CATHODE RAY TUBE
The CRT is a vacuum tube that emits a focused electron beam from the back of the tube to the front of the tube. The front of the tube is coated with phosphors that glow when they are struck by the electron beam. An image is created by moving the electron beam back and forth across the back of the screen. The beam moves in a pattern from left to right, top to bottom and then it repeats. Each time the beam makes a pass across the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. The intensity of the beam is modulated thus causing the screen phosphors to glow with different intensities or to even not glow at all. The desired images to be displayed are actually retraced between 30 to 70 times each second. This keeps the images continually refreshed in the glowing screen phosphors without a flicker being perceivable to the eye.
|The electron beam is generated from a filament and electrically charged cathode in the back neck of the CRT. The electron beam is first passed through a control grid. The control grid modulates the intensity of the electron beam. The higher the intensity the brighter the phosphor dot it strikes will glow. Next the beam passes through an accelerating electrode, this will speed up the electron beam. Then the beam passes through a focusing anode. This will focus or tighten the stream of electrons. All of these elements comprise the electron gun structure housed in the neck of the CRT.|
he structure on the neck of the CRT is the yoke. The yoke contains four electromagnets placed around the neck of the CRT in 90 degree increments. By varying the voltage of these four electromagnets, the electron beam can be deflected or bent to reach any location on the phosphor coated screen.
|T A final stage of acceleration is achieved with the high voltage anode. The familiar suction cup wire that attaches to the side of the CRT is connected to this anode. This anode is often a metalized surface on the inside of the picture tube. Many thousands of volts are applied to the anode to pull the electrons towards the phosphor coated screen. Phosphors can be formulated to emit many colors. Additional circuitry in the computer can create numbers, letters, and other symbols by using the control grid to turn the electron beam on and off, while simultaneously using the electromagnets to deflect the beam to the desired locations on the screen.|
In an industry in which development is so rapid, it is somewhat surprising that the technology behind monitors and televisions is a 100 years old.
|The cathode-ray tube, or CRT, was developed by Ferdinand Braun, a German scientist, in 1897 but
was not used in the first television sets until the late 1940s. Although the
CRTs found in modern monitors have undergone modifications to improve
picture quality, they still follow the same basic principles.
CRT is essentially vacuum tube. It begins with a slim neck and tapers outward until it forms a large base. The base is the viewing portion and is coated on the inside with a matrix of thousands of tiny phosphor dots. Phosphors are chemicals which emit light when excited by a stream of electrons: different phosphors emit different colored light.
A Each dot consists of three blobs of colored phosphor: one red, one green, one blue. These groups of three phosphors make up what is known as a single pixel.
In the neck of the CRT is the electron gun, which is composed of a cathode, heat source and focusing elements. Color monitors have three separate guns, one for each phosphor color. Combinations of different intensities of red green and blue phosphors can create the illusion of millions of colors. This is called additive color mixing and is the basis for all color CRT displays.
Images are created when electrons, fired from the electron gun, converge to strike their respective phosphor blobs (pixel) and each is illuminated, to a greater or lesser extent. When this happens, light is emitted, in the color of the individual pixel. The gun radiates electrons when the heater is hot enough to liberate electrons (negatively charged) from the cathode, which are then narrowed into a tiny beam by the focus elements. The electrons are drawn toward the phosphor dots by a powerful, positively charged anode, located near the screen.
The phosphors in a group are so close together that the human eye perceives the combination as a single colored pixel. Before the electron beam strikes the phosphor dots, it travels thorough a perforated sheet located directly in front of the phosphor layer known as the "shadow mask". Its purpose is to shape the electron beam, forming a smaller, more rounded point that can strike individual phosphor dots cleanly and minimize overspill, where the electron beam illuminates more than one dot.
The beam is moved around the screen by magnetic fields generated through deflection coils. It starts in the top left corner (as viewed from the front) and flashes on and off as it moves across the row, or "raster".
When it hits the front of the screen, the electrons collide with the phosphors that correlate to the pixels of the image that is to be created on the screen. Once a pass has been completed, the electron beam moves down one raster and begins again. This process is repeated until an entire screen is drawn, at which point the beam returns to the top to start again.
The most important aspect of a monitor is that it should give a stable display at the chosen resolution and color palette. A screen that shimmers or flickers, can cause itchy or painful eyes, headaches and migraines. It is also important that the performance characteristics of a monitor be carefully matched with those of the graphics card driving it.
It is no good having an extremely high performance graphics accelerator, capable of ultra high resolutions at high flicker-free refresh rates, if the monitor cannot lock onto the signal.
A monitor's three key specifications are:
the maximum resolution it will display
at what refresh rate
whether this is in interlaced or non-interlaced mode
Resolution and refresh rate
Resolution is the number of pixels the graphics card is describing the desktop with, expressed as a horizontal by vertical figure. Standard VGA's resolution is 640 x 480 pixels. The most common SVGA's resolutions are 800 x 600 and 1024 x 768 pixels.
Refresh rate, or vertical frequency, is measured in Hertz (Hz) and represents the number of frames displayed on the screen per second. Too few, and the eye will notice the intervals in between and perceive a flickering display. The world-wide accepted refresh rate for a flicker-free display is 70 Hz and above, although standards bodies such as (Video Electronic Standards Association) VESA are pushing for higher rates of 75 Hz or 80 Hz.
A computer's graphics circuitry creates a signal based on the Windows desktop resolution and refresh rate. This signal is known as the horizontal scanning frequency, HSF, and is measured in KHz. Raising the resolution and/or refresh rate increases the HSF signal. A multi-scanning or "autoscan" monitor is capable of locking on to any signal which lies between a minimum and maximum HSF. If the signal falls out of the monitor's range, it will not be displayed.
An interlaced monitor is one in which the electron beam draws every other line, say one, three and five until the screen is full, then returns to the top to fill in the even blanks (say lines two, four, six and so on).
An interlaced monitor offering a 100 Hz refresh rate only refreshes any given line 50 times a second, giving an obvious shimmer. Non-interlaced (NI) is where every line is drawn before returning to the top for the next frame, resulting in a far steadier display. A non-interlaced monitor with a refresh rate of 70 Hz or over is necessary to be sure of a stable display.
Masks and dot pitch
The maximum resolution of a monitor is dependent on more than just its highest scanning frequencies. It is also limited by the physical distance between adjacent groups of phosphors, known as the dot pitch and is typically between 0.25 mm and 0.28 mm. The smaller the number, the finer and better resolved the detail. However, trying to supply too many pixels to a monitor without a sufficient dot pitch to cope causes very fine details, such as the writing beneath icons, to appear blurred.
There's more than one way to group three blobs of colored phosphor - indeed, there's no reason why they should even be circular blobs. A number of different schemes are currently in use, and care needs to be taken in comparing the dot pitch specification of the different types. With standard dot masks, the dot pitch is the center-to-center distance between two nearest-neighbor phosphor dots of the same color, which is measured along a diagonal. The horizontal distance between the dots is 0.866 times the dot pitch. For masks which use stripes rather than dots, the pitch equals the horizontal distance.
This means that the dot pitch on a standard dot-mask CRT should be multiplied by 0.866 before it is compared with the dot pitch of these other types of monitor.
The vast majority of computer monitors use circular blobs of phosphor and arrange them in triangular formation. These groups are known as "triads", and the arrangement is a dot trio design. The shadow mask is located directly in front of the phosphor layer - each perforation corresponding with phosphor dot trios - and assists in masking unnecessary electrons, avoiding overspill and resultant blurring of the final picture.
Because the distance between the source and the destination of the electron stream towards the middle of the screen is smaller than at the edges, the corresponding area of the shadow mask get hotter. To prevent it from distorting - and redirecting the electrons incorrectly - manufacturers typically construct it from Invar, an alloy with a very low coefficient of expansion. This is all very well, except that the shadow mask used to avoid overspill occupies a large percentage of the screen area. Where there are portions of mask, there's no phosphor to glow and less light means a duller image.
The brightness of an image matters most for full-motion video and with multimedia becoming an increasing important market consideration a number of improvements have been made to make dot-trio mask designs brighter.
Most approaches to minimizing glare involve filters that also affect brightness. The new schemes filter out the glare without affecting brightness as much.
Toshiba's Micro filter CRT places a separate filter over each phosphor dot and makes it possible to use a different color filter for each color dot. Filters over the red dots, for example, let red light shine through, but they also absorb other colors from ambient light shining on screen - colors that would otherwise reflect off as glare. The result is brighter, purer colors with less glare. Other companies are offering similar improvements. Panasonic's Crystal Vision CRTs use a technology called dye-encapsulated phosphor, which wraps each phosphor particle in its own filter and ViewSonic offers an equivalent capability as part of its new SuperClear screens.
In the 1960s, Sony developed an alternative tube technology known as Trinitron. It combined the three separate electron guns into one device: Sony refers to this as a Pan Focus gun. Most interesting of all, Trinitron tubes were made from sections of a cylinder, vertically flat and horizontally curved, as opposed to conventional tubes using sections of a sphere which are curved in both axes. Rather than grouping dots of red, green and blue phosphor in triads, Trinitron tubes lay their colored phosphors down in uninterrupted vertical stripes.
Consequently, rather than use a solid perforated sheet, Trinitron tubes use masks which separate the entire stripes instead of each dot - and Sony calls this the aperture grill. This replaces the shadow mask with a series of narrow alloy strips that run vertically across the inside of the tube. Rather than using conventional phosphor dot triplets, aperture grill-based tubes have phosphor lines with no horizontal breaks, and so rely on the accuracy of the electron beam to define the top and bottom edges of a pixel. Since less of the screen area is occupied by the mask and the phosphor is uninterrupted vertically, more of it can glow, resulting in a brighter, more vibrant display. With aperture grill monitors the equivalent measure to dot pitch is known as 'stripe pitch'.
Because aperture grill strips are very narrow, there's a possibility that they might move, due to expansion or vibration. In an attempt to eliminate this, horizontal damper wires are fitted to increase stability. This reduces the chances of aperture grill misalignment, which can cause vertical streaking and blurring. The down side is that because the damper wires obstruct the flow of electrons to the phosphors, they are just visible upon close inspection. Trinitron tubes below 17 in or so get away with one wire, while the larger model require two. A further down side is mechanical instability. A tap on the side of a Trinitron monitor can cause the image wobble noticeably for a moment. This is understandable given that the aperture grill's fine vertical wires are held steady in only one or two places, horizontally. Mitsubishi followed Sony's lead with the design of its similar Diamondtron tube.
Capitalizing on the advantages of both the shadow mask and aperture grill approaches, NEC has developed a hybrid mask type which uses a slot-mask design borrowed from a TV monitor technology originated in the late 1970s by RCA and Thorn. Virtually all non-Trinitron TV sets use elliptically-shaped phosphors grouped vertically and separated by a slotted mask.
In order to allow a greater amount of electrons through the shadow mask, the standard round perforations are replaced with vertically-aligned slots. The design of the trios is also different, and features rectilinear phosphors that are arranged to make best use of the increased electron throughput.
The slotted mask design is mechanically stable due to the crisscross of horizontal mask sections but exposes more phosphor than a conventional dot-trio design. The result is not quite as bright as with an aperture grill but much more stable and still brighter than dot-trio. It is unique to NEC, and the company capitalized on the design's improved stability in early 1996 when it fit the first ChromaClear monitors to come to market with speakers and microphones and claimed them to be 'the new multimedia standard'.
Enhanced Dot Pitch
Developed by Hitachi, the largest designer and manufacturer of CRTs in the world, EDP is the newest mask technology, coming to market in late 1997. This takes a slightly different approach, concentrating more on the phosphor implementation than the shadow mask or aperture grill.
On a typical shadow mask CRT, the phosphor trios are more or less arranged equilaterally, creating triangular groups that are distributed evenly across the inside surface of the tube. Hitachi has reduced the distance between the phosphor dots on the horizontal, creating a dot trio that's more akin to an isosceles triangle. To avoid leaving gaps between the trios, which might reduce the advantages of this arrangement, the dots themselves are elongated, so are oval rather than round.
The main advantage of the EDP design is most noticeable in the representation of fine vertical lines. In conventional CRTs, a line drawn from the top of the screen to the bottom will sometimes 'zigzag' from one dot trio to the next group below, and then back to the one below that. Bringing adjacent horizontal dots closer together reduces this and has an effect on the clarity of all images.
Simplified Block Diagram
Refer to the next block diagram when reading this description:
The Video Interface (A) is designed around a custom IC and will accept DC or AC coupled positive analog video signals. It can also be used with negative analog and 4 line TTL. This IC has a built in multiplier circuit for the master gain control and blanking functions. Resistors are used to protect the IC and to set the gain. The programmed gain is dependent on the input signal amplitude except with the TTL mode. Solder jumpers and component substations are used to program the Video Interface for the type of input signal to be received. The output of the IC drives the video amplifiers. This drive is a current where 0 mA is black and 5.5 mA is a saturated color.
The Video Amplifiers (B) are of the push pull type. They are built partly on thick films and partly on the video PCB. Spreading out the amplifier reduces the component heat and improves the life of the unit. The bandwidth is 25 MHZ with 40Vp-p output. The rise and fall times are 20nS.
The Beam Current Feedback (C) circuit directs most of the beam current of each amplifier to the beam current buffer. The only time this current is measured, by the auto bias circuit, is during the time of the three faint lines at the top of the screen and three lines thereafter. The CRT auto bias circuit is designed to adjust the video amplifier bias voltage such that the beam current of each of the three guns is set to a specific programmed value.
The Beam Current Buffer (D) converts the, high impedance low current, beam current signal into a low impedance voltage. This voltage is applied to the auto bias IC through a 200 ohm resistor. After the three lines of beam current are measured, the program pulse from the auto bias IC, produces a voltage drop across this 200 ohm resistor that equals the amplitude of the beam current voltage.
The CRT Auto Bias IC (E) is a combination of digital and analog circuitry. The digital part is a counter and control logic which steps the analog circuits through a sequence of sample and hold conditions. The analog part uses a transconductance amplifier to control the voltage on a 10uF capacitor (one per gun). This voltage is buffered and sent to the video amplifiers as the bias voltage. In monitors without CRT auto bias, this voltage is adjusted manually using a setup procedure to set the color balance. With CRT auto bias, the color balance is set during the end of each vertical blanking time.
The control sequence is:
1. The cycle starts with a sync pulse from the horizontal oscillator (15.7 KHz) or from the vertical sync delay (31 KHz operation).
2. The grid pulse on G1 causes cathode current which can be seen as the three faint white lines at the top of the screen. This cathode current is transmitted by the beam current feedback to the beam current buffer where it is converted to a voltage and applied to the CRT auto bias input pin. At this time the CRT auto bias IC outputs a reference voltage at its input pin which sets the voltage across the coupling capacitor. This coupling capacitor voltage is directly dependent on beam current.
3. After the grid pulse is over, the program pulse matches the voltage from the beam current buffer. If the voltage from the beam current buffer, during the grid pulse, is the same as the voltage from the program pulse, the bias is correct and no bias correction is produced.
The timing of the auto bias IC is synchronized to the vertical oscillator and the flyback pulses. For horizontal frequencies higher than 15.7 KHz a Vertical Sync Delay (F) is needed to position the grid pulse, generated 3 gray lines, at the top of the screen.
The aging of the picture tube (CRT) not only affects the balance of the cathode cutoff voltage, which is corrected by the auto bias circuit, but it also affects the gain of the CRT. The Auto Bright (G) circuit actively corrects for CRT gain changes by sensing any common bias voltage change, from the auto bias circuit, and adjusts the screen voltage to hold the average bias voltage constant. The lower adjustment on the flyback transformer which is the screen voltage, is used to set the auto bright voltage to the center of its range. Therefore, the auto bright circuits sets up a second control feedback loop to reduce picture variation due to CRT aging. The auto bright circuit is also used to turn off the beam current when the monitor power is turned off.
The CRT (H) for the 1493 and 2093 monitors are 90° deflection type color picture tubes. The 2793 CRT incorporates 110° deflection. These three picture tubes have integral implosion protection and a EHT of 25KV.
Blanking (I) is accomplished by setting the gain of the interface IC to zero during blank time. The Horizontal Blanking pulse is generated by amplifying the flyback pulse. The Vertical Blanking pulse is started by the vertical oscillator one shot and ended by the counter in the auto bias IC via the "bias out" pulse. The Master Gain control, located on the remote PCB, sets the gain of the video signal when blanking is not active.
The Beam Current Limiter circuit, which is designed to keep the FBT from overloading, will reduce the video gain if the maximum average beam current is exceeded.
The Sync Interface (J) can accept separate or composite sync. Two comparators are used to receive sync, one for vertical sync and the other for horizontal sync. Resistor dividers are used to protect the comparator IC from over voltage damage. For customers who do not require interlace, an additional vertical sync stabilization circuit is included. This circuit synchronizes the vertical sync to the horizontal cycle.
The Vertical Oscillator (K) generates the vertical free running frequency when no vertical sync is present. When sync is applied, the vertical oscillator synchronizes to the leading edge of the sync pulse.
The Vertical Control & Output (L) circuit consists of:
1. One shot.
2. Ramp generator.
3. Vertical drive.
4. Vertical output.
The sync pulse from the LA7851 triggers a one shot in the LA7838 which clamps the vertical ramp generation capacitor to 5V during the first half of vertical retrace. The ramp generation capacitor then charges via a constant current set by an external resistor. This resistor is connected to the V SIZE pot, located on the remote control board, for the vertical size adjustment. The vertical drive is a differential amplifier which compares the ramp voltage to the yoke return feedback current. The yoke feedback current and voltage circuits are used to set the vertical linearity. The vertical Output is a power driver, with thermal protection, which drives the vertical deflection yoke. It also has a special pump up circuit which doubles the output voltage during vertical retrace. This voltage doubler also increases the efficiency of the circuit since the high retrace voltage is not present across the power driver during the trace time.
The Horizontal Control (M) incorporates a variable sync delay and a phase locked loop to generate the horizontal timing. The H POS. adjustment, on the remote control board, sets the sync delay time which controls the picture position. The phase locked loop uses the flyback pulse to generate a sawtooth wave which is gated with the delayed sync pulse to control the horizontal oscillator.
The Horizontal Driver (N) supplies the high base current necessary to drive the horizontal output transistor (HOT) which has a beta as low as three. A transformer is used to step up the current from the driver circuit and also protects the horizontal output transistor from a continuous turned on state. A special clamp circuit is connected to the transformer which reduces the turnoff time of the horizontal output transistor for reduced power dissipation.
The Horizontal Output transistor (HOT) (O) is mounted to the rear frame which acts as a heat sink. The collector conducts the 900 volt primary flyback pulses which should not be measured unless the equipment is specifically designed to withstand this type of stress. A linear ramp current is produced in the horizontal yoke by the conduction of the horizontal output transistor (trace time). A fast current reversal (retrace time) is achieved by the high voltage pulse that follows the turn off of the horizontal output transistor. This pulse is due to the inductive action of the yoke and flyback transformer.
The main function of the Flyback Transformer (FBT) (P) is to generate a 25,000 volt (EHT) potential for the anode of the picture tube. This voltage times the beam current is the power that lights up the phosphor on the face of the picture tube. At 1.5mA beam current, for the 2793 monitor, the FBT is producing almost 38 watts of high voltage power. The FBT also sources the focus voltage, screen grid voltage, filament power, and has two more secondaries which are used for control functions. The FBT has a built in high voltage load resistor which stabilizes the EHT, for the low beam current condition. This resistor also discharges the EHT, when the monitor is turned off, which improves the safety of handling the monitor.
The Remote Control PCB (Q) houses the:
CONTROL DESCRIPTION CIRCUIT
1. H SIZE ----------- Horizontal raster size --------- Diode modulator
2. V SIZE ----------- Vertical raster size ------------- Vertical control
3. V RAS. POS. --- Vertical raster position ------- DC current to V. yoke
4. H POS ------------ Horizontal picture position -- H. sync delay
The Horizontal Size Control (R) circuit has four inputs:
# SIGNAL FUNCTION
1. Horizontal size ------------------------------ Horizontal size control
2. Beam current -------------------------------- Blooming control
3. Vertical linear ramp -----------------------
4. Vertical parabolic + V. linear ramp --- (#4)-(#3)=Vertical parabolic (Pincushion)
The horizontal size control circuit sums the four signals at one node plus the feedback from the diode modulator to drive a switching mode power driver. The output of the power driver is then connected to the diode modulator through an inductor to complete the control loop.
The Diode Modulator (S) is a series element of the horizontal tuned circuit. It forms a node between GND and the normal yoke return circuit. If this node is shorted to GND, the result is maximum horizontal size. A forward conducting diode that suddenly receives reverse current will stay in the conducting state until its internal charge is depleted. The reverse conduction time is dependent on the forward current. A diode, placed in series with the yoke, is then used to control the retrace pulse amplitude across the yoke. The horizontal size, therefore, is controlled by controlling the current to this diode via the horizontal size control circuit.
A Voltage Doubler (T) is used in the power supply for two reasons:
1. To improve the efficiency of the power supply.
2. To permit 120 volt and 230 volt operation. For the 230 volt operation the voltage doubler is replaced with a bridge rectifier.
The Switching Regulator (U) is synchronized to the horizontal pulse and drives a power MOSFET. Unlike most regulators that have a common GND, this power supply has a common V+ and current is supplied from V- to GND. The MOSFET is connected to V- and signal ground (GND) through a transformer which is used as an inductor for series switch mode regulation. An operational amplifier, voltage reference, comparator, and oscillator in the power supply controller IC are used to accomplished regulation by means of pulse width modulation.
The transformer has two taps on the main winding which are used to generate the +16 volt and +24 volt supplies. It also has a secondary which is referenced to V- and supplies the power supply. Since the power supply is generating its own power, a special start up circuit is built into the power supply controller IC that delays start up until the capacitor which supplies the IC is charged up enough to furnish the current to start the power supply. This capacitor is charged with current through a high value resistor from the raw dc supply. This self sustaining action is why the power supply chirps when an overload or underload occurs. Additional secondaries to drive the horizontal raster shift circuit and the video amplifiers are also included in the power transformer.
The Load (V) consists primarily of the horizontal flyback circuit. The power supply will not operate without the load since the voltage that sustains the power supply comes from a secondary in the power transformer and depends on some primary current to generate secondary current.
A +12V regulator (W) is used to supply current, to all the control circuits in the monitor, with the exception of the power supply. Many of the control circuits are decoupled from the +12 volt line with a resistor or diode to minimize noise from common current loops.
The Over Voltage Protect (X) circuit is built into the power supply and monitors the flyback transformer peak pulse voltage. This circuit will turn off the power supply and hold it off if the EHT exceeds its maximum rated value. Since excessive X-ray output occurs with excessive EHT, this circuit provides X-ray protection.
The Fault Detector (Y) senses beam current and temperature. This circuit will activate the power supply shutdown circuit if either the maximum temperature is sensed or if the beam current becomes large enough to threaten the FBT.
The Degaussing circuit (Z) is connected across the isolated AC line. A posistor is used to allow a large current to flow, in the degaussing coil, on power up. This current is then gradually reduced by the increased temperature of the positive temperature coefficient thermistor in the posistor. A relay is used to short the degaussing coil after the degaussing operation. This greatly reduces posistor residual current in the degaussing coil. When repairing a monitor, the degaussing coil should be unplugged, to avoid possible damage to the degaussing coil shorting relay.