Square wave question. - Page 1
Forum section: electronics / testgear

author: Jesse1973
author_rank: Newbie
timestamp: February 17, 2020, 05:50:32 am
content: Recently purchased a Kenwood CS4125 scope.Scope powers up fine.Controls work.Following the user manual for pre power up settings.So far so good.Attach probes.Follow probe compensation. Then square wave I get using the onboard test point or separate freq generator isnt exactly square.I am a noob when it comes this so bare with me.The peak is flat and parallel to the x axis .Then rise and fall are not perpendicular to the x axis.Actually they are slightly angled .The both lean in the same direction from left to right.I am guessing something is wrong with the scope .What could it be and can it be fixed.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 17, 2020, 07:14:57 am
content: This is the reality of how signals work. If you zoom out to see several periods of the waveform it will look pretty square, if you zoom in on the edge it will always be slanted.This is usually called "rise time" in specifications, and it's effected by the switching speed of the source and the input bandwidth of the measurement device. If both of those are infinite, it looks like a perfectly vertical line, but in reality, that cannot exist, so provided your equipment is fast enough, you will always be able to see that rise time.That said, designers of scopes understand this, so even if your scope measures a single point on the low side and a single point after the full rise - what would appear as an instantaneous rise - the interpolation algorithm that draws the line between those points factors in the limited bandwidth of the scope and will always be at least slightly slanted.

author: newbrain
author_rank: Super Contributor
timestamp: February 17, 2020, 07:57:44 am
content: Quote from: Jesse1973 on February 17, 2020, 05:50:32 amCrude diagrams follow:This, as DaJMasta says, is normal.This is definitely not, and means something is wrong/misaligned in your scope.

author: Brumby
author_rank: Unknown
timestamp: February 17, 2020, 08:29:10 am
content: Thank you for those diagrams, newbrain. That clearly shows the concern I had.

author: Jesse1973
author_rank: Newbie
timestamp: February 17, 2020, 04:12:39 pm
content: The wave looks like the second picture.I guess there is something wrong then.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 17, 2020, 04:32:15 pm
content: Wonder if a magnetic disturbance is possible?

author: Jesse1973
author_rank: Newbie
timestamp: February 17, 2020, 06:13:11 pm
content: Is that something that can be fixed ?If so who would I send an old school scope to?

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 17, 2020, 06:36:44 pm
content: Switch the input to GND and ensure the trace is aligned with the X-axis. If it isn't then use trace rotation to adjust it.

author: ebastler
author_rank: Super Contributor
timestamp: February 17, 2020, 06:46:30 pm
content: I struggle to see how this slanted signal shape, with the X axis not orthogonal to the Y, could be caused by a flaw or misadjustment in the electronics alone. Looks more like the CRT's deflection electrodes for the Y beam deflection have been moved out of whack?

author: newbrain
author_rank: Super Contributor
timestamp: February 17, 2020, 07:15:53 pm
content: Quote from: ebastler on February 17, 2020, 06:46:30 pmFrankly, I am as puzzled as you.I do not think electrodes on a scope CRT can be moved. Aren't they embedded in the glass structure?I would rather suspect a crosstalk between horizontal and vertical deflection circuits, quite possibly through the supply.EtA: on a lighter note you can compete on the "Show us your square wave" thread with a negative fall time!

author: ebastler
author_rank: Super Contributor
timestamp: February 17, 2020, 07:28:54 pm
content: Thanks, newbrain. Right, the Y circuit cross-talking into the X circuit would also explain it.

author: StillTrying
author_rank: Super Contributor
timestamp: February 17, 2020, 08:31:47 pm
content: I think a magnet near the gun end of the CRT could cause that.Have you tried the scope in a different physical position.Is the square wave the same shape when it's in the top 3 Y divisions and bottom 3 Y divisions.A small image of the screen might help.

author: Jesse1973
author_rank: Newbie
timestamp: February 17, 2020, 11:53:54 pm
content: Okay here it is .Not sure why it is all fuzzy in the pic the screen is quite sharp.

author: 0culus
author_rank: Super Contributor
timestamp: February 18, 2020, 12:12:31 am
content: Exposing CRTs in your photographs is certainly an artform...no worries there. It's hard to tell but it almost looks like the trace rotation is off...have you tried turning that adjustment with a small screwdriver? Nevermind, looking at it again the peaks appear to be aligned with the graticule.I'd expect a strong magnetic field distortion to affect everything.

author: james_s
author_rank: Super Contributor
timestamp: February 18, 2020, 12:44:07 am
content: Quote from: newbrain on February 17, 2020, 07:15:53 pmNo, the deflection plates are spot welded to the metal structure of the electron gun but they are otherwise mostly unsupported and being relatively large they are quite fragile. I've seen bent deflection plates on multiple occasions, it can happen if the scope is dropped or knocked over. An xray of the CRT can verify the condition.

author: frogg
author_rank: Regular Contributor
timestamp: February 18, 2020, 12:58:38 am
content: I'm sure others would already say this, but given that the Kenwood CS4125 has a CRT, and the CRT is scanned line by line, it's pretty obvious to me that whatever is vertically syncing with the horizontal scan is slightly off.You'd have to find the schematics of the scope to know what's syncing the horizontal and vertical scans.

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 18, 2020, 01:27:26 am
content: Quote from: frogg on February 18, 2020, 12:58:38 amI'm not sure about that... Not sure about what would cause such a display either...In an CRT scope the syncing business is usually called the trigger if my understanding is right.

author: Circlotron
author_rank: Super Contributor
timestamp: February 18, 2020, 01:37:16 am
content: Quote from: frogg on February 18, 2020, 12:58:38 amWhat you say might apply to early DSOs with CRTs but analog scopes like this one draw the trace as one continuous wiggly line from left to right. No raster scanning here.

author: Brumby
author_rank: Unknown
timestamp: February 18, 2020, 05:08:25 am
content: Quote from: Circlotron on February 18, 2020, 01:37:16 amWorth repeating!Yes little Jimmy. Thereissuch a thing as 'analogue'!

author: Brumby
author_rank: Unknown
timestamp: February 18, 2020, 05:54:19 am
content: Quote from: newbrain on February 17, 2020, 07:15:53 pmThatdoesexplain what is displayed. Specifically, the X-axis getting a sniff of the Y-axis deflection.I see the other suggestions as having a poor fit.

author: Fungus
author_rank: Super Contributor
timestamp: February 18, 2020, 08:26:31 am
content: Quote from: Brumby on February 18, 2020, 05:54:19 amYep, and it sounds like a very difficult thing to fix.

author: james_s
author_rank: Super Contributor
timestamp: February 18, 2020, 08:48:54 am
content: Quote from: frogg on February 18, 2020, 12:58:38 amThis is just plain wrong.You are describing a raster display, while an analog oscilloscope uses a vector display. The image is not scanned line by line, the timebase directly controls the horizontal sweep while the signal you want to look at drives the vertical deflection and the result is the trace is drawn in real time. Scopes that have a readout use a time sharing technique where the beam shoots off to trace out the characters then returns to the trace.There's more than one way to drive a CRT. Analog scopes use electrostatic deflection tubes but there are magnetic deflection vector monitors too, mostly used in vintage arcade games like Asteroids and Tempest.

author: rf-loop
author_rank: Super Contributor
timestamp: February 18, 2020, 02:19:25 pm
content: Quote from: frogg on February 18, 2020, 12:58:38 amNo. It is absolutely sure normal xy vector tube as in nearly all pure analog scopes what I know over tens of years. It can also see even nearly without eyes when look trace in image. Raster monitor do not draw like this. Never.--------------------For OPIt looks bit like magnetic error if not like some others have told about some crosstalk (what I bit suspect when I look trace shape, but ... all is possible.). I do not know Kenwood internals but some scopes have small permanent magnets for fine adjust symmetry (of course expensive higher class scopes have also other methods for adjust some linearity and trace bending like example in many old Tektronix) but also I have seen scopes what have been in strong magnetic field and some parts around tube are magnetized so it may need perhaps demagnetize.

author: trevatxtal
author_rank: Contributor
timestamp: February 18, 2020, 05:31:55 pm
content: Simple the crt tube has been rotated in its clamps.If you have ever replace a crt tube then this is the first thing you set up.

author: StillTrying
author_rank: Super Contributor
timestamp: February 18, 2020, 06:10:59 pm
content: If the tube was rotated to fix it, the whole width of the scan would then slope upwards, and the straight tops and bottoms of the square wave would then slope in an unusual way. I don't think interaction between the Y and supplies would that perfect, so I'm sticking with magnetism.

author: james_s
author_rank: Super Contributor
timestamp: February 18, 2020, 08:31:04 pm
content: How do you rotate a rectangular tube? It can only fit in two orientations and only one will put the anode terminal in the right place.I'm betting bent deflection plate however I suppose magnetism is possible.If you can swap wires between the horizontal and vertical plates that should make it obvious whether it's anything electrical.

author: newbrain
author_rank: Super Contributor
timestamp: February 18, 2020, 10:38:51 pm
content: Quote from: james_s on February 18, 2020, 08:31:04 pmOf course you don't.In fact, AFAIK, rotation controls on CRO with the graticule etched on the tube are (electro)magnetic.I think this is the case for the scope in question, see page 7 ofthe manual, "Large Aperture" and "Trace Rotation" bullet points.As for the magnetism hypothesis, I struggle to see how a magnetic field could have so precise a gradient to generate the effect in the picture....But what do I know? The only analog scope I own isthis behemot(though mine is marked FAIRCHILD)...

author: james_s
author_rank: Super Contributor
timestamp: February 19, 2020, 12:06:27 am
content: Yes trace rotation uses a coil around the neck with DC current controlled by a pot.That would rotate the entire Trace though, not just one axis.Try unplugging the horizontal deflection if you can, and see if the beam moves straight up and down with the vertical position. Be sure to turn the brightness down first.

author: Jesse1973
author_rank: Newbie
timestamp: February 23, 2020, 04:48:18 am
content: Stupid question now.What does the horizontal deflection look like ?

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: February 23, 2020, 10:37:25 am
content: Quote from: newbrain on February 17, 2020, 07:57:44 amThat signal has three different voltages at some points in time! Quantum-mechanical phenomena !?!?

author: newbrain
author_rank: Super Contributor
timestamp: February 23, 2020, 11:07:51 am
content: Quote from: Jesse1973 on February 23, 2020, 04:48:18 amFirst, a word of warning: inside a CRT oscilloscope there are high and dangerous voltages which can result in serious injuries or death.Do notattempt repair, if you are not confident with or don't know safety procedures (or ignore that you don't know).That said, the service manual can be found onelectrotanya, and on page 52, coordinates BE-4 to BF-5, there is a cable (CN7 to CN407) to the horizontal deflection plates.I also see a capacitor, C401, between the horizontal and vertical deflection plates but it seems to be only used in the 4135 model - a leakage of that cap might have the observed effect.

author: frogg
author_rank: Regular Contributor
timestamp: February 24, 2020, 01:13:53 am
content: Quote from: james_s on February 18, 2020, 08:48:54 amI'm not talking about a raster display. I understand very well that oscilloscopes draw in the vertical axis and sweep from left to right.