oscilloscope calibration - Page 1
Forum section: electronics / general

author: Slania87
author_rank: Newbie
timestamp: July 09, 2025, 10:54:31 am
content: Hello,I started to work with the following oscilloscope: Multicomp pro Digital Storage Oscilloscope 100MHz MP720665I used a function generator: PM 5127 PHILIPS to generate functions. (Yes, it is pretty old.)I executed the self calibration and also the factory reset.I gave a 2000 Hz sinusoidal wave and the result is around 1366 Hz.How can I solve this issue?Thanks a lot.

author: chilternview
author_rank: Frequent Contributor
timestamp: July 09, 2025, 12:05:40 pm
content: Your PM5127 dates from 1975, and is probably out of cal by a long way. I'd trust the scope, at least that will have a crystal controlled frequency reference, unlike the PM5127. The service manual will probably have info on how you can calibrate your PM5127.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 09, 2025, 03:00:05 pm
content: Usually to calibrate a scope, you need at least a time mark generator, precision DC source, a fast-rise square wave generator, and a leveled sine-wave generator.You also need to make sure that the equipment is properly adjusted and calibrated too, or you risk making the adjustment on your scope worse than when you started.....

author: EC8010
author_rank: Frequent Contributor
timestamp: July 09, 2025, 05:49:41 pm
content: Even by function generator standards, that's a pretty poor sine wave. It's clipped on the top and it appears to be slew rate limiting. I'd say that function generator isn't merely out of calibration (insofar as function generators are ever reallycalibrated), it's got a fault.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 09, 2025, 05:50:35 pm
content: Does your Multicomp scope have a function generator built in? I think some do.If it does you can compare it with the ext generator using two channels.Your Sig gen is probably way off but you will not know unless you have an accurate freq standard. It is not necessary to have a Metrology grade Freq Standard but in today's world there are very good GPSDO units that are pretty cheap. Perhaps you could borrow one. Also the sine wave is not symmetrical and this is probably the fault of the old generator also.Terra is right about proper cal but you should be able to get away with less.Do you have any other instruments that have a 10 MHz output?Scopes are not usually "right on" but should be close on their freq and volt readouts. All the scopes I have seen do not have anything near a good freq standard built in. Perhaps there are some but I have not seen one. But your reading is not even close.If you have a short wave radio you could see if your freq gen is close by tuning in to an AM station and then putting a small wire into the output of the generator as an antenna and seeing where the freq gen competes with the AM station.I bet the freq gen is way off.How does the scope probe cal work? These things are not very accurate in freq but make a good square wave. But see if the scope freq readout with the probe cal is close to the scope probe cal freq in the manual.

author: Slania87
author_rank: Newbie
timestamp: July 10, 2025, 07:21:41 am
content: Thank You for the answers! I will try out the built in function generator.

author: Kean
author_rank: Unknown
timestamp: July 10, 2025, 07:54:48 am
content: Or you could even use the sound card in your computer.Software like Audacity can generate tones up to 22kHz or so which should be quite accurate to validate the scope isn't the problem.* For higher frequencies you'll need to adjust the audio sample rate - e.g. a 22kHz sine isn't going to be great when using the default 44.1kHz sample rate, much better at 192kHz.

author: J-R
author_rank: Super Contributor
timestamp: July 10, 2025, 08:23:11 am
content: The MP720665 scope appears to be the Owon XDS3104E with the 2 channel AWG and touch screen option:https://www.owontech.com/digital-oscilloscopes/benchtop-oscilloscopes/xds3000-e-series-4ch-12-14bit-touchscreen.htmlAgreed, that PM 5127 is ancient and likely faulty. I would not trust it.It's very unlikely that all four scope channels and both generator channels would be faulty in the scope, so you could perform some tests with multiple output-input combinations to verify operations.Do you have any multimeters with a frequency (Hz) function? Many do have this feature and the 2kHz output we are discussing should be well within the DMM's capabilities.Besides a GPSDO, another item that might be of interest is something like the NanoVNA. I have the H4 which can generate a test signal from 10kHz to 1.5GHz. This is normally used to characterize the item you are testing, but you can also use it as a very basic generator by setting the span to zero.

author: 2N3055
author_rank: Super Contributor
timestamp: July 10, 2025, 09:07:20 am
content: New function generator..https://eleshop.eu/uni-t-utg932e-function-generator.htmlThat is very decent cheap device. It will run circles around old clunker.Whenever device you have is listed in Radio Museum Web site you know you will need to replace it eventually.I am not recommending serious devices from Siglent or Rigol, because they are much more expensive, and are overkill in your case, if that old analog function gen served your purpose so far.

author: J-R
author_rank: Super Contributor
timestamp: July 10, 2025, 09:49:55 am
content: I don't think it makes a lot of sense to get a separate, low-end function generator, since the scope already has that built-in. Either get something much better, or get a different product that might be of use but can also create some signals. (RF generator?)The UNI-T UTG932E is generally similar to the Owon's AWG. Yes, the UNI-T has a higher sample rate and higher output voltage, but half the waveform length and half the included waveforms.When using the Owon's built-in generator, you also get FRA to generate Bode plots.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 10, 2025, 11:31:55 am
content: Yea, I think its a bad old function generator.Does anyone know if the scope function generator and the scope itself use the same time base?

author: J-R
author_rank: Super Contributor
timestamp: July 10, 2025, 08:46:59 pm
content: Quote from: Wallace Gasiewicz on July 10, 2025, 11:31:55 amI was also thinking this could be a possibility, so using a DMM as a backup to check the scope's frequency readings might be a good idea. 2000Hz versus 1366Hz should be easy to figure out.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 10, 2025, 11:05:28 pm
content: Yes, it should be easy to tell the difference from 2000 to 1366 Hz. That is why I suggested using an AM radio to sample the output of the Function Generator. You should be able to pick up 1366 Hz on the AM radio. You could also set the Function Generator to 1000 Hz and see where that appears on the AM radio. Not very accurate but should answer our question about the output of the generator. Also the output of the scopes generator.This would help if no DMM with Freq meter is available.

author: 2N3055
author_rank: Super Contributor
timestamp: July 11, 2025, 05:43:07 am
content: 2000 Hz and 1366 HZ are audio frequencies.1366 Hz is between E6 and F6 notes, and 2000 Hz is between B6 and C7, in standard A=440 Hz tune.So definitely not AM radio range, but simple direct audio.You could check it with musical tuner, or if you have a piano you could tell by listening and comparing with error this big. It is almost an octave error.