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G8MNY > TECH 21.09.08 11:03l 322 Lines 16706 Bytes #999 (0) @ WW
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Subj: Oscilloscopes
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Sent: 080920/2144Z @:GB7CIP.#32.GBR.EU #:3311 [Caterham] $:3311_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Nov 07)
(8 Bit ASCII Graphics use code page 437 or 850)
This bul tells you about the basics of Oscilloscopes.
COORDINATES
The display uses 2 coordinates Horizontal X & Vertical Y.
Y The 3D Depth Z coordinate can't be displayed on the 2
/³\ _ Z axis system, but it is sometimes used as a brilliance
³ /| input. (e.g. grid 1 voltage for TV work).
³ /
³/ The X coordinate is usually used for time display with
ÅÄÄÄÄÄÄ>X an internal time ramp generator.
DISPLAY SYSTEM
Until recently this has always been a Cathode Ray Tube, usually electrostaticly
deflected.
Deflection The beam of electrons from the gun are
EHT Plates attracted or repelled by the pair of X
Phosphor _|_ X Y & Y plates, that have a differential
coated ³ "~Ä-.________________ deflection voltage across them. The more
front ³ Glass []ÄÁÄ Electron³ð sensitive Y plates are nearest the gun.
screen ³ Bulb [] ÄÂÄ Gun ³ð The gun voltages are usually @ -1kV to
with ³ _.-Ä"~~~~~~~~~~~~~~~~ the final gun anode & deflection plates.
Graticule ~~~PDA X Y This means some of the front panel gun
controls may be @ -1kV!
The glass bulb normally uses a Post Deflection Acceleration system where Extra
Height Tension of a few +kV is applied to the screen end of the bulb & a spiral
high value resistance coated to the inside eventually reaches the gun anode.
The effect of this is to further accelerate the electron beam, but at the
direction already set by the plates, it greatly enhances the brilliance
(electron energy) without needing kilo volts of deflection voltage.
With magnetic defection types (for audio bandwidths) the plates are replaced by
external coils mounted at 90 degrees to their deflection axis. Larger forces on
the electron beam can be applied this way so PDA is not used & deflection
angles much greater (shorter tubes). However the coils are inductive & needs
drive voltage proportional to frequency & it is this that limits the usefulness
for wide bandwidth scope use, as the drive circuits become a very inefficient
constant current system. A TV CRT magnetic deflection has fixed scan
frequencies & that is a different matter.
Phosphors can be any colour, but green is the brightest to the eye & blue the
best for photography, so some are blue-green (e.g. P7). Phosphor persistence
times can be quite slow for scope phosphors to reduce flicker & for you to
follow a very slow trace.
There where more specialist CRTs that permit image storage, these are very
useful for very slow events as well as fast one offs. They use a 2nd gun to
spay re-energizing electrons that are attracted to the static charge left on
the screen by the 1st gun & keep the screen dimley lit for as long as required.
Modern LCD systems such as on PCs offer a more flexible system, but the A-D
generally used is very limited & the quantisation & pixilation of some of the
cheaper offerings is far inferior to a good CRT display.
CONTROLS
Graticule. Controls a light that ÚÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄ¿
eliminates the etched ³ ³ ³ ³ ³ Spot is 3.0
graticule (like graph ÃÄÄÄÄÅÄÄÄÄÅÄÄÄÄÅÄùÄÄ´ divisions
paper) engraved on ³ ³ ³ ³ ³ up & 3.4
the tube or just in ÃÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ´ divisions
front of it. ³ ³ ³ ³ ³ across.
ÃÄÄÄÄÅÄÄÄÄÅÄÄÄÄÅÄÄÄÄ´
³ ³ ³ ³ ³
ÀÄÄÄÄÁÄÄÄÄÁÄÄÄÄÁÄÄÄÄÙ
Brilliance. Alters the brightness by controlling the CRT's electron gun
current, too bright will burn the phosphor over time, especially if
left at as spot or line! Blanking signals used in the scope turn
the gun off!
ELECTRON GUN
Astig
ÚÄ Á ÄÄÄÄÄÄ¿³ÚÄ__Cathode
Electron beam |[<Heaters
À¿ Ä ÄÂÄÄÄÄÙ³ÀÄÄÄGrid
³ Focus ³
AnodeÀÄÄÄÄÄÄÄÄÄÄÙAnode
A bright spot will have faint rings around it due to the electron
wave length effect at the voltage used in the CRT.
³ ( ù ) ³
Focus. Controls the gun mid repelling tube electrode voltage. The effect
is to make a weak electrostatic lens that focuses the divergent
electron bean from the cathode to a spot on the screen.
³ Electrostatic
Screen³ÄÄÄÄÄ=====ððððð±±ð=ÄCathode
³ Lens
ú ù @
Small dim Brighter defocussed
spot spot blob
A defocussed blob often shows a picture of the cathode surface!
Astig. Astigmatism control is similar to focus, but applied to a pair of
(internal) plates in the focus tube wall, so it causes the spot to change from
a horizontal oval to vertical oval shape, enabling a really tight
small round spot to be achieved. ÜÜ ù Ý
Rotation. Controls current put on a coil around the tube that puts a small
(internal) twist on the electron beam, to rotate the whole display so the X &
Y axises are true to the rectangular faceplate. Round tubes you
just unclamp & rotate the tube! _ . -Ä
Ä Ä Ä_Ä.-Ä~Ä Ä Ä Ä
-Ä ~
Geometry. Controls an additional beam plate voltage used to distort the
(internal) display to make it (e.g. a carrier producing a rectangle) exactly
fit the graticule for perfect geometry. _
_ . -Ä ~ ³
ÞÛÛÛÛÛÛÛÛÛÛ ³ ³
ÛÛÛÛÛÛÛÛÛÛÝ ³ ³
ÞÛÛÛÛÛÛÛÛÛÛ or ³ _³
ÛÛÛÛÛÛÛÛÛÛÝ ³_ . -Ä ~
Y Shift. Controls the standing DC on the Y plates to set the vertical beam
position. Use with input grounded for display calibration. For +ve
only signals set the beam to the bottom graticule, for AC or ñ DC
use the middle. For 2 channels either superimpose (confusing) or
use a near top & bottom reference graticule.
ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄ¿
³ ³ ³ /\ ³ ³ ³ Ã-------´Y1
³ /\ ³+ ô ³ ôAC ÆÍÍÍÍÍÍ͵Y1Y2 ³ ³
³_³ ³__³DC ³ \/ ³ ³ ³ Ã-------´Y2
ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÙ
Internal balance & bias presets may affect the shift offset positon
as the gain is altered.
Y Gain. A wide range stepped input attenuator in front of the Y pre-
amplifier that through the Y defection amp drives the Y plates.
Volts per division is in 10 3 1 steps or 10 5 2 1 steps per decade.
__ __
__ __ ³ ³ ³ ³ good square
ÚÄÄ¿__ÚÄÄ¿ ÚÄÄ¿ ÚÄÄ¿ ³ ³ ³ ³ ³ ³ ³ ³ waves have
ÀÄÄÙ ³__³ ³ ³ invisable
³__³ verticals!
10V/div 5V/div 2V/Div 1V/Div
There is often an off calibration variable amplifier variable gain
control as well. A higher gain (e.g. pull for 10x) option switches
come at the loss of display bandwidth (e.g. from 20MHz - 8MHz @10x)
Y Input Selects input DC coupled or AC coupled that removes DC components
from Y input amplifier. Note there will be a DC limit (e.g. 300V)
An input grounded option may be used for shift calibration.
The BNC input is normally 1Mê//30pF (DC open circuit on AC mode),
a 10:1 scope probe is designed to use this as its calibrated input
load.
Y Select With scopes with more than one Y channel, you can select which one
of both are used. Sometimes the 2 can be added or subtracted.
.-. ___
/ \ / \ Subtracted _
Y1³ ³ Y2 ÚÙ À¿ ÁÄÄ¿_ÚÄÄÂÄÄÙ ÀÄÄ
AF Amp \ / Distorted \___/ Distortion after gain
input 100mV '-' AF O/P 50V 4ê adjusts cancel signal
X Input This may be an option when the timebase is off. Normally fixed
gain in the timebase external trigger input, or using the 2nd Y
channel amp with all its gain options. Bandwidth less than the
normal Y channel. ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ . ú '³
³ .ú ' ³
³ .ú'X Y plot ³
³ .' ³
³: ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
Lissajous figures can be obtained with an X in & different but
related frequencies.
_ _ _
(_X_X_) ì O 8
X = 3xY X = 2xY X = Y X = Y/2
X Shift This has the same function as the Y shifts but often used to move a
waveform to a convenient graticule for measurement.
X Gain x5 Often a fixed gain increase rather than variable. Higher gain may
reduce the X bandwidth & make the trace dimmer. Used to zoom in to
part of the waveform without upsetting the timebase.
X Timebase. This is a re-triggerable ramp oscillator that is used to scan the
spot across the screen.
___ ___Gun Blanking
|___| |___|
/³ /³
/ ³ / ³ Xamp input
ramp/ ³ / ³
___/ ³___/ ³___
hold off awaiting trigger
The time per division can be set over a large range of decade sub
steps 10 3 1 or 10 5 2 1.
Ú¿Ú¿Ú¿Ú¿Ú¿Ú¿Ú ÚÄ¿ ÚÄ¿ ÚÄ¿ Ú ÚÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿ Ú
ÀÙÀÙÀÙÀÙÀÙÀÙ ÀÄÙ ÀÄÙ ÀÄÙ ÀÄÄÄÙ ÀÄÄ ÀÄÄÄÄÄÄÄÄÙ
20mS/Div 10mS/Div 5mS/Div 2mS/Div
Off calibration speed variable usually provided to make waveforms
fit the display better & for % measurement.
During spot flyback the gun is turned off (blanked) to stop any
misleading traces.
X Trigger Selected from Y channels, external input or mains line. Trigger can
be DC or AC input & +/- edge trigger, Variable trigger Level or
Stability enables the exact height or slope to determine the
trigger point.
+ve slope AC
.Á. -DC Zero trigger
+DC trigger -/ \ /
+DC Zero trigger - ³ ³Ù ³
/ \ / -DC trigger
-ve slope '-'
AC trigger/
HF mode can help recover HF trigger signals better, as can triggers
filters for line & frame TV waveforms. Some scopes allow for
alternate channel triggering.
Chop/Alt With 2 channels being displayed on a singe beam CRT there are 2
ways to do this. Either chop between them at a high frequency
(e.g. @ 100kHz) to show low frequency waveforms, where gun blanking
is done to hide the chop mode edges..
_ _ Gun Blanking
____| |___| |____ signal
_____ _____
ch1 ³ ch2 ³ ch1 Chop Mode Astable
ÀÄÄÄÄÄÙ Switch matrix control voltage
Or for higher frequencies, use alternate & change channels every
timebase sweep. The timebase now triggers a bistable.
_____ _______
ch1 ³ ch2 ³ ch1 ³ Switch matrix
ÀÄÄÄÄÄÄÄÙ control voltage
/³ /³
/ ³ / ³ Xamp input
ramp/ ³ / ³
___/ ³___/ ³____
hold off awaiting trigger
The switching is done with balanced low impedance lines with a
diode matrix where only the signal current is switched.
Some scopes do the choice for you depending on timebase setting.
Delay Line. After the switching & before the Y display amp, fast scopes fit a
signal delay line say around 20nS (6M of 300ê twisted wire). This
gives the timebase time to be triggered before the event reaches
the display.
Timebase ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ Y delay ÄÄÄÄÄÄÄÄÄ¿ Exaggerated
delay _ ³ Ú¿_ ³ > < Ú¿_ ³ delay here!
> < ³ ³ ³ À¿_ ³ ³ À¿_ ³ But you are
_Ü_³ ³ Ü_³ À¿_ ³ ³ _Ü_³ À¿_ ³ able to see
³_³ ß ³ ß Video ~³_ ³ ³_³ ß Video ~³_³ before the
| ³_________________³ ³_________________³ trigger
Trigger With no Y delay With Y delay line point!
point
Dual timebase. For more advanced scopes a 2nd timebase running at a faster rate
can be used to select a small part of the waveform. Typical example
is to display one line from a TV frame waveform, e.g. a test line.
Bright up highlight mode indicates the waveform section that can be
expanded up. Extreme brightness is needed for this!
_ ___
ºººÝº|||... ³ À¿_ ³ ³ ³
ºººÝººººº³³³||||.. _Ü_³ À¿_ _Ü__³__³ ³
_²²²Û²²²²²²²²²²²²²²_² ³_³ ß ~³_³ ß ³_
TV Frame 20mS 2 highlighted lines 128uS
Main frame triggered timebase Line triggered 2nd timebase
Calibrate. This is usually a square wave of fixed height, sometimes at mains
frequency, that is used for checking Y gain & timebase, &
especially for calibrating scope probes. (see Scope & DMM
Calibrator bul)
__
__³ ³__³ e.g. 500mV peak to peak @ 1kHz.
OVERALL SYSTEM
DC _______ _______
Y1ÄÂÄ\. ³Stepped³ ³ Y1 Ã>Trigger Graticule---Lamp
in À´ÃÁÄ´ Atten ÃÄ´preamp ³ Delay ______
AC ÀÄÄÄÄÄÄÄÙ ³x10 optÃÄÄÄÄ¿ line_ ³ Y ÃÄY+ /\/\/
Y1gain------------ÁÂÄÄÄÄÄÄÙ \__________³ ³_³output³Plates
Y1shift------------Ù ³ Balanced ³_³ ³ amp ÃÄY- \/\/\
DC _______ _______ ³ diode matrix ÀÄÄÄÄÄÄÙ
Y2ÄÂÄ\. ³Stepped³ ³ Y2 ÃÄÄÄÄÙ switch Rotate--Coil
in À´ÃÁÄ´ Atten ÃÄ´preamp Ã>Xamp | Brill---CRT G1
AC ÀÄÄÄÄÄÄÄÙ ³x10 optÃ>Trigger| Focus---CRT A2
Y2gain------------ÁÂÄÄÄÄÄÄÙ | Astig---CRT A3
Y2shift------------Ù ______³___ Z in--ÂÄÄÄÄÄÅ--CRT Anode
³ Chopper ³ ³ CRT ³--CRT Cathode
Y Mode----------------------´Astable/ö2ÃÄÄÂÄÄÄÄÄ´ PSU ³--CRT Heaters
Y1¿ÚY2 _______ ÀÄÄÄÄÄÄÂÄÄÄÙ ³ ÀÄÄÄÄÄÙ--CRT EHT
TrigÄÄÄÄ \____³Trigger³ ________ ³ Blank³ Y2 ______
in ³ Gate ³ ³TimebaseÃÄÙ ³ ³ __³ X ÃÄX+ /³_/³
+/- ----------³ ÃÄ´ ÃÄÄÄÄÄÄÄÄÙ / ³output³Plates _
DC/AC---------³ ³ ³ ÃÄÄÄÄÄÄÄÄÄÄÙ³ ³ amp ÃÄX- \³ \³
Level---------ÁÂÄÄÄÄÄÄÙ ÀÄÂÄÄÂÄÄÄÙ Ramp ³ ÀÄÂÄÄÂÄÙ
TV/HF----------Ù ³ ³ ³ ³ ³
Time----------------------Ù ³ X in ÄÄÙ ³ ³
Variable---------------------Ù ³ ³
Xshift--------------------------------------------Ù ³
Xgain------------------------------------------------Ù
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73 De G8MNY @ GB7CIP
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