understand if old oscilloscope needs calibration - Page 1
Forum section: electronics / testgear

author: Unknown
author_rank: Unknown
timestamp: April 07, 2013, 03:43:14 pm
content: Hi to all.I want to buy an old oscilloscope, so I would like to know which are the tests and where to look to detect that it's out of tune and needs to be calibrated.I mean, if i see an analog oscilloscope at a good price, how can I understand if it's a good deal or if i get a brick to be tuned?Maybe I can start to look at the 1kHz waveform signal it internally generates but what should I look at? Loss in Vpp, rising-falling edges deformed, incorrect frequency or...?Or using some external signal generator?Searching for tricks.Obviously, all of this, apart of checking the brightness of CRT and the funcionality of the knobs.TIA

author: ddavidebor
author_rank: Super Contributor
timestamp: April 07, 2013, 07:38:05 pm
content: you can find all this info in the service manual, in the performance check list.in simple version, just look if everything in every scale and mode is equal to what you see on a DSO.

author: Unknown
author_rank: Unknown
timestamp: April 07, 2013, 07:50:38 pm
content: But this is the problem: I haven't those docs neither a DSO to perform this check.In fact I want to understand if some oscilloscopes aren't out of tune in some local electronic market or electronic fair.I can't know which brand or model I can find. So I was looking if there is some simple test to do to get some clues about its calibration. I think this restricts to 1kHz square wave or, if present, using an external signal generator (often there are).To help me decide if buy it or leave.Or what do you do to decide if an oscilloscope is good or not, apart the brand and model?

author: c4757p
author_rank: Super Contributor
timestamp: April 07, 2013, 07:58:58 pm
content: Calibration? Doesn't usually drift much, it's really only an issue if you replace parts. I pretty much never have to adjust a scope that still has all of its functions. Just check the functions.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: April 07, 2013, 11:51:52 pm
content: One thing you could do to check voltage accuracy would be to measure a battery with the scope and cross check that with a DMM. Just a quick check to be sure nothing is too far off. Do this for both + and -, and they should obviously be the same.Checking the time base can be bit more difficult, because who is to say that the function generator they have available is accurate? I would just check the 1kHz compensation output on the scope and make sure it reads close. As c4757p said, if it's close, it is likely very close. I recently checked a 22 year old HP that has never (to my knowledge) been calibrated, and it was so close I didn't bother changing a thing.

author: c4757p
author_rank: Super Contributor
timestamp: April 08, 2013, 12:28:41 am
content: One thing I'm usually picky about is centering of the controls. If you focus the display and the focus knob is most of the way through its travel distance, it's probably drifted and might drift more (overheating resistors in that section are not uncommon). Make sure you can turn the display all the way from invisible to blinding, and make sure that the vertical/horizontal position knobs roughly agree with the position on the CRT. If they've drifted, they may continue.

author: Unknown
author_rank: Unknown
timestamp: April 08, 2013, 07:30:22 am
content: Ok, these are useful tips.QuoteMe too thought of the problem of the external generator's calibration. But isn't the 1kHz too much low freq to make a valid test? I mean, should the problems arise at higher frequency or also at that so low freq? As for example the deformed rising edges towards the end of the bandwith.I've seen in an ad, one sells a LeCroy 9424 and says 2 of 4ch are to be tuned.What does it mean? Which operation does it need? And if 2ch are untuned, could also the other two follow soon?

author: grumpydoc
author_rank: Super Contributor
timestamp: April 08, 2013, 07:41:12 am
content: QuoteSounds like one to avoid TBH not sure about the 94 series without trying to dig out a manual but these are fairly complex digital 'scopes so unless you're fairly experienced I wouldn't go for one which isn't working properly.The 1kHz output on a 'scope isn't there to calibrate the instrument. High end 'scopes do have accurate outputs - I sometimes use the cal output on my LeCroy 9354 to measure the rise time for other 'scopes as it's the most convenient fast rising pulse generator I have near the bench (I do have an older Tek pulse generator but it badly needs a service so it's in the loft at the moment).However older analogue 'scopes generate something "around" 1kHz, perhaps within 10% - they're usually tighter on voltage.That's not to say they can't be used as a go/no go test and the fast rising edge of the square wave has a lot of high frequency components so they can give some information about the high frequency response of the 'scopeWhat's your budget - you might be better off with one of the entry level digital 'scopes such as the Rigols

author: vk6zgo
author_rank: Super Contributor
timestamp: April 08, 2013, 07:56:40 am
content: People get all hot & bothered about "Calibration" of Oscilloscopes,which betrays a fundamental misunderstanding of their function.They arenota substitute for a DMM voltage scale,or a frequency counter!Analog 'scopes do not offer extremely high accuracy measurement of voltage amplitudes,normally best accuracy of a few % for a close to full screen display.(Plus your reading errors).DSOs have voltage readouts which may or may not be accurate.The "measure a battery with a DMM,then with the 'scope on DC coupled,see what deflection you get" trick is a good one.Timing accuracy is usually fairly good ,but remember,the things cannot offer "crystal controlled accuracy".The 1kHz calibration output isn't crystal locked,either.You could knock up a tiny crystal oscillator,check its frequency, work out what 1/f is,then carry it round in your pocket when you go looking at 'scopes.

author: Unknown
author_rank: Unknown
timestamp: April 08, 2013, 05:14:06 pm
content: I understand the concept that oscilloscope isn't a subtitute of precision instruments, but what I meant (maybe with the incorrect term 'calibrated') was to understand which is the limit under which the displayed waveform is not useful anymore.To have some hints for buy or leave an old one.I seem to understand the important thing is the overall knobs and display and a quite tolerant measure in V and Freq.Good idea the crystal in pocket. Could we say a 10-15% tolerance?I think suffice to me 100Mhz.I thought maybe I could buy a 100Mhz DSO like Owen or Rigol (I looked at the other threads) and maybe another analog one if I find one minimum 2ch 20Mhz at around 50€. So I will have the best of two worlds in the low freq and only digital for high freq. I don't think it should better give 300€ for analog 100-200Mhz (i.e. Tek 475 at 340€ or Tek 465m at 280€, including probes).Does it make sense?

author: ddavidebor
author_rank: Super Contributor
timestamp: April 08, 2013, 08:47:06 pm
content: if you can, buy an DSO! rigol, hantek, owon... they're better than any analog scope.

author: grumpydoc
author_rank: Super Contributor
timestamp: April 08, 2013, 09:21:47 pm
content: QuoteSure, but past 10kHz my 'scope is (at present) also my best AC voltmeter and I suspect this is true of many others as well. So while I don't mind it being 1 or 2% out I'd rather it not be 10 or 20% out on voltage.QuoteIf you can afford a 100MHz DSO I would buy yourself one.After that if you want an analogue 'scope look around for something like a Tek 2225 (Dave did a review in episode 208) - you might have to wait a while until one turns up at €50 but you might get one for that sort of money if you're patient.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 09, 2013, 01:23:31 am
content: Previously,I said "They are not a substitute for a DMM voltage scale,or a frequency counter!"I need to qualify that,as in service work,we do use them as a rough substitute for those instruments all the time!For instance,with the 'scope DC coupled,we can look at the supply rails to see if they are roughly correct.That done,we may check signal levels--for instance,if at some test point, we are supposed to be seeing a sine wave of 200mV p-p at 1kHz,& we see a sine wave ofabout4 vertical divisions at 50mV/div,& 5 horizontal divisions at 200uS/div,we are pretty happy that we have a functioning circuit to that point.Actually,we do this sort of thing with a DMM all the time----if the "5 volt" rail is actually 4.88v or 5.12v we don't lose much sleep.The difference is that a DMM can be fooled & show "reasonably close" readings for an incorrect waveform.whereas an Oscilloscope of any kind will give you a good idea of the waveshape of a signal.Even on DC supplies,the DMM may happily integrate any hum or noise spikes which are present.On one occasion,I was working on my Mother-in-law's TV,looking for the reason behind a picture with compressed vertical scans.The vertical output circuit obtained its supply voltage from a rectified overwind on the Horizontal Output transformer,giving a DC voltage of around 160v.The DMM said 130v---low,but not low enough for the symptoms.Borrowed an old BWD 'scope from work,& the "130v DC" was revealed to be a lot of unfiltered 1/2 waves at 15kHz line rate---- faulty filter cap.The DMM was quite happy to tell me it was DC!

author: ddavidebor
author_rank: Super Contributor
timestamp: April 09, 2013, 05:01:19 am
content: Yeah, but you can't calibrate a power supply with a scope.Ananalog scope has 80 values, a digital one 256, crap mutimeter 2000, an high end multimeter 20000

author: vk6zgo
author_rank: Super Contributor
timestamp: April 09, 2013, 06:04:56 am
content: If you have good eyesight,you may be able to interpolate (guess!) to maybe 200,but I agree,for most of ushalf a small division is "it"---hence your "80 values".But this is the useable resolution,& can't be compared with the "number of values" the device displays.An analog 'scope stillattemptsto display an infinite number of voltage levels ,but is limited by instrument linearity, noise & beam diameter.For the same screen size,a DSO won't be any better--it is still limited by our vision.Weare the limiting factor,when it comes to visual interpretation of the 'scope display.A larger DSO screen will allow you to see better than half a small division,allowing you to make a closer "guess".Even so,resolution doesn't help if the accuracy of the reading is lousy,whichis,perhaps a point in favour of a DSO,as the ADC steps may be more accurate than the linearity of the analog device.If it has a built in voltmeter function,it will be nearly as good as a "crap multimeter",but that is a different argument.Where both types of 'scopes shine is in seeing things that even the best DMM has problems with,I would suggest that you need a DMM to set the volts correctly,& an Oscilloscope to check for any crud which shouldn't be there.By the way,how accurately does the voltage adjust control on your power supply let you set the output?How stable is it after you set it?In most cases,you could probably get away with the "crap multimeter".

author: kfitch42
author_rank: Frequent Contributor
timestamp: April 09, 2013, 03:13:58 pm
content: Quote from: frank10 on April 07, 2013, 07:50:38 pmThere are a few tools out there that can help do a quick spot check on a scope (I haven't used these, but I have heard good things about them):This one has tektronix in the name, but the only tek specific part is the color scheme:http://www.ebay.com/itm/CALIBRATOR-AND-TESTER-FOR-TEKTRONIX-OSCILLOSCOPES-NEW-WARRANTED-CALIBRATED-/130864417080?pt=LH_DefaultDomain_0&hash=item1e78208938Has DMM in the name, but for a quick spot check ... it is more than nothing.http://www.voltagestandard.com/DMMCheck_Plus.html