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G8MNY  > TECHNI   24.11.21 09:20l 384 Lines 17543 Bytes #999 (0) @ WW
BID : 53951_GB7CIP
Subj: Oscilloscopes
Sent: 211124/0759Z @:GB7CIP.#32.GBR.EURO #:53951 [Caterham Surrey GBR]

By G8MNY                                 (Updated Jan 13)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This bull tells you about the basics of Oscilloscopes.

               The display uses 2 co-ordinates Horizontal X & Vertical Y.
  Y            The 3D Depth Z co-ordinate can't be displayed on the 2
 /│\ _ Z       axis system, but it is sometimes used as a brilliance
  │  /|        input. (e.g. grid 1 voltage for TV work).
  │ /
  │/           The X co-ordinate is usually used for time display with
  ┼──────>X    an internal time ramp generator (Timebase).

Until recently this has always been a Cathode Ray Tube, usually electrostaticly
                 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  │   "~─-.________________    deflection voltage across them. The more
 front   │ Glass    []─┴─ Electron│­  sensitive Y plates are nearest the gun.
 screen  │  Bulb   [] ─┬─   Gun   │­  The gun voltages are usually @ -1kV to
  with   │   _.-─"~~~~~~~~~~~~~~~~    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 often uses a Post Deflection Acceleration system where Extra
High Tension of several kV is applied to the screen end of the bulb & a spiral
high value resistance coated to the inside eventually reaches the lower voltage
gun anode. The effect of this is to further accelerate the electron beam, but
at the direction already set by the plates, this greatly enhances the
brilliance (electron energy) without needing kilo volts of deflection voltage.

With magnetic defection types (See below) 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 can also be much greater (shorter tubes). However as the
coils are inductive & need the 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 can be made quite efficient.

Phosphors can be any colour, but green is the brightest to the eye & blue the
best for photography, so blue-green (e.g. P7) colour is common. Phosphor
persistence times or afterglow can be quite slow for scope phosphors to reduce
flicker & for you to follow a very slow trace. The afterglow can be a different

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-energising electrons that are attracted to the static charge left on
the screen by the 1st gun & keep the screen trace dimly lit for as long as

Modern LCD systems such as on PCs offer a more flexible system, but the A-D
generally used is limited & the quantisation & pixilation of some of the
cheaper offerings are far inferior to a good old CRT display. But different
trace Colours, Storage features, & also Spectrum Analyser display are now
standard features.

Controls a light that    ┌────┬────┬────┬────┐
eliminates the etched    │    │    │    │    │ Spot is 3.0
graticule (like graph    ├────┼────┼────┼─¨──┤ divisions
paper) engraved on       │    │    │    │    │ up & 3.4
the tube or just in      ├┼┼┼┼┼┼┼┼┼┼┼┼┼┼┼┼┼┼┼┤ divisions
front of it. (avoid      │    │    │    │    │ across from
parallax error when      ├────┼────┼────┼────┤ the left
taking measurements)     │    │    │    │    │ hand corner.
Alters the brightness by controlling the CRT's electron gun current too bright
will burn the phosphor over time, especially if left as a bright spot or line!
Blanking signals are used in the scope turn the gun off!

   SCREEN                                  ELECTRON GUN
  /~~~~~~~\                         Astig
 /  /~~~\  \                         ┌─ ┴ ──────┐│┌─__Cathode
│  (  ¨  )  │      Electron beam                 |[<Heaters
2nd Spot 1st                         └┐ ─ ─┬────┘│└───Grid
ring    ring                          │   Focus  │

A bright spot will have faint rings around it due to the electron wave length
effect at the voltage used in the CRT.

Controls the gun mid repelling (focus) tube electrode voltage (see GUN diagram
above) The effect is to make a weak electrostatic lens that focuses the
divergent electron beam from the cathode to a small spot on the screen.

              │          Electrostatic
              │              Lens

A defocussed blob often shows a picture of the cathode surface!

             ˙          ¨           @
         Small dim   Brighter   defocused
            spot        spot        blob

Astig. (internal)
Astigmatism control is similar to focus, but applied to a pair of 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.
    ▄▄    ¨    Ţ

Rotation. (internal)
Controls current put on a coil around the tube that puts a small 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!
               _ . -─
  ─ ─ ─_─.─˙─~─ ─ ─ ─
  -─ ~
Geometry. (internal)
Controls an additional beam plate voltage used to correct the display to make
it exactly fit the graticule for perfect geometry, sometimes a pincushion
shape. (a carrier will produce a rectangle)
                    _ . -─ ~  │       │~──----──~│    ..--──--..
  Ů██████████      │          │       │          │   │          │
   ██████████Ţ     │          │   or  │          │   │          │
   Ů██████████     │         _│       │          │   │          │
    ██████████Ţ    │_ . -─ ~          │_.--──--._│    ~──----──~

Beam Find.
Only found on some scopes, it can be useful if the X or Y shifts are extreme.
It reduces the final X & Y amp gains & preset the brilliance, to let you see
where the spot has gone.

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 ˝ DC use the middle. For 2 channels either
superimpose (confusing) or use a near top & bottom reference graticule.
 ┌───────┐       ┌───────┐       ┌───────┐       ┌───────┐
 │       │       │ /\    │       │       │       ├-------┤Y1
 │  /\   │+      ├┤  │  ├┤AC     Ă═══════ÁY1Y2   │       │
 │_│  │__│DC     │    \/ │       │       │       ├-------┤Y2
 └───────┘       └───────┘       └───────┘       └───────┘
Internal balance & bias presets may affect the shift offset position as the
gain is altered. (A calibration round robin.)

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.
                                         __    __
                            __    __    │  │  │  │ good square
 ┌──┐__┌──┐   ┌──┐  ┌──┐   │  │  │  │   │  │  │  │ waves have
                 └──┘         │__│         │  │    invisible
                                           │__│    verticals!
  10V/div       5V/div        2V/Div      1V/Div

There is often an off calibration variable gain control as well. A higher gain
(e.g. pull for 10x) option switches come at the loss of display bandwidth (e.g.
20MHz reduced to 8MHz @ 10x)

Y Bandwidth can also non linear with small displays OK & large ones poor, due
to high voltage output amp slew rate limitations.

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
is used for shift calibration.

The BNC input is normally 1Mŕ//30pF (DC open circuit on AC mode), a 10:1 scope
probe is designed to use this as its calibrated input load. (See ref below)

Y Select.
Scopes with more than one Y channel, you can select which one to use or both.
Sometimes the 2 can be added or subtracted (ADD with an inverted channel).
       .-.                 ___
      /   \               /   \           Subtracted    _
   Y1│     │         Y2 ┌┘     └┐          ┐/└─┐_┌─┘\┌─┘ └─
 AF Amp     \   /   Distorted    \___/    Distortion after gain
 input 100mV '-'    AF Output 50V 4ŕ      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 quite a bit less than the normal Y channels.
  │          . ˙ '│
  │      .˙ '     │
  │   .˙'X Y plot │
  │ ,'            │
  │:              │
Lissajous figures can be obtained with an X in & different but related
   _ _ _        _ _            _ 
  (_X_X_)      (_X_)     \ to (_) to /         8

 X = 3x Y    X = 2x Y       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. Again a higher gain will
normally reduce the X bandwidth further & also make the trace proportionally
dimmer. It is used to zoom in to part of the waveform.

X Timebase.
This is a re-triggerable ramp oscillator that is used to scan the spot across
the screen.

 off__     ___     Gun Blanking
      |___|   |___|

         /│      /│
        / │     / │ Xamp input
   ramp/  │    /  │
   ___/   │___/   │___
 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.

 ┌┐┌┐┌┐┌┐┌┐┌┐┌  ┌─┐ ┌─┐ ┌─┐ ┌  ┌───┐   ┌───┐    ┌────────┐        ┌
  └┘└┘└┘└┘└┘└┘    └─┘ └─┘ └─┘      └───┘   └──           └────────┘
    20mS/Div       10mS/Div          5mS/Div           2mS/Div

Off calibration speed variable usually provided to make waveforms fit the
display better & for % measurement. A "hold off" control on some scopes lets
you vary the free run timebase frequency without altering sweep calibration.

During spot flyback the gun is turned off (blanked) to stop any misleading

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 control" enables
the exact height or slope to determine the trigger point.
                +ve slope AC
                      .┴.    -DC Zero trigger
        +DC trigger -/   \  /
 +DC Zero trigger - │     │┘    │
                   /       \   / -DC trigger
                  -ve slope '-'
                  AC trigger/

An "HF mode" can help recover HF trigger signals better, as can triggers
filters for LF, or TV line & frame waveforms. Some scopes allow for alternate
channel triggering. And some scopes even show the trigger channel on screen.

With 2 channels being displayed on a single 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 │ ch2 │ ch1  Chop Mode Astable
      └─────┘ Switch matrix control voltage

Or for higher Y frequencies say above 50kHz, use alternate & change channel
every timebase sweep. The timebase now triggers a bistable.
 _____         _______
 ch1  │  ch2  │  ch1  │ Y switch matrix
      └───────┘         control voltage
           /│      /│
          / │     / │ Xamp input
     ramp/  │    /  │
     ___/   │___/   │____
 hold off awaiting trigger

The switching is done with balanced low impedance lines with a diode matrix
where only the signal currents are switched. Simple scopes do this 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 (e.g. 6M of 100ŕ twisted wire). This gives the timebase
time to be triggered, before the event reaches the display, so you can see it!

 Timebase  ┌─────────────────┐   Y delay  ─────────┐   Exaggerated
 delay  _  │   ┌┐_           │   >   < ┌┐_         │   delay here!
   > < │   │   │  └┐_        │         │  └┐_      │   But you are
    _▄_│   │ ▄_│     └┐_     │   │  _▄_│     └┐_   │   able to see
 │_│ ▀     │ ▀  Video   ~│_  │   │_│ ▀  Video   ~│_│   before the
   |       │_________________│   │_________________│   trigger
 Trigger      With no Y delay      With Y delay line   point!

Dual timebase.
For more advanced scopes a 2nd timebase able to run 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!
                                       _                  ___
   ║║║Ţ║|||...                        │ └┐_           │  │   │
   ║║║Ţ║║║║║│││||||..              _▄_│    └┐_    _▄__│__│   │
  _▓▓▓█▓▓▓▓▓▓▓▓▓▓▓▓▓▓_▓         │_│ ▀         ~│_│ ▀          │_
     TV  Frame  20mS               2 highlighted lines  128uS
 Main frame triggered A timebase     Line triggered B timebase

Delay Control.
More advanced scopes have precision time delay that lets you delay the 2nd
time base start point, as a % of the first. See above.

This is usually a square wave of fixed voltage, at 1kHz or mains frequency). It
is used for checking Y gain & timebase, & especially for calibrating scope
probes. (see Ref. below)
  __│  │__│ e.g. 500mV peak to peak @ 1kHz.


   DC    _______   _______
Y1─┬─\. │Stepped│ │  Y1   ├>Trigger          Graticule---Lamp
in └┤├┴─┤ Atten ├─┤preamp │              Delay   ______
    AC  └───────┘ │x10 opt├────┐         line_  │  Y   ├─Y+    /\/\/
Y1gain------------┴┬──────┘     \___________│ │_│output│Plates
Y1shift------------┘           │ Balanced   │_│ │ amp  ├─Y-    \/\/\
   DC    _______   _______     │ diode matrix   └──────┘
Y2─┬─\. │Stepped│ │  Y2   ├────┘ switch          Rotate--Coil
in └┤├┴─┤ Atten ├─┤preamp ├>Xamp   |             Brill---CRT G1
    AC  └───────┘ │x10 opt├>Trigger|             Focus---CRT A2
Y2gain------------┴┬──────┘        |             Astig---CRT A3
Y2shift------------┘         ______│___   Z in--┬─────┼--CRT Anode
                            │  Chopper │        │ CRT │--CRT Cathode
Y Mode----------------------┤Astable/÷2├──┬─────┤ PSU │--CRT Heaters
      Y1┐┌Y2   _______      └──────┬───┘  │     └─────┘--CRT EHT
Trig──── \____│Trigger│  ________  │ Blank│ Y2   ______
  in          │ Gate  │ │Timebase├─┘      │ │ __│  X   ├─X+    /│_/│
+/- ----------│       ├─┤        ├────────┘  /  │output│Plates   _
DC/AC---------│       │ │        ├──────────┘│  │ amp  ├─X-    \│ \│
Level---------┴┬──────┘ └─┬──┬───┘ Ramp      │  └─┬──┬─┘
TV/HF----------┘          │  │               │    │  │
Time----------------------┘  │        X in ──┘    │  │
Variable---------------------┘                    │  │
Xshift--------------------------------------------┘  │

I have a 12" one of these, I use for displaying a spectrum analyser adaptor.
It uses constant current power output drivers with current NFB. The whole thing
runs terribly hot, as any display offset (e.g. trace at the bottom of screen)
needs huge standing current from a low voltage supply, & at only a few 10s of
kHz the supply is far to low to give much deflection.
                                        │     +10V
             +10V @8A             Drive │._    Supply
Display Amp     │                    │ _│  │ _            DISPLAY
   _   Input ─┤\│                    │'    │'              ____
└─┘ └─┘       │˝ >──┬────┐           │     │  _10v        Ů    Ţ
            ┌─┤/│  220  1ŕ)|| Y Yoke    Clipped           Ţ    Ů
            │   │   │     )|| Coil     Transient    Ţ    Ů      Ţ    Ů
            └───)───┴────┤                          Ů____Ţ      Ů____Ţ
                │       0.1R                     Slopping sides on large
             -10V @8A   _│_                    verticals due to lack of volts

Putting the yoke coil in the NFB loop gives the required voltage to frequency
uplift of 6dB/Octave, but due to the coil there is a time delay that causes
instability, so the amp gain has to be reduced at HF to make this circuit work.

Unlike with audio, the signal phase is important on scopes. so no bodges really
work well!

These are specialist Colour TV displays that can decode & show the phase &
amplitude of the colour signal.

Also see my buls on "Scope & DMM Calibrator", & "Scope Probes".

Why Don't U send an interesting Bul?

73 De G8MNY @ GB7CIP

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