Calibrating analog scope with minimal equipment , only built-in generator  - Page 1
Forum section: electronics / testgear

author: osmax_br
author_rank: Regular Contributor
timestamp: February 11, 2025, 01:26:50 am
content: Welcome to episode 2 of me asking dumb questionsMy analog scope (os9020g 20mhz plus 1 mhz function generator) that I bought is out of calibration, it was in a humid storage environment for 30 yearsHOW DO I CALIBRATE THIS THING WITHOUT SEPARATE FUNCTION GENERATOR?I have the service manualHowever I only have a dmm (cheap one) and this scopeMy stupid question is : can I do a BASIC calibration using the built-in function gen, or it is a stupid ideaAnd if it is a stupid idea (most probably is) , what is the least expensive thing I can buy to calibrate this thing (I am cash stripped) , should I just do a simple crystal oscillator circuit and call it a day?!!Or just leave it as is since I might do more de-calibration than calibration with my not so good equipment (or non existent at this point)Also one more stupid question, when I push times 10 knob, the crt becomes very dim that I barely see a trace , any suggestions about the problem ?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 11, 2025, 09:29:15 am
content: Unless there is really a serious problem, I would leave most of the the trim parts untouched. It needs good knowledge and instructions to do the compensation adjustment. A service manual seems to be available for this scope.The error could well be with the build in function generator as well. One can still use it for a few test, especially comparing the 2 channels relative to each other. For a clean square wave, there is usually an extra test signal for the probe compensation.A first point would anyway be measuring how far off the scope actually is.To get a frequency reference one could consider things like the TV horizontal frequency, sound from a PC or phone, mains frequency and a crystal oscillator of any kind.For the voltage scale the DMM is probably best for low frequencies. If a square wave is not disturbed changes are good that the gain has little frequency dependence.A point one could consider is a look inside and cleaning of dust - somehing like a spider in the high voltage part could be an issue.It is normal that the picture becomes rather dark when using the x10 (for the horizontal). Many scopes don't increase the intensity and most of the time the beam is outside the screen. So getting 1/10 the intensity is kind of normal.Often one can than manually increase the intensity a bit or use a not so bright room.

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 11, 2025, 11:36:03 am
content: To adjust and calibrate any equipment, you need appropriate known references.Without that, you are just swapping one unknown for another.For oscilloscopes, this usually means at least a leveled sinewave generator, and square wave and time mark generators, as well as DC voltage (that one is easy with your DMM).How do you know the unit is out of adjustment? Does it not perform as expected compared to a reference signal?

author: tggzzz
author_rank: Super Contributor
timestamp: February 11, 2025, 11:40:24 am
content: Quote from: TERRA Operative on February 11, 2025, 11:36:03 amMost (analogue) scope service manuals don't require a levelled sinewave generator. A step generator with suitably fast risetime and minimal overshoot is sufficient to get appropriate waveform shape - and is directly comparable with the principal scope use cases.

author: forrestc
author_rank: Unknown
timestamp: February 11, 2025, 12:00:55 pm
content: Welcome to life with an analog scope.As others have mentioned, why do you think it's out of calibration? Some things are more issues with probing (or the probes) than the scope itself.Also, (for some types of miscalibration) why do you care? I've used oscilloscopes for many many years (probably getting close to 35 or 40 now), and I'd have to think really hard to come up with a point where I really needed a calibrated oscilloscope. Usually you're just wondering if a signal is there or not, or roughly what shape it is. Heck, I used to use the variable gain knob on the analog ones so signals would fill the screen. Unless it's way, way out of calibration, it should be usable for a good portion of what you want a scope for. And by way out I mean things like it's reading 2V when it should be 5V. Or something like that.Just to be clear: I'm not saying a calibrated oscilloscope isn't nice to have - it's just that the types of things you'd pull a 20Mhz analog scope out for aren't typically the types of things where calibration is needed.So, I guess to summarize my question: What unexpected thing is it doing? If you can describe it, some of us who have been around the block way too many times might be able to let you know some way to deal with it. I can't tell you the number of times I thought my scope was out of whack when it was just a weird setting.Oh, and as someone else mentioned, if you tell the horizontal to go x10 the speed it's going to be much dimmer. How an analog scope works is that it basically controls the X axis using the horizontal knob such that it takes however long is appropriate for the setting for the line to sweep across the screen. Set it down to as slow as possible and you'll see what I mean. When you get really fast, it goes across the screen so fast that the beam of electrons doesn't have time to light up the phosphor very much. That's what the "intensity" or "brightness" knob is for - to adjust how "bright" the display is. You generally will turn it down when dealing with slower signals but up when dealing with fast ones. With really fast signals it wasn't uncommon to use these in a darkened room as there just wasn't enough brightness even with the knob turned all the way.

author: tunk
author_rank: Super Contributor
timestamp: February 11, 2025, 12:32:22 pm
content: No expert, but I guess you could use a 1.5 celland a 9V battery to check DC accuracy (first testthem with your DMM). An arduino could be usedto create and check square waves.

author: tggzzz
author_rank: Super Contributor
timestamp: February 11, 2025, 12:43:40 pm
content: Quote from: osmax_br on February 11, 2025, 01:26:50 am(I responded to this elsewhere, and have copied ithereso that it can diethere.https://www.eevblog.com/forum/testgear/should-i-buy-the-finrsi-dso510-on-a-tight-budget/msg5814097/#msg5814097)The principal function of a scope is to show a waveform's shape, not the absolute voltage, not the absolute timing. Analogue scopes are about as accurate as an analogue multimeter, around 2%.To calibrate the vertical deflection, use a PSU as a voltage source, and measure the voltage with a DVM (which should be much better than 2%).To calibrate the horizontal deflection, you need a known frequency. Mains frequency will be accurate to better than 1%, except in exceptional conditions. Do not connect to the mains; just connect a wire to the input and you will see (very distorted) EMI pickup at mains frequency.If your internal function generator is based on a crystal oscillator, then it will be accurate. If, as I expect, it is not then its frequency will be suspect. Use your scope to compare it's output frequency accuracy with the mains using the lissajous figures X Y method; that does not guarantee frequency accuracy at other frequencies.So, creating your own crystal oscillator and digital divider is probably the best route.When the scope's horizontal is *10, then it is sweeping ten times faster (doh!), so only 10% of the electrons are hitting each spot. That will make it dimmer.

author: osmax_br
author_rank: Regular Contributor
timestamp: February 11, 2025, 01:09:20 pm
content: Quote from: tggzzz on February 11, 2025, 12:43:40 pmGot it , sorry for the inconvenience I madeThanks!

author: osmax_br
author_rank: Regular Contributor
timestamp: February 11, 2025, 01:18:34 pm
content: Quote from: forrestc on February 11, 2025, 12:00:55 pmWell, it works fine for the vertical line , I tested itCouldn't test the horizontal lineHowever what made me think it is out of calibration is that when I received it it had no trace , the problem was with the crt bias knob on the crt board , and every now and then the display returns being dim so I have to change it internally, I guessed if the crt bias is always wrong that means everything else is wrongSometimes I have to fix trace rotation every time I move the machine , and I was worried that it might give me wrong readingsLike what you said , I don't really need those precise readings since it is an analog scopeThe internal function generator for example at square frequencies above 100khz starts to give weird square signals , it is not a very high frequency , so I wanted to do the calibrationThe signal is a slope instead of a straight line , but a 100khz is not much I think , I use it normally as a crystal

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2025, 01:32:27 pm
content: Set the function generator to produce a square wave and measure that square wave with the AC mode of your multimeter. Assuming your multimeter measures average AC rather than root mean square AC, it will read 11.1% high when measuring the square wave. The peak-to-peak value of the square wave will then be 2 times the AC reading, minus 11.1%. This then can be used as a vertical calibration source.For timebase calibration, measure the output of the function generator with a frequency counter.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 11, 2025, 05:21:58 pm
content: The frequency generator square wave may not be good, as it goes though an amplifier and amplitude adjust. A simple 74HC based fixed amplitude generator gives a better vertical signal. The test signal at most scopes is usually a similar type. The amplitude is set by the DC supply.

author: David Hess
author_rank: Super Contributor
timestamp: February 11, 2025, 11:01:08 pm
content: Quote from: Kleinstein on February 11, 2025, 05:21:58 pmSome function generators produce a very clean output, with the majority of error coming from not having a perfect 50% duty cycle for the AC voltage measurement.The output from a CMOS gate or multiplexor is almost ideal and trivial to implement.

author: mawyatt
author_rank: Super Contributor
timestamp: February 11, 2025, 11:21:51 pm
content: This might be of interest.https://www.eevblog.com/forum/testgear/ac-rms-dmm-tests/Best