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G8MNY > TECH 23.01.17 11:25l 474 Lines 21584 Bytes #999 (0) @ WW
BID : 32448_GB7CIP
Read: GUEST
Subj: Strange Scope Hum Update
Path: IW8PGT<CX2SA<GB7CIP
Sent: 170123/0952Z @:GB7CIP.#32.GBR.EURO #:32448 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Mar 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have been working on an old Telequipment D61 scope. It had a few minor
problems which I have fixed OK, but one seems to baffle me...
It has a hummy Y1 trace on max sensitivity with the input open circuit (1Mê
impedance) WHEN HOT, it goes if you earth/terminate the input into lower than
10Kê. You can see the smooth 50Hz hum on the Y1 FET amp with a 2nd scope. There
is nothing nearby to cause it that I can see. The hum is much bigger than on
the circuits supply rails etc. Heating the PCB greatly increases the hum level.
The identical Y2 preamp is in the far corner of the PCB & does not suffer this
problem.
Removing components to isolate the circuit up, I found the hum is ON the PCB
tracks, in fact any track at high Z track has the hum. (plug in transistors use
in this kit make testing easier!) So I did some further testing, the 1kV AC for
the +2.5kV doubler & -1.2kV rails is also on the same PCB, but 5cm away & well
screened with a wall of large soothing caps.
What surprised me more is the 1kV AC is the reverse phase to the hum pickup!
PCB LAYOUT
--------------------------------------------------------------
+2.5kV* ²²²²²²²²²²²²² +EHT CAP
Diode Û
1 Û ÜÜÜÜÜ Diode
² 2
² AC Cap
²
* <--1kV AC
Diode Û EARTH POST
2 Û
/~~\/~~\ 3 LARGE SERIES
\__/\__//~~\ CAPS FOR -1.2kV
NPN -7.5V Reg D \__/
/\ /~\ Low Voltage
\/ \_/ Caps
() Y2 FET Y1 FET ()
PREAMP PREAMP
-------------------------------------------------------------
FURTHER TESTING
Removing the 1kV AC & hum goes looking on a 2nd scope. Cooling the PCB the hum
also goes. So I conclude the hum is travelling inside the PCB & the out of
phase pickup is some strange capacitance field effect see below.
FIXES?
1/ I tried an insulated earthed screening plate to no avail, as my scope probe
investigations already showed me there was little electric hum field in the
air around the FET preamp!
2/ I cleaned the PCB, both sides removing dirt from the high voltage area, but
there was no hum level change.
3/ I noted the maker had punched anti-tracking large holes in the PCB under
diodes 1 & 2, so I drilled another one under diode 3. Some hum reduction,
4/ Then I added 2 metal plates from the earth post to clamp the PCB & take away
some of the PCB electric field near the AC source. This does seem to have
fixed it.
WHAT HAPPENING?
I think the old PCB is slightly conductive, why it should be so heat sensitive
I don't know.
I think it electrically looks like this according to my computer model...
COLD Y1 input>ÄÂÄ100KÄ¿
³ ³
1kVÄÄÄÄÄ2GêÄÄÄÄÂÄÄ2GêÄÄÂÄÄ2GêÄÄÂÄÄ2GêÄÄÁÂÄÄ´ÃÄÁÄ>FET NO
AC ===15pF ===15pF ===15pF 1Mê 100pF HUM
ÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ (< 1mV)
Caps made of PCB tracks & components
HOT Y1 input>ÄÂÄ100KÄ¿
³ ³
1kVÄÄÄÄÄ1GêÄÄÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÁÂÄÄ´ÃÄÁÄ>FET > 5mV OF
AC ===15pF ===15pF ===15pF 1Mê 100pF HUM 180ø
ÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ out of phase
Caps made of PCB tracks & components
FIXED Y1 input>ÄÂÄ100KÄ¿
³ ³
1kVÄÄÄÄ1GêÄÂÄÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÁÂÄÄ´ÃÄÁÄ>FET NO
AC 200pF=== ===15pF ===15pF ===15pF 1Mê 100pF HUM
ÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ (< 1mV)
SB TECH @ WW
Oscilloscopes
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.
COORDINATES
The display uses 2 co-ordinates Horizontal X & Vertical Y.
Y The 3D Depth Z co-ordinate 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 co-ordinate is usually used for time display with
ÅÄÄÄÄÄÄ>X an internal time ramp generator (Timebase).
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 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
colour!
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
required.
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
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. (avoid ³ ³ ³ ³ ³ across from
parallax error when ÃÄÄÄÄÅÄÄÄÄÅÄÄÄÄÅÄÄÄÄ´ the left
taking measurements) ³ ³ ³ ³ ³ hand corner.
ÀÄÄÄÄÁÄÄÄÄÁÄÄÄÄÁÄÄÄÄÙ
Brilliance.
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!
ELECTRON GUN
SCREEN Astig
ÚÄ Á ÄÄÄÄÄÄ¿³ÚÄ__Cathode
³ ( ù ) ³ 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.
Focus.
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.
³ Electrostatic
Screen³ÄÄÄÄÄ=====ððððð±±ð=ÄCathode
³ Lens
A defocussed blob often shows a picture of the cathode surface!
ú ù @
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.
ÜÜ ù Ý
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!
_ . -Ä
Ä Ä Ä_Ä.ÄúÄ~Ä Ä Ä Ä
-Ä ~
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)
_
_ . -Ä ~ ³ ³~ÄÄ----ÄÄ~³ ..--ÄÄ--..
ÞÛÛÛÛÛÛÛÛÛÛ ³ ³ ³ ³ ³ ³
ÛÛÛÛÛÛÛÛÛÛÝ ³ ³ or ³ ³ ³ ³
ÞÛÛÛÛÛÛÛÛÛÛ ³ _³ ³ ³ ³ ³
ÛÛÛÛÛÛÛÛÛÛÝ ³_ . -Ä ~ ³_.--ÄÄ--._³ ~ÄÄ----ÄÄ~
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 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 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.
__ __
__ __ 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 invisible
³__³ 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 1Mê//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 _
Y1³ ³ Y2 ÚÙ À¿ ¿/ÀÄ¿_ÚÄÙ\ÚÄÙ ÀÄ
AF Amp \ / Distorted \___/ Distortion after gain
input 100mV '-' AF Output 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 quite a bit less than the normal Y channels.
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_) (_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
|___| |___|
on
/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. 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
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 control" 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/
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.
Chop/Alt.
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 ³ ch2 ³ ch1 Chop Mode Astable
ÀÄÄÄÄÄÙ 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 ³ ch2 ³ ch1 ³ Y 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 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 100ê twisted wire). This gives the timebase
time to be triggered, before the event reaches the display, so you can see it!
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 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!
_ ___
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 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.
Calibrate.
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)
__
__³ ³__³ e.g. 500mV peak to peak @ 1kHz.
OVERALL SCOPE 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
MAGNETICLY DEFLECTED X/Y SCOPE DISPLAY
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.
_
³ +10V
+10V @8A Drive ³._ Supply
Display Amp ³ ³ _³ ³ _ DISPLAY
_ Input Ä´\³ ³' ³' ____
ÀÄÙ ÀÄÙ ³ñ >ÄÄÂÄÄÄÄ¿ ³ ³ _10v Þ Ý
ÚÄ´/³ 220 1ê)|| Y Yoke Clipped Ý Þ
³ ³ ³ )|| Coil Transient Ý Þ Ý Þ
ÀÄÄÄ)ÄÄÄÁÄÄÄÄ´ Þ____Ý Þ____Ý
³ 0.1R Slopping sides on large
-10V @8A _³_ 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!
PAL VECTOR SCOPE
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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