2 _CRT Display Design_A_

CRT Display Design
© Display Laboratories Inc.
Session 2
Video System

Featured Seminars
 Introduction to CRT Displays
 Video and Tube Biasing
 Deflection and High Voltage
 Micro-Control and Waveform Generators
 Special Topics and Miscellaneous Circuits

CRT Display
Video Amplifier
Assembly

Intro to the Video System.
 Block diagrams
 General description
 Input signals and timings
 CRT gun characteristics
 Video amplifiers
 Bias and Blanking amplifiers
 Focus and Horizontal Static Convergence
 Other Considerations

Block Diagram
Power Supply
Cathode Ray Tube
Vertical & Horizontal
Deflection Amplifiers
Video Amplifier
& Blanking
R
G
B
H+V
Sync.
Sepr.
Focus and
Convergence

Block Diagram (Video System)
Power Supply
Cathode Ray Gun
Bias & Focus Amplifiers
Video Amplifier
R
G
B
G2
Focus
R G B
Heater
G1
Back Porch
Clamp and
Blanking

Block Diagram (Input Signals)
 RGB Analog Video
 Separate Sync.
 Composite Sync.
 DVI Digital Video
Video Amplifier
R
G
B
Deflection Amplifier
H
V
Video Amplifier
R
G
B
H+V Sync. Separator

General Description
 The video system controls the operation of the gun
 Provides interface of video source to the CRT gun
 Controls intensity and color balance of the image
 Performs gain adjustment of drive level
 Blanking amplifier provides retrace blanking
 Bias amplifiers establish operating point & spot kill
 Focus amplifier controls spot shape and size
 Electrostatic Horizontal Convergence (Static)

Video System Components
 CRT Gun
 Gun Structure
 Triode Section
 Lens Section
 Cathode Drive
 Spot Size
 Cutoff
 Drive requirements
 Operating Point
 Color Balance
 Black Level Balance
 Video Amplifiers
 Preamplifier
 Output Amplifier
 Black level Amplifier
 Back Porch Clamp
 Other Services
 Retrace Blanking
 Astigmatism Correction
 Sync. Separation
 Input Selection
 On Screen Display

Future Seminars
 Deflection and High Voltage
 Micro-Control and Waveform Generators
 Special Topics and Miscellaneous Circuits

Video System
 Block diagram
 Preamp
 Input select
 Termination
 Contrast
 Sync Separator

Video System
 Sync tips
 Black level
 Back Porch Clamp
 Output Amp
 Cathode voltage swing
 T-rise/T-fall
 Cathode capacitance

Video System
 Beam current
 CRT Bias
 Cutoff
 Brightness
 Black level
 Arc suppression

Video System
 Color Tracking
 White balance
 Preset Temp
 Variable Temp
 White to Black color tint
 White uniformity
 Circuit considerations

Input Signals
Typical sync on green input signal.

Composite Sync. Types
 Serrations
 Half line
 Interlace
 Doubles
 Equalizing pulses
 Missing sync. pulses

Input Connectors
 BNC – RGB analog video
 DB-15 – RGB analog video
 DVI – Digital CRT video
 Timing and resolutions •

Sync. Types
 Sync. on Video/Green
 Separate Sync.
 Composite 1 wire
 Separate 2 wire

Input Signals and Specs.
 Display Industry (VESA)
 Input Signal Specification (VESA)
 Input Signal Definition (VESA)
 Video Signals and Sync Types
 DVI Digital Video Interface
 GTF General Timing Format
 P&D Plug and Display
 DMT Default Monitor Timings

VESA Monitor Timing Specifications Version
1.0 Rev. 0.8

Standards and Guidelines Summary

Sync Signals during Vertical
 Double H Sync during Vertical
 Vertical “Or” with Horizontal
 Vertical “XOR” with Horizontal
 Proper Composite Sync.
 Separate H and V sync.

D V I Digital Video Interface
 Shown is a connector by JAE Electronics for the Dual
protocol video interface.
 Both Digital and Analog may exist simultaneously on this
connector.
 The Digital is made of up to two 3bit low voltage differential
channels with about 180MPixel/Sec each. They share a
common Clock.
 DDC Data and Clock, Vertical Sync and +5v are also
supplied.
 RGB and Horizontal Sync are provided over Coaxial cable.
 DVI should open a new range of display improvements.

Video Rates, Dot Clock, Bandwidth
 Dot Frequency & Bandwidth
 Dot Frequency & Rise Time
 Specifying CRT Drivers
 Rise Time of Cascaded Stages

Dot Frequency
 Dot Rate is a property of the Signal.
 Bandwidth is a property of the Amplifier.
 Dot Time = 1/Dot Clock Frequency.
 Dot Clock Frequency = (Active H Pixels)x
(Active V Pixels)x( Vertical Refresh Rate)x(t total/t active)
 If the video signal contains alternating white and black pixels the
resulting square wave will be ½ the Pixel Rate.
 The video amp should reproduce this adequately.

Dot Frequency & Bandwidth
 90% Rule; The high frequency dot amplitude should not be
less than 90% of the low frequency dot amplitude.
 The response of an amplifier is generally down to 90% at
½ the –3db frequency.
 Amp 3db BW = ½ to 1 times the Maximum Dot Clock
frequency.
 The designer may pick within this range depending on the
overall performance requirements of the monitor.

Dot Frequency & Rise Time
 The dot period = 1/Dot Frequency.
 The rise time of the signal at the cathode should be less
than 1/3 the dot period.
 Designers should specify CRT drive requirements in terms
of rise and fall time at 50 Vp-p.

Bandwidth Requirements
 Determine timing to be supported
 Determine trace time (Active Raster)
 Compute Dot rate and Pixel period
 Rise time should be 1/3 of Dot period
 Determine Large Signal Bandwidth
 Design or Choose Amp. components

Rise Time of Cascaded Stages
 It has been said that the whole is the sum of it’s parts. Or at least
the square root of the sum of it’s squares.
 For a generator, amp, load, and probe t rise and t fall is given by;
T meas = [T gen2 + T amp2 + T scope2]½
 For the amplifier the times are;
T amp = [T meas2 - T gen2 - T scope2]½
 4 ~= [72
- 42
- 42
]½
 4 ~= [49 - 16 - 16]½

Video Amplifier Sections
 Transistor Amplifier General Theory
 Pre-Amplifiers
 Output Amplifiers
 Open ‘Loop’ Amplifiers
 Closed ‘Loop’ Amplifiers
 Black Level
 On Screen Display

Video Channel Block Diagram
CRT Gun
Black Level Amp
Output AmpPre Amp
Input select
and Termination
Bias and Focus
On Screen
Display

Pre-Amplifier System
 The pre-amp has become much more than a simple
variable gain wideband amplifier.
 Modern Chips include;
 RGB three channel integration
 Common Gain ‘CONTRAST’
 Separate ‘DRIVE’

Voltage or I2
C buss control
 Black level clamp DC restoration
 OSD input selection or integrated OSD
 Blanking input
 Brightness insertion into gain path

Variable Gain Test Circuit
 Using a Spice model of the
proposed gain stage the transfer
function can be plotted.
 In addition to plotting the gain /
control voltage, the effects of
R4/R5 can be studied.
 R4/R5 will adjust the range of linear
control of the V2 input voltage.
 R2/R1 determines the max/min
variable gain range.
 For Drive control a 2 to 1 (6db)
range is sufficient for color
balancing.
 General Contrast range of 10:1
(20db) or more my be needed.

Contrast and Drive Gain Stages
 This is a simple cascode Contrast
and Drive gain stage.
 Input video is AC coupled to an
internal bias supply.
 Then buffered by an emitter
follower driving a cascode
differential transistor pair that has
maximum gain range.
 Then into a second differential pair
with a 2:1 voltage range.
 Buffered by a Darlington emitter
follower with an active current
mirror load that is used to
reestablish DC bias across R13.
 Note: that the maximum AC gain
of this stage is;
(R10+R11)/R9 = 2.
 Low AC gain and wide band!

Black Level Clamp
 Offset voltage
between pins 18
and 19 produces
a + or – current at
pin 12.
 This current
charges or
discharges the
bias cap that
reduces the
offset.
 The DC level of
the pre-amplified
signal is sampled
for the clamp time
and set to the
desired level.

Preamp Output Buffer
 Negative feedback amp
with a single voltage gain
inversion stage Q22.
 Input is buffered with an
emitter follower.
 Non-inverting summing
junction Q20 with
Darlington connection Q21
to inversion stage Q22.
 Output is buffered with an
emitter follower and an
active load.
 Gain set at R21/R18 = 16.

O S D Mixer
 The OSD On Screen Display
signal needs to be inserted
into the video during active
trace time.
 This is generally done over
the normal video. That is
covering or masking the
normal video. A mux or mixer
is used to switch from one
source to the other.
 Care must be used as to not
inject unwanted switching
noise or blanking artifacts
into the image.

O S D Mixer
 Some times the video must be blanked to provide the
desired LOOK.

Blank Insertion
 Black level clamping
occurs during blanking.
 The output of the pre-
amplifier can be switched
to a ‘blacker than black’
level during blank time to
drive the output amplifier
to a ‘Blank’ level.
 The Black level clamp
should clamp to the
preamp and not the input
to the output amplifier.

High End Peaking
 When G2 is moved blacker the slowest channel of video will
disappear first. When G2 is further reduced only the over
peaked channel appears.
 The most accurate way to set
high frequency peaking is to
(after color balance) set G2
so that the white block is
about 1/4 as bright.
 Contrast should be set for 2/3
of maximum.
 Adjust the high frequency capacitors on the video board for
the same shade of color in the high frequency block.
 This diagram shows what happens when G2 is set to cover
up most of the video.

Cathode Connection
G2
Video Output
Amplifier
Black Level
Amplifier
G1
R
Single point ground
Carbon Comp. Resistors

Output Amplifiers
 Open Loop
 Class A
 Class A with Class B Load buffer
 Closed Loop
 Class A with Class B Load buffer
 Class A with charge pump
 Class A with Active Load
 Dual Class B

General Theory
 Speed Limitations
 Flash-Over susceptibility
 ESD protection
 Negative Feedback
 Open Loop
 Closed Loop

Open “Loop” Amplifiers
Current Feedback
Made Easy

General Description
 Amplifiers that are not corrected with voltage
feedback
 The Cascode Amplifier
 Negative “Feedback”
 Voltage verses Current Feedback
 Simple Wide Bandwidth Topology

The Cascode Amplifier
 The Cascode amp is two
amps in one.
 Input is buffered by an
emitter follower Q1.
 Output is a grounded base
configuration Q2.
 This combines high input
impedance and wide
bandwidth characteristics.
 Passive speed up can be
used in the collector.
 Emitter “Peeking” will help
offset some of the
capacitive load effects.

Cascode Amplifier Current Feedback
 The current in R3 is from the
emitter of Q1.
 This current is the sum of the
base current and the collector
current.
 The collector current is almost
exactly the same as the output
voltage. V=Ic*(R2||Cload).
 If Cload is small then Ic and
Vout are similar.
 And so is V-R3.
 The base of Q1 and its emitter
act like - & + inputs to a
differential amp.
 Hence V-R3 is “Negative
Feedback”

Cascode Amplifier Current Feedback
 This type of Class A linear Amp
is amongst the fastest and most
stable topology available.
 It’s main draw back is the load
current required to charge Cload
at high slew rates.
 The fastest amps SONY has
used are in the DDM 20”x20”
monitors. Tr, Tf < 1.5nS
 These were specially developed
by Tektronix for the 1GHz
Oscilloscopes in the late 1980’s.
 The PCB and collector cathode
connection use strip line layout.

 Transistor Q2 is operated in the
grounded base configuration, which
does not have a Ccb feedback effect.
 The collector base capacitance appears
from collector to ground and does not
have a multiplication of 1+Vgain.
 Transistor Q1 is also operated as an
emitter follower that reduces the Ccb
effect. Node 9 is a 12 volts supply.
 Node 2 is at about 11.3 volts and
appears much like a voltage supply.
 There is very little A.C. signal at node 2.
 The input signal is at node 7, 3 and 4.
 The Ccb effect of an emitter follower
appears as input capacitance
⇒Why R5? We just learned that a
grounded base amplifier is fast.
Why slow down the amplifier with
base resistance?

 There are two functions of a base resistor
for Q2.
 This type of amplifier is being pushed to
have more gain at high frequencies by the
peeking capacitor C1 and inductor L1. At
the same time the hFE
of Q2 is dropping
because of fT
. The two effects collide and
cause instability and possible oscillation.
Resistor R5 roles off the amplifier gain at
a point higher than its operating range.
 The second reason for base resistance in
the output stage of a video amplifier
transistor is for CRT tube arc protection.
Five to ten ohms of base resistance
reduces the base current under arc
conditions.
 Generally try to keep R4 in the range of
1/5 to 1/10 of R3. Once again C1
(peeking capacitor) pushes the gain and
phase upward while capacitance and fT
causes the gain and phase to drop. If the
peeking capacitor is allowed to push the
gain up uncontrolled there is likely to be
 Transistor Q2 will have a longer
life with base resistance.
⇒Why R4? It is obvious that R2
and Cload are compensated for by
R3 and C1. Why limit the effect of
C1 with R4?
It is hard to see
300mhz oscillation
in a video amplifier
using a 100mhz
oscilloscope

Cascode with Emitter Followers
 This circuit buffers the capacitive load from the
collector of Q2.
 The buffer only improves the speed if Cload is
larger than that added by Q3 + Q4!
 In a B/W monitor there is a direct connection from
video amplifier to CRT, cathode Cload may be
too small to justify the buffer stage.
 Color monitors have additional circuitry added
between video amp and cathode. The Cload is
large and emitter flower buffers are common.
 Remember;
1.)      Minimize the load on Q2 collector.
2.)      Ic of Q3 and Q4 are due only to Cload
charging current.
3.)      Q3 and Q4 heat sinks are connected to
supply and ground and do not add capacitance.
4.)      Cload charging current is not limited by R2.
Current from R2 is multiplied by hFE
of Q3.
5.)      Cload charging current does not pass
through Q2 thus minimizing beta limiting.

Closed “Loop” Amplifiers
Voltage Feedback

General Description
 Amplifiers that are corrected with voltage feedback
 Evolution
 Complementary Class B
 Voltage verses Current Feedback
 Gain Bandwidth
 Frequency Compensation

Cascode with Charge Pump
 The Large signal
bandwidth is limited by
R1’s ability to charge the
capacitive load.
 By adding Q5 and driving
It on through C1 Q5 can
increase the slew rate of
the rise time at D1.
 This increases the
performance without
adding to DC power
consumption
 Gain = R1/R4

Feedback with Active Load
 In an effort to reduce parts
and power dissipation a
dual stage
complementarily design
evolved.
 It had fewer parts but was
unstable without negative
feedback.
 It also lacked a natural
operating bias point.
 R2 is needed to “Close “
the loop.
 With R1 and R9 a DC
operating point could be
established.

Feedback with Active Load
 Q2 is biased with R4,5,&6
to form a current source.
 C1 can drive Q2 to act as
an active load when Q1 is
driven off.
 Vin is essentially a current
input node.
 Vin is really an Iin summing
junction.
 Frequency compensation
networks are used to
control the response of this
type of amplifier.

Closed ‘Loop” Amplifiers
 Now is a good time for a
network analyzer.
 If you have one it is easy
to determine the response
of the basic output amp
under load conditions.
 Simply hook up input and
output and plot frequency
response.
 Then compute or look up
values for the RC’s and
get a cup of coffee.

Closed ‘Loop” Amplifiers
 If your lab is not so well equipped
or you don’t have the time to
figure how to use the thing you
can use the
 “cut and try” method.
 First input a test signal. Square
waves are best.
 With only Rin look at the
response at the cathode.
 If it is under peaked, and it will be,
start by adding a small pF
trimmer 3-30 pF at C1. And
adjust.
 The leading edge of the
waveform should rise. Bring it up
to level with the body of the
waveform.

Frequency Response
 Now it will fall back for a short time.
 Next you need a RC at R2,C2. Use the same kind of variable cap as
above but add a small value resistor in series say 10 ohms.
 Adjust. This should start to bring the droop up to flat and over peak
the leading edge. Back off C1.
 If you were very lucky this will do it.
 For the rest of us we look to find if the new effective peaking budges
or droops.
 If it holds for a short time and then droops before it returns. increase
the resistance.
 If not decrease it.
 In bad cases a second RC may be needed to give further mid-band
peaking.
 An RC may be placed across the Amp if the response is over peaked
at mid band.

 Two types of video peaking are used to get the best video response. High
band peaking is used to set the response of the first pixel or a single pixel.
The mid-band peaking sets the response of pixels 2 through about 7.
 Between the preamp and the
video output amplifier are the
video peaking components.
 At low frequency CA, CB and RB
do not effect the circuit.
 The drive for low frequency signals, is sent by RA.
 At a high frequency determined by CB, the amount of drive is set by RA
and RB in parallel.
 At a very high frequency CB increases the drive even more.
 We have experience that the value of CB will be the same for all video
power amplifiers with the same date code. Amplifiers made at a different
time may require a different value.
Peaking

High End Peaking
 There are three video preamplifiers and
three power amplifiers each with it's own
frequency response at the very high end.
 There is no way to get three perfectly
matched preamps and three matched
power amplifiers.
 On the video board there are trimmer
capacitors to set the very high end
frequency response for each channel.
 If the back ground is set for black then the
variations is high end frequency response
is hard to see. In the black ground is set
very black then the differences in
frequency response is very noticeable.

High End Peaking
 The video generator produces a large
white block and a block of every other pixel
on next to each other.
 The vertical line on the right would show
the frequency of the large white block.
The vertical line on the right shows the
frequency of the every other pixel region.
 If the color balance is done correctly then
the white block will look white. If the high
frequency peaking is not adjusted correctly
then there will be a color shift in the high
frequency part if the test pattern.
 If G2 is adjusted so only the tips of video
can bee seen then the differences in video
will be easy seen.

High End Peaking
 When G2 is moved blacker the slowest
channel of video will disappear first. When
G2 is further reduced only the over peaked
channel appears.
 The most accurate way to set high
frequency peaking is to (after color
balance) set G2 so that the white block is
about 1/4 as bright.
 Contrast should be set for 2/3 of
maximum.
 Adjust the high frequency capacitors on
the video board for the same shade of
color in the high frequency block.
 This diagram shows what happens when
G2 is set to cover up most of the video.

The Electron Gun
 The uni-potential gun can be broken down into
two sections, the triode and the lens section.
 The triode is the electron emitting and shaping
section formed by the electron source (cathode),
the control grid (G1) and the accelerator (G2).
 The lens of the uni-potential design has three
elements G3, G4 and G5.
 Cathode Life
 Flash-Over protection

The Triode
 A 6.3 or 12-volt filament heats the cathode.
Operating at an elevated temperature causes
electrons to "boil off" the surface of the cathode
and form a cloud.
 The electron-emitting surface of the cathode is
often impregnated with tungsten and barium to
lower the work function of the surface.
 The effective area of the cathode is determined
by the aperture size of G1 (first control grid).

The Triode
 The video signal is typically applied to the
cathode along with video blanking. This permits
the G1 and G2 elements to be biased relative to
the cathode for uniform operation from CRT to
CRT.
 The physical relationship of G1, G2 and the
Cathode, influence the lower beam angle, center
focus voltage, and the spot size for a given gun
design.

The Triode
 Electrons carry a negative charge. They are
attracted to positive elements within the tube like
G2 and the faceplate (1st and 2nd anodes) and
are repelled by the relative negative charge of
G1.
 The more negative G1 appears relative to the
cathode, the greater the reduction in the flow of
electrons.

The Triode
 Beam current is attracted through a small hole in
G1, called the aperture, by a positive voltage
applied to grid two (G2, the first accelerator or
anode).
 The bias voltage applied to G2 is set in
conjunction with G1 and Cathode to establish
“cut-off” of electron flow, at black levels in the
picture.
 The bias voltage between the cathode (+) and G1
(-) has the largest influence on spot size after
physical gun design.

Triode Cross Section (G1 ref.)
Cathode
Heater
G1
G2
0v 600-900v
130v70v
70v
100v
100 v
30v
110v

Determining Operating Point
 Select spot size (Line/in.) & Intensity
 Determine Ik from the charts
 Determine Vk/G1 bias point from chart
 Determine Drive requirements from chart
 Determine G2 range for Vk/G1 from chart
 Using signal ref (Green Default)
 Determine R/B .vs. G cutoff limits
 Black level amps cover these values

Cathode Current / Drive Voltage
 Drive curves are
shown on this chart.
 Note that the three
lines do not match
exactly.
 This must be taken
into account when
setting black level and
white balance.
 Each gun will have a
different black (cutoff
voltage) level and
video gain setting.
400uA
40 volts drive

Bias Chart Ik .vs. Vk @ G1
 This chart is for an older
Trinitron tube.
 It shows three different
lines for Ekco voltages.
 The more “remote” the
cutoff (to the right) the
more drive voltage is
required to attain a Ik
current.

Adjustable G2 for Fixed G1
 This is for an older Trinitron
Tube.
 It shows the range of G2
voltages for Ekco voltage.
 All tubes made of this gun
type should have G2
voltages within this range.
 The chassis must be able to
generate voltages that more
than cover this range.

The Lens
 Electrostatic lenses are used to focus the electron
bean onto the phosphor screen in the front of the
tube.
 The combination of G3, G4, and G5 form the
lenses. G3 and G5 are connected to the fixed
high voltage of the anode (originally called the
2nd anode).
 The voltage on G4 is varied to control the focal
distance. Static focus voltage and any dynamic
voltage, if required, are applied to G4 through the
base connector.

The Lens
 High-resolution displays require dynamic focus to
maintain pixel quality into the corners of the CRT.
This is even more critical on flat profile CRTs.
 Electron guns can be optimized for dynamic focus
or flat focus applications. A flat focus gun will
provide a compromise of focus quality from center
to edge and is appropriate for many low to
medium resolution applications.
 Displays requiring higher resolution such as those
in desktop publishing, graphics terminals,
document processing, and medical imaging,
require dynamic focus.

The Lens
 The function of each of these gun elements and
their interaction is critical to the overall
performance of the CRT.
 From cathode to the face of the CRT, it is the
relationship between each of these individual
elements that determines the final appearance of
the un-deflected spot in the center of the tube
face.

The Electron Gun (Color PIL)
 Example of Hitachi Elliptical Aperture, Dynamic
focus (A-EADF) Electron Gun

Hitachi Elliptical Aperture, Dynamic
 At the heart of Hitachi's high performance
monitors is the EADF electron gun which ensures
the sharp focus, high definition, distortion free
image.
 The elliptical aperture lenses produce maximum
focusing control while minimizing distortion effects
due to centerline offset.

Hitachi Elliptical Aperture, Dynamic
 Hitachi's dynamic focus capability means that
even FST screens have consistently sharp focus,
right into the corners where the beam path length
is substantially greater than at the center.
 An electro-static Quadra-pole lens which makes
constant adjustments to the cross sectional shape
of each beam ensures that the landing spot is
precisely circular whatever the deflection of the
beam or the position on the screen surface.

Focus Voltage and Modulation
 The type of gun determines static focus voltage
requirements.
 Uni – potential
 Bi – potential
 High Bi – potential
 Dual – potential
 FBT provides the large static DC for gun
 Focus Amplifier drives AC waveform

Beam Shape & Focus
 The electron beam leaving an electron gun will
normally be circular in shape. If the gun were at
the radius of curvature of the faceplate then the
spot at the faceplate would be round.
 The problem is that most CRTs are short, the gun
is very close to the faceplate.
 This causes the electron beam to strike the
phosphor at a non right angle.
 The corners will have an elliptical shaped spot-
landing pattern.

Beam Shape & Focus
 A Dynamic Quadra-pole Lens found in some
electron gun enables the beam to be made
elliptical as it leaves the gun.
 Naturally the Quadra-pole lens will distort the
beam in a direction that is at a right angle to the
normal elliptical effect.
 The addition of the two elliptical effects will result
in a round but larger spot.
 The beam’s spot shape, and therefore picture
sharpness, remain the same all over the screen.

Beam Shape & Focus
 Dynamic Focus voltages may be applied to the
gun of the CRT to optimize the spot size.
 A complex 3- D waveform is often needed on very
flat and/or large CRT’s.
 Several hundreds of volts of drive are used to
effect the beam at this point in the gun.
 The voltage may be different for the four corners
of the tube.

Quadrapole Focus Lens
 As Gun design evolves new
and more elaborate methods
will be devised to correct
spot performance.
 This is one example in the
evolution that shows the
effect of electrostatic field
shaping on beam shape.
 One set of focus electrodes
are held at a static voltage
while another set adds a
dynamic waveform.

Focus Chart
 Focus control voltages may
not be symmetrical in all
directions.
 Focus is dependent on the
“throw” distance from the gun
to the inside of the face plate.
 The outside surface we now
view may be flat but inside it is
still curved.
 Check the chart for your tube.

Thermionic Emission
 Thermionic emission is to electrons what
evaporation is to the water molecules in a hot cup
of coffee.
 It is a process by which some of the electrons
inside a piece of metal can 'boil away', leaving the
surface of the metal into the surrounding space.

Thermionic Emission
 Inside a metal the electrons are not stationary,
but are constantly moving, with an average speed
that is controlled by the temperature of the metal.
 It is important to realize that this is only the
average speed of the electrons, that some of
them will have speeds that are significantly larger
than this.
 It is these higher speeds, and therefore higher
energy electrons, which have enough energy to
escape from the metal.

Thermionic Emission
 Since the speed of the electrons increases with
temperature, the number of electrons with
sufficient energy to escape also increases with
temperature, in fact exponentially so.
 At room temperature (300 K) the number is very
small, but if the wire is heated to 1000K the
number of electrons escaping is dramatically
increased.

Heater & Cathode Condition
 Heater voltage affects the life
of the display tube as shown
in Fig 1.
 DO NOT operate heater
beyond the proper voltage.
 This voltage should be
regulated to a range of 95%
to 102% of nominal spec.
 The heater bias voltage with
respect to the cathode
should be +0v to –200v.
 Power save state must
completely turn off the heater
supply.
The Heater and/or G1
must NEVER be biased
more positive than the
Cathode.

CRT Life Expectancy
 "What is the life expectancy of a CRT"?
 There is no ‘Single’ answer to that question.
 This information will provide some reasonably accurate
guidelines so that you may form your own opinion taking
into account assumptions about variables that apply to your
application.
 Many factors determine the useful life of a CRT.
 The two major contributing factors are cathode emission
and phosphor aging, and are the only two that will be
addressed here.
 It should be noted that references to phosphor aging do not
include localized burns such as logos, stationary data and
statistics.

Cathode Emission
 The two most common types of
cathodes in use today are the
oxide cathode and the dispenser
cathode.
 This chart shows the approximate
decrease in emission under very
heavy loading conditions for both
types in an accelerated life test.
 As you can see, for a given beam
current, the oxide cathode will be
at 50% of its initial emission value
after approximately 3,500 hours of
operation.
 Under the same loading
conditions, a dispenser cathode
will have lost only 7% of its initial
emission value after 7,500 hours.

Phosphor Emission
 All phosphors age with use, each
phosphor having its own aging
characteristics.
 Phosphor aging manifests itself in
two ways: a reduction in luminance
and a visible discoloration of the
screen.
 An accepted theorem used to
characterize phosphor aging states
that, other factors remaining
constant, aging of a phosphor is a
function of the accumulated charge
deposited on it.
 This is known as coulomb aging.
The chart to the right shows
approximately the long-term aging
characteristics of P4, P104 and P45
phosphors.

Re-calibration
 In addition to the effect of phosphor aging there will also be a small
reduction in luminance caused by cathode degradation.
 The loss of luminance caused by both phosphor aging and cathode
degradation can be recovered by re-calibration of the monitor.
 Monitors may be designed with sufficient reserves in the drive circuitry
to allow re-calibration several times.
 This will eventually affect spot size as the cathode current must
increase to overcome the luminance loss due to phosphor aging.
 Calibration to the original luminance can be repeated until one of the
following limits are reached:
1. The reserves in the drive electronics have been depleted.
2. The spot size has become objectionable.
3. The luminance uniformity has become unacceptable.

Contents
 G2 Bias Amplifier
 Vertical Focus Amplifier
 Blanking Amplifier
 Horizontal Static Convergence
 Video Reference
 Bypassing Gun Supplies

Bias Amplifiers
 The CRT has two grids that determine the
operating point of the triode section.
 Once the Cathode / G1 voltage is set, G2 needs to
be adjusted to establish ‘Cutoff”
 G2 is controlled by the micro-controller through a
high voltage amplifier
 G2 must be properly filtered to maintain luminance
uniformity

G2 Bias Amplifier
 This figure shows a typical G2
amplifier
 The grounded base configuration
provides sufficient flash-over
protection.
 Due to the high supply voltage
required and flash-over susceptibility
proper resistors and layout are
required.
 Often series resistors are used in the
collector to meet the ‘SOA’ save
operating area.
 Due to the high impedance involved
Collector leakage/temperature must
be considered as well as input
leakage of the error amplifier.

Focus Modulator
 Dynamic focus
 Vertical
 Horizontal
 Mixer
 Flyback transformer

Vertical Focus Amplifier
 200 to 400v P-P are used to focus the
vertical.
 A high voltage bias amp like for G2 may
be used.
 The amp is required to have a
bandwidth that will reproduce the
vertical focus waveform.
 This is typically a parabolic shape and
needs about 3 times the fundamental to
produce the tips.
 The capacitive loading and slew rate
determines the collector resistance
needed.
 Flash-over and layout must be
considered.

DC Bias for V focus amp
 When Focus Modulation is AC coupled
to the CRT the absolute DC bias point of
this amp is not critical.
 Simple bias the amp for DC out of 400 –
600 v by setting ratio of R1 and R2.
 If Waveform applied to R3 is AC
coupled then we can find R2 after we
pick R1.
 R1 must provide enough current to get
the slew rate needed to charge Cload
@ the collector of Q1.
 VR1 = Ic*R1 = 500
VR2 = Ie*R2 = 12 - 0.7
Ic = Ie
R1/R2 = 500/(12 - 0.7)
R2 = R1*11.3/500

DC Bias for V focus amp
 The AC gain is R1/R3
 If the CRT requires 300vp-p for
Vfocus and the waveform
generator output is 3vp-p then;
R1/R3 = 300/3
R3 = R1/100
 There will be some thermal drift
to the amplifier but it will be small
enough to be ignored.
 A special group of transistors
have been designed for this high
voltage low current application.
 Horizontal modulation is
sometimes done with this kind of
amp. But the H freq. is limited by
Ic and Cload.

Horizontal Component
 The horizontal parabola is mixed
with the vertical component.
 This combination is level shifted
in the Flyback focus assembly
and applied to the CRT.
 Due to the high voltages present
attention to materials used and
mechanical clearances must be
observed.
 The horizontal waveform is often
2 times the voltage of the vertical
and comes from the horizontal
section of the chassis.
 Generation and modulation of
the horizontal waveform will be
covered in the section on
Deflection.

Retrace Blanking
 Retract blanking is used to hide from view the scanning beam
during times when it is traveling from the end of one scan line
to the start of the next or from the bottom of the screen to the
top.
 During these times the Video signal should be inactive.
 However this is not always true!
 New proposals are being offered to use this ‘dead’ time to
communicate information between the HOST and the
Display.
 Some of the proposals include White and Black Reference
levels, Border information as well as remote display control
commands.
 Retrace blanking is needed to mask these signals from view.

Separate Blanking Amplifier
 Some Video amplifier
configurations require an
additional amplifier to
completely blank the image.
 The inserted signal must be at
least as large as the maximum
video to insure a blank screen.
 The rise and fall times must be
close to that of the video rate.
Otherwise the edge of the
image may look soft or twinkle.
 The source of the blanking
signal can also cause jitter on
the edges of the raster being
blanked.
 Newer designs blank the video
amplifiers.

Output Amp Blanking
 Blanking may be inserted into the video before the output amp.
 By clamping the input of the output amp the output is forced as positive (Black)
as possible.
 Care must be taken to insure that the amp can handle this offset and recover
instantly.

Preamp Blanking
 Another option is to switch
the preamp to supply a
Black or Blank level output
during the Blanking interval
 This had the added
advantage of using this as a
‘pedestal’ to diode clamp
the DC Cutoff voltage for
each gun.
 Black and white color
tracking is enhanced by
passing both through the
Drive and Contrast sections
of the preamp.

Horizontal Static Convergence
 H Stat amplifier to be adjusted by the micro-processor.
H stat, G2 and Vfocus
amplifiers have similar
topology.

Video Bias Reference
 The CRT cathode, G1 and G2
control the flow if electrons to
the screen.
 The voltages between these
elements determine the flow.
 Variations on any one of these
will be seen.
 G1 and G2 may be ‘static’
voltages but they must share a
common reference with the
video amp.
 In this example of a simple
video amp. The +80 v supply
can have bad ripple and noise
that will never be seen because
of the common mode
bypassing of G1 and G2.

Black Level Cathode Connection
G2
Video Output
Amplifier
Black Level
Amplifier
G1
R
Single point ground
Carbon Comp. Resistors

Black Level Amp
 The black level amp will
need an output range of
Vcutoff +- 20% (~80v to
~120v).
 The actual Black level at
the cathodes will be this
voltage – Blank insertion
voltage.
 This amp will need to be
DC stable but adjustable.
 To accommodate the three
guns Ekco’s.
 An amp much like G2’s but
lower voltage.

Flash-Over Protection
DANGER
30,000 volts
Do Not Touch!

Flash-Over Protection
 Flash-Over Current
 Soft-flash guns and tubes
 Limiting resistors
 Aqua-dag return path
 P C B Layout
 Spark Gap Tubes
 Protection Diodes
 Surge limiting Resistors
 Spark gap sockets

Soft Flash-Over Gun
Structures
 Most Tube manufacturers today use
guns that incorporate ceramic limiting
resistors between gun elements and
the socket.
 This impedance greatly reduces the
surge currents in the G2, Focus and
HCV Circuits.
 Limiting the flashover in gun elements
that are operated at on near the
Anode voltage is critical.
 Clean assembly techniques reduce
the contaminates loose in the tube.
 It is recommended that the CRT
NEVER be shipped, stored, or used
with the gun down.
 Loose phosphor and other
contamination can lodge between gun
elements.

P C B Layout
 One my clients had a monitor that lost
vertical sync after a hard tube arc.
 Their repair group replaced U2, the
vertical sync buffer on the video
board.
 Their engineers had added resistors
and diodes to the sync lines with
minor effect.
 Adding MOVs and small capacitors
did not help. Nor did adding diodes
on both sides of the input resistor.
 Here is the schematic, as traced from
the printed circuit board.
 “A picture is worth a thousand words”.

Output Amplifier Protection
 The most common and effective Flash-Over protection devices are Clamp Diodes
and a special Spark Gap.
 Most low cost high speed switching signal diodes have the speed and can handle
the surge current with limiting by R1 of 33 to 100 ohms.
 One of the best and lowest cost Spark Gap is made by MMC in Japan
 It is made like a metal film resistor except that the trimming laser cuts a precise
gap completely around the middle of the ceramic body.
 The lead and path lengths to supplies and Ark return MUST be SHORT!

Desired arc current flow is from PCB ground below the CRT connector
through the shield and then through the braids and/or metal chassis to
the CRT DAG and HVPS.
Desired Arc Current
Return Paths
Braid
DAG
Straps
DAG
Coating
Anode
HV
PSChassis
Arc Protection System Design
Goal: Keep High Currents Away From Video
Board Electronics

Arc Protection and the
Recommended Application Circuit
 Good arc protection is required for reliable operation
 All products should be tested using a bench top tester similar to the figure
below. With the Output Amp installed in the neck board for the testing,
Apply the test voltage of 25 kV, it passes if no failures after 25 discharges
to each channel.
 Check the data sheet for recommended values for each device.
High Voltage
Power Supply
Variable
Transformer
1 2
S
AC
Input
R
C Output to
Unit Under Test
50 Meg Ohms
2000 pF
dc
High Voltage Source for Arc Testing

The Application Circuit
Device R1 (Ohms) L (nH) R2 (1/2W, Ohms)
LM2409 110 0.82 100
LM2407 91 0.56 33
LM2405 91 0.22 33
LM2415 91 0.39 33
LM2403 100 0.12 33
LM2413* ? ? ?
LM2402 75 0.05 33
LM2412* ? ? ?

 Inductor L1 reduces the voltage stress on the outputs of the device during
the initial High frequency ringing of the arc.
 Resistor R1 reduces voltage stress and limits short circuit current that flows
from the device while the spark gap is still active after the initial burst of the
arc.
 Diodes D1 and D2 reduce voltage stress on the outputs by clamping the
the voltage to the Vcc supply and ground respectively.
 Resistor R2 limits the current into the protection diodes and also limits short
circuit current.
 Capacitor C3 minimizes the voltage rise at high frequencies at the cathode
of clamp diode D1.
Arc Protection and the
Recommended Application Circuit

RLC Network Transient Response
• Case 1- Overdamped (R/2L)2
> 1/LC
• Case 2 - Critically Damped (R/2L)2
= 1/LC
• Case 3 - Underdamped (R/2L)2
< 1/LC

R=100, 200, 300, 400
100
300
400
L=0.22 uH
C=10 pf

C=8pf, 10pf, 12pf
R=200 Ohms
L=0.22 uH
8pf
12pf

L=0.12, 0.22, 0.32
R=200
Ohms
L=0.22 uH
0.12
0.32

Determining the Output Network
 Use a RL network as shown below to dial in the network.
 For a given L Value Adjust R for best/desired response
 Start with L’C resonant frequency, f=1/(2πSQRT(LC)), at about the
-3dB BW of the Device
 L’ includes trace inductance on the PCB
 After determining a good network, replace the R with a fixed value and
retest.

High Voltage Arcs in CRT
Monitors

RFI and EMI
 Shielding
 Filtering
 Minimizing

Shielding
 Shield around strong emitters
 Thickness and material
 Holes < 1/20 wavelength
 1Ghz =~ 0.5”
 Bond seams < 0.5”
 Al or Cu for Electrical fields
 High Perm for Magnetic fields
 Signal returns inside coaxial shields
 Ground both ends to chassis
 Ferrite cores around cables
 Perrite cores in connector shells

Filtering
 Ferrite beads for decoupling and loss
 Filter caps
 Perrite* beads for signal leads and surge suppression
 Filter G2, it couples to Rk and Gk at tube socket
 SHORT leads on all bypass caps!
*Green Tree Technology 1 612-473-3700

Minimizing EMI
 Generate Less!
 Use slowest rise times as possible
 Short Signal path lengths
 Close return paths
 Keep it in the BOX
 Use shields and ground planes
 Common mode filters around cables

Color
 Colors are rays of light, i.e. electromagnetic waves
with wavelengths between 380 nm and 780 nm.
 We perceive them with our eyes and our brain
translates them into what we call "colors". In other
words: colors are products of our brain.
 This means that one person may perceive colors
slightly differently from another.

Color
 To display colors, monitors use what is called
"additive color mixing", using red, green and blue
light. If we mix red, green and blue light together,
we get white light.
 When white is required on the screen, three
electron guns hit the red, green and blue dots, or
different shapes, of phosphor that coat the inside
of the screen, which in turn glow together and
produce white light.

Color Metrics
 The idea of tri-receptor
vision was worked out far
before the physical
mechanism of retinal
pigments was understood.
 A common diagram for
describing human color
perception was developed
by the International
Commission on Illumination
(CIE).
 The CIE diagram is an
attempt to precisely
quantify the tri-receptor
nature of human vision.

Color Balance
 Color Temperature is controlled by the relative level
of intensity of Red, Green and Blue light generated.
 These in turn are controlled by the relative voltage
of the three cathodes verses G1.
 Each Cathode has its own unique characteristics.
 The specification sheets give data that has been
gathered from many tubes and represents the limits
for that type of tube.
 The Video section must adjust to accommodate the
entire range of values.

White Point
 A popular way of defining a color
is by color temperature.
 Max Planck established a scale
of color from heated material.
 When a black body is heated to
a high temperature it begins to
emit light.
 The higher the temperature the
brighter and bluer the color.
 The Planchian Locus of Color
temperatures are plotted on this
CIE chart.

Color Tracking
 White temperature
 Three Drive (gain) curves
 Black level and color temperature
 Three Cutoff voltages
 Hue
 Saturation

Ik verses Ek
80 70 60 50 4085Ek Volts Eg1 = 0
Ik
uA
100
500
200
20
10
50
Ik
uA
1
5
2
0.2
0.1
0.5

Color Tracking
 For accurate Color at both
highlight and lowlight points
both the cutoff and drive
must be set.
 If the Color temperature is to
be changed all 6 of these
values must be changed.
 As seen previously all three
guns behave differently.
 The start and end range for
each Cathode will depend on
the desired Temperature.
10v 20v 40v 80v

Black Color and Ekco
 Each gun has a different Ekco.
 The three guns share a common G1.
 Ekco must be set on each cathode.
 The ratio of highest to lowest is given in the data sheet but
generally runs about 20%.

Black Color and Ekco
 The designer could set a range of as small as 25% and then adjust the
most remote Cathode for cutoff at the highest voltage.
 This would cause the cathodes tp be at random settings from tube to
tube.
 As we shown that the spot size depends more on Ekco.
 I like to set the most visible color (green) to the design Ekco and then
set the other colors.
 This causes a greater range for the Ek amps of +- 20%Ek.

DC Setting Ekco
 The simplest DC setter is to clip
the blackest part of the video
against a diode to the Ek bias
amp.
 The AC coupled Video must be
returned to the Ek supply to keep
the DC up against the Diode.
 There is one problem with this
approach. If the video black/blank
level is not the same, in the case
of Sync on Green the black color
will shift to Green in that case.
 The preamp must insert true blank
level.

Other Considerations
Circuit components they won’t
show you in the
‘SCHEMATIC’
And Other Tricks

Topics
 P C B Effects
 Stray Capacitance
 Transmission Lines
 Oscilloscope Probes
 Handy Tips

P C B E f f e c t s
 As video speeds get into the RF region the
printed circuit board becomes a major part
of the design.
 PCB traces add Capacitance, Inductance,
Delay and possible Reflections to signals.
 For the formulas below we define a PCB as
folows:
 Trace width W and thickness T and PCB
thickness is H.
 The dielectric coefficient ( d ) of the
insulating material.
2 < d <6.
 d ≈ FR4 fiberglass = 4.7
d ≈ G-10 fiberglass = 5.0 to 5.3
 T ≈ 0.0015” for 1 oz. Cu or
T ≈ 0.0030” for 2 oz. Cu
 H ≈ 0.062” or 0.031”

P C B E f f e c t s
( )






+
+
=
TW
H
Ln
d
Co
8.0
98.5
14.167.0






+
=
TW
H
LnLo
8.0
98.5
67.0475.0017.1 += dtpd
Co
Cl
increase += 1
The above example is for a trace without capacitive loading. If the far
end of the trace is connected to a transistor with capacitance then the
delay is larger.
Cl= load cap. Co= cap. of trace
nS/foot
Delay
It is of interest that the propagation delay of the line is dependent only
on the dielectric constant and is not a function of the width or spacing.
1.3ns/foot
nH/inch
Trace inductance
At just above the video rate inductors and even transformers are
routinely built using PCB traces!
Typically 9 to 10nH per inch
pF/inch
Trace Capacitance
Printed circuit trace capacitance runs around 1.67pF per inch. It is very
typical to find 1 to 3pF on most nodes.
1.67pF/inch

P C B E f f e c t s
ohms
Strip line Impedance
Impedance of a strip line. If a 75Ω video
needs to move across a PCB with out
reflections then a strip line should be used.
75 Ω ≈ 0.062” board, one ounce Cu, 0.100
trace width
50 Ω ≈ 0.062” board, one ounce Cu, 0.049
trace width
Delay with Loading
If a trace having a delay of 1.77ns/ft and a
length of 2 inches having a capacitance of
3.5pF, is terminated in a load of 2pF the
resulting delay is 2.21ns/ft.
Co
Cl
dtpd +•+= 167.0475.0017.1
ftnsftnstpd /21.2
5.3
2
1/77.1 =+•=






++
=
TW
H
Ln
d
Zo
8.0
98.5
41.1
87

Transmission Lines
 A transmission line (either coaxial or
as a strip line on a PCB) is a way of
moving a signal over a distance with
minimum distortion.
 The maximum distance that a signal
should be moved without a
transmission line is the rise time of
the signal divided by twice the
propagation delay of the line.
 A video signal with a rise time of
1.7ns should use a transmission line
to travel over 6 inches.
 Video boards rarely have any traces
6 inches long. With that logic it looks
like we will never need to make a
strip line.
td
tr
L
2
max ≤

Transmission Lines
 Wrong! The video DAC in the
computer is at least three feet away
from the video board in the monitor.
 The transmission line should not
end at the back of the monitor.
 It should not end at the edge of the
video board.
 It must continue with 75 ohms
impedance across the PCB right up
to the video pre-amp to the
termination resistor.
 In high speed strip lines, the shape
of the trace is important.
 The impedance of a sharp corners
causes as much as 7.5% reflection.
It is a good idea to
have smooth, rounded
lines and constant line
widths.

P C B Capacitance
 It was found that there is a large
amount of capacitance between power
resistor R1 and the ground plane.
 The ground is removed under the
resistor to reduce the stray
capacitance.
Resistor R1 is the output of the video amplifier.
It has also been found that during tube ark conditions the
voltage on R1 gets very high.
A spark jumps through the side of R1 into the ground plane
cracking the resistor’s insulation.
The spark builds up a path of carbon leaving a conductive
path from R1 to ground.
If voltages can exceed 1,000 volts then it is a good idea to
clear out the ground plane under carbon film and metal film
resistors!

P C B Capacitance
 In this printed circuit board example the
dark color is the topside ground plane. The
light color trace is the bottom side traces.
 There is a signal that passes through R2,
C2, D3, D4 and R4.
 The ground plane has been cleared out
around the traces in an attempt to reduce
trace capacitance.
 Care must be taken to keep the ground
plane intact.
The second example shows a ground
planes with a crosshatch pattern in the
sensitive area.
In this example the trace to ground
capacitance is reduced to 50%.
0.007” traces set at 0.020” spacing that
should have resulted in a 58% fill.
The fill factor is typically 50%. Any other
amounts of fill can be used (25% or 10%).

Oscilloscope Probes
 A typical oscilloscope probe may
have 10MΩ resistance in parallel
with 12pF capacitance.
 That is 79Ω at 200MHz!
 The tube cathode capacitance is
another 12pF.
 Stray capacitance found in the tube
socket, arc protection circuitry,
black level clamp, D.C. bias amp,
wires and traces could add another
10pF.
 The nominal loading of the video
amplifier is 22pF.
 With the probe the loading
increases to 34pF.
 The amplifier will not operate the
same!
With a 30 volt peak to peak
signal there may be as much as
¼ amp of current in the
oscilloscope probe depending on
the rise time of the signal!
Choose your probe wisely.

Oscilloscope Probes
 The 12pf is the problem so we must find a
way of looking at the video signal without
adding capacitance.
 There are available several low
capacitance probes.
 They come in two types; active and
passive.
 The active probe I use has a built in
amplifier. The problem is that it can not
handle high voltage signals.
 All probes have a problem handling high
voltage signals.
 When you mix high voltage and high
frequency the probe’s A.C. plus D.C.
voltage can easily be exceeded.
 The passive probe is 1pf and 5000 ohms.
 It works very will if the amplifier can handle
the 5000 ohms load.
This probe can not handle
more than 70 volts D.C. it
typically needs to be A.C.
coupled to look directly at
the cathode of a CRT!

Home Made Probes
 Passive low capacitance probes can
easy be built.
 The oscilloscope must be switched to
50 ohms internal terminating
impedance.
 Use a short length of good 50Ω coax.
 On the end of the coax solder a 4950 or
450 ohm resistor. Any divider ratio can
be used.
 Watch the power rating on the resistor.
 It is not uncommon to find 100 volts of
bias on the cathode of a CRT tube.
 You may want to A.C. couple the probe
by adding a D.C. blocking capacitor in
series with the 4950Ω resistor.
 Keep all lead lengths s h o r t !!!!
Keep the ground lead
length very short!
Use non-inductive
resistors!

Home Made Probes
 Many resistors will have a small
amount of stray capacitance that forms
a peaking capacitor.
 You may need to add the tiniest
amount of peaking capacitor. Amateur
Radio Operators commonly build high
voltage sub-pf capacitors by twisting
together insulated wire.
 The first resistor in the photo has
added three twists of insulated wire
held together with clear heat shrink
tubing. The wire can be cut shorter to
reduce the capacitance. Twisting the
wire tighter will increase the
capacitance.
 The second resistor has the two wires
run in parallel inside clear tubing.
The wires can be pulled back to
reduce the capacitance.

Home Made Probes ÷ 100
 To get a low voltage version of what
is happening at the output of a video
amplifier solder a 100:1 divider onto
the amplifier.
 Remember the leads must be very
short.
 To measure the rise and fall time of
the amplifier an A.C. only divider can
be made with two capacitors.
 If the D.C. level is important then
resistors must be added.
 The 10MΩ/12pf is the oscilloscope
probe.
 C1,C2,R1 and R2 are soldered onto
the amplifier.

Handy Tips Sniff the Video (no direct connection)
 Another option for viewing the output of a video amplifier without adding
substantial loading;
Wrap the cathode wire around the scope probe.
 The wire to probe capacitance is only a fraction of a pico-Farad.
 The results is a divide by many thousand.
 The gain of the scope will have to be turned up all the way.
 The probe is A.C. coupled!
 No low-frequency effects can be seen.
This method is only good for
looking at edges! Trise / Tfall

Handy Tips Proper Probe Use
 When using high frequency probes their connection to the circuit is
critical to accurate measurements
 First remove the protective plastic “CLIP” cover.
 Make or buy small loops of bare wire (or spring wire) that hold the
probe in place and ground the probe.
 The ground ring on the probe makes an excellent connection point.
BEWARE the probe tip ground may short the
point being measured. BYE - BUY Amplifier!!!

Handy Tips Ground Lead
 First throw away the alligator clip ground wire!
 A 10X oscilloscope probe that adds 10pF and
10MΩ which might not cause trouble.
 A six inch ground lead has about 700nH of
inductance.
 You have just inserted a LC resonant circuit
into your amplifier (or should I say your
oscillator)!
 If the point you are looking at has fast edges it
will cause the LC to ring.
 The oscilloscope will see ring that is not there.
Below is the frequency and phase response of
a 10X probe with a 6” ground lead.
 The actual ring may be different than this
spice model. The ground lead is more
complicated than a simple inductor and the
capacitance is also complex. Move the path
of the ground lead an inch and the look of the
ring will change.

Handy Tips Ground Lead
 Here is the frequency and phase response of a 10X probe with a 6” ground lead.
Would you use a probe with this frequency
response? Have you seen this ring before?

Handy Tips Without a Probe
 Do not trust your oscilloscope! The important
thing is to trust your eyes.
 That is how the end customer will judge the
product.
 The output of a video amplifier that drives a
CRT tube has a high level for black and a
low level for white.
 Normally the grid bias is set so that the high
level makes black. 10% video ringing is
almost impossible to see.
 To see the details of the video set the bias
very black; so just the tips of video will be
beyond the black level.
 Turn the room lights are turned out to see
the imperfections in the video level.
 In the above example; ringing becomes
apparent.
 Notice the slope to the video.
 It shows a low frequency response problem.

2 _CRT Display Design_A_

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