there were three primary clurs (Red, Green and Blue) that culd be adjusted in different amunts t create any clur. The clurs are frmed as a result f the shadw mask, which is a layer with hles in it that cntrls the angle f the incming electrn beams. This is because the flurescent screen is separated int multi-clured phsphrs that are placed adjacent t each ther at small intervals. Thus it isn't actually a single clured pixel, but rather 3 very small pixels that jin tgether t frm a larger dt. CATHODE RAY OSCILLOSCOPES A Cathde Ray Oscillscpe (CRO) is a diagnstic device that allws ne t "see" vltage. It is essential a Cathde Ray Tube with tw perpendicular sets f deflecting electric plates. The vertical set is where an input vltage is plugged in fr the scillscpe t display. Hwever, the hrizntal set is cnnected t a "sweep generatr" This is what prvides a cnstant, but adjustable, timebase fr the sweeping.

4 It essentially creates a "sawtth vltage." This is what causes the image t be animated, and measured with a linear scale. DIGITAL STORAGE OSCILLOSCOPE Traditinal strage CROs have several limitatins like shrt strage duratin, lw data writing rate, expensive than a cnventinal CRT, and the inability t stre multiple images. Digital Strage Oscillscpe (DSO) is used t limit these limitatins. In DSO, the wavefrm t be stred is digitized, stred in a digital memry, and retrieved fr display n the strage scillscpe. BLOCK DIAGRAM OF DSO: It cnsists f data acquisitin, strage, and data display blcks. WORKING : Digital strage scillscpes wrk in varius mdes which are discussed belw. 1. Rll Mde It is the mst basic mde f peratin which is similar t that f a general purpse CRO. When an input is applied, its trace is displayed n the screen. A user can use this mde t keep an eye n the wavefrm and its varius characteristics. 2. Refresh Mde It is used when the sample rate f a wavefrm becmes t high, and when the wavefrm f interest is repetitive r nearly s. The DSO prduces a stale, triggered display with a higher sweep time.

5 3. Single Sht Mde The DSO is in an inactive state in this mde, displaying the last trace captured, until a given sequence f events take place. 4. Equivalent Time Mde It basically arranges the ADC t wrk n limited sweeps, s as t allw particular signals t sync prperly and give ut a clearer trace. Additinally, there are three acquisitin mdes in a DSO, which are explained briefly belw - Sample - In this acquisitin mde, the scillscpe samples the signal in evenly spaced intervals t cnstruct the wavefrm. This mde accurately represents signals mst f the time. (can result in aliasing) Peak Detect - In this acquisitin mde, the scillscpe finds the highest and lwest values f the input signal ver each sample interval and uses these values t display the wavefrm. (t display narrw pulses, while nise can be higher in this mde) Average - In this acquisitin mde, the scillscpe acquires several wavefrms, averages them, and displays the resulting wavefrm. (Used t reduce randm nise). Wrking f a DSO is similar t that f a CRO, save fr the invlvement f memry, as is described belw. When the memry is full, the next data pint frm the ADC is stred in the first memry lcatin writing ver the ld data. The data acquisitin and the strage prcess cntinue till the cntrl circuit receive a trigger signal frm either the input wavefrm r an external trigger surce. When the triggering ccurs, the system stps and enters int the display mde f peratin in which all r sme part f the memry data is repetitively displayed n the cathde ray tube. In display peratin, tw DACs are used which gives hrizntal and vertical deflectin vltage fr the CRT. Data frm the memry gives the vertical deflectin f the electrn beam, while the time base cunter gives the hrizntal deflectin in the frm f staircase sweep signal. The screen display cnsists f discrete dts representing the varius data pints, but the number f dts is s large that they tend t blend tgether and appear t be a smth cntinuus wavefrm. The display peratin ends when the peratr presses a frnt-panel buttn and cmmands the digital strage scillscpe t begin a new data acquisitin cycle. Applicatins f DSO: 1. Take cursr and pulse width readings 2. Measuring rise time and prpagatin delay 3. Implementing math functins like subtractin and additin 4. Acting as a simple signal tracer, a DSO enables technicians t prbe electrnic device s individual cnnectins and cmpnents, t determine the malfunctining part. 5. In measuring the functins f the individual cmpnent f the device, the DSO lcates where an expected signal is incrrect r absent.

6 6. The DSO can als measure cmpnents minr variatins in peratins and alert the technician f the need fr fine-tuning r replacement. 7. T prevent errneus replacement f parts, the DSO als helps technicians identify the parts that are still wrking.