Troubles measuring video RF signal: analog vs digital oscilloscope issue? - Page 1
Forum section: electronics / projects

author: kinap35
author_rank: Contributor
timestamp: January 15, 2025, 04:20:37 pm
content: Hi,I'm trying to adjust some levels inside a VTR, and i'm currently having a look at RF video signals.But i'm confronting something i'm not sure to understand and how to deal with: my waveform looks different, and with (maybe) 50% more amplitude on my digital oscilloscope compared to an anlogue one.Digital: Rigol DS4014 with all options activated.Analog: Tektronix 2445What is strange (for me at least), is that on the Rigol, i can see a more dense/opaque central zone surrounded by upper and lower little transparent "bands". The central opaque zone is about 200 mVpp, but the complete signal including "bands" is 50% higher at 300 mVpp. I've tried to activate bandwidth limitation to 20 MHz (since RF on this U-Matic SP PAL VTR is no more than 10 MHz), but not so much change: the whole signal (including "bands") is a little lowered (about 20 mVpp less).But on the Tektronix 2445, the same signal, with same probe, the display is less clear/more blurry than on the Rigol, because of the scanning/remanence of the display's tube (and maybe its venerable age and not so pristine condition), but most importantly, the whole displayed signal is just a little over 200mVpp, without visible additional side bands (graphically displayed band, not modulation bands of course), even without any BW limitation. With 20 MHz limitation, it is a little less than that.I've also tested to connect a signal generator directly to both scope, and injecting a 1Vpp sine signal is correctly measured on both at least upto 20 MHz, so it doesn't seems to me that it could be a frequency response of one scope compared to the other.So where this huge difference in displayed/measured RF signal comes from ?And which one is the right one? Or sayed differently: if i need to adjust my RF level to 300 mVpp, preferably on the Rigol, should i ignore or include the stranger upper/lower semi-transparent bands?Attached to this message, some pictures to illustrate what i can see:- Picture on the Rigol of the overall displayed signal- Measurements on the Rigol of the center signal only and including the side bands- Same signal with the same probe on the Tektro 2445, without and with BWL @20MHz- A capture of the service manual with a drawing of the expected signalWhat do you think about that? Is the digital Rigol has some more sensitivity/responsiveness than old analog ones, and so is displaying more signals? Is it an unwanted side effect of digital domain (aliasing maybe)?Are the level specifications in old service manuals inaccurate when using digital scopes? Or should the semi-transparent bands be ignored when measuring? I'm completely lost!Thanks for your help.

author: NE666
author_rank: Frequent Contributor
timestamp: January 15, 2025, 04:47:26 pm
content: Just because you can't see it on the CRT-based scope, doesn't mean that it's not there.You've just discovered a major advantage of digital scopes over the older phosphor technology - they can visualise for you that which your eyes otherwise couldn't, with faint phosphor.Dave produced an episode on this. Coming from analogue scopes, digital ones can appear 'worse', since they actually give a more accurate representation of the signal, warts 'n all.Correct me if I'm wrong, but it looks as if you've an AM signal there, with the carrier frequency being more prominent as expected.

author: NE666
author_rank: Frequent Contributor
timestamp: January 15, 2025, 04:51:10 pm
content: Episode #601

author: kinap35
author_rank: Contributor
timestamp: January 15, 2025, 05:18:57 pm
content: Thanks NE666.I've not watched the video yet, but you are telling me that what i can see on digital scope is more accurate that what a analog phosphor one was able to display? But then it also means that the adjustment values in the 80's service manual, when only analog scopes were available, are nowadays wrong when using a digital scope?Is there a rule of thumb way to extrapolate the "new" adjustment value suitable for digital era scopes? Or should i simply ignore those extraneous bands when measuring?About the signal: this RF video signal is FM modulated, and the prominent part is corresponding to the synchronization signal which is constant during a certain amount of time and at a lower voltage (so lower frequency) than the black to white level part of video signal.

author: G0HZU
author_rank: Super Contributor
timestamp: January 15, 2025, 06:00:00 pm
content: I'm a bit out of date in terms of what the very latest DSOs can do but when probing audio and video signals:A decent analogue scope (with a decent CRT tube) from the 1980s should still (just) be best when merely 'observing' a video signal for fine detail and fine changes in the detail. It will miss things that the DSO can display but overall, I think it should still give the most rewarding experience in terms of observing changes in the signal.A decent DSO should be much better when it comes to actually 'measuring' anything to do with the video signal in terms of amplitude or timing.Most people will prefer the DSO overall, but I do think there will be some die hard video gurus who would only want to use an analogue scope for video stuff.Note: Some of the attachments in the OP were missing for me?

author: kinap35
author_rank: Contributor
timestamp: January 15, 2025, 06:46:51 pm
content: Thanks G0HZU.Until now, DSO experience was much butter for most level or timing adjustments, and apparently OK judging by what i was seeing and results. But specifically for this RF amplitude adjustment, it seems it introduces unexpected problems!About missing attachments: i don't know why, but i can see now only 3 out of the 6 i originally posted and where visible. So i'm reposting here the 3 missing attachments.

author: NE666
author_rank: Frequent Contributor
timestamp: January 15, 2025, 07:50:21 pm
content: Quote from: kinap35 on January 15, 2025, 05:18:57 pmAll things being equal, by which I mean chiefly that both are operating correctly, in calibration and are setup correctly; yesBear in mind though, that oscilloscopes aren't hugely accurate when it comes to voltage measurements. Other, more specialised instruments, much more so. However, given that's what your service manual specifies, we can be confident that these general levels of accuracy are considered good enough here.Quote from: kinap35 on January 15, 2025, 05:18:57 pmIt's difficult to say, without knowing what kind of tolerances the circuit requires, and the service manual specifies. I'd be inclined to be conservative, given the context, and set for the lower limit first and see what results that yields elsewhere. I'd also turn on any averaging function the DSO has, to get some more 'sense' from the noise.

author: G0HZU
author_rank: Super Contributor
timestamp: January 15, 2025, 08:22:38 pm
content: It's also worth it to revisit (double check!) that any x10 scope probes are properly compensated to suit the (analogue or digital) scope they are fitted to. Obviously, this could cause amplitude variations up at RF frequencies if the compensation was slightly out for the internal square wave test signal..

author: Someone
author_rank: Super Contributor
timestamp: January 16, 2025, 05:11:48 am
content: Quote from: kinap35 on January 15, 2025, 05:18:57 pmThis is why various pieces of antiquated/historic test equipment was kept in production, although modern equipment could get a more accurate result the inaccuracies of specific methods (or even models of equipment) could be necessary to get the alignment/adjustment/testing functionally correct.There is rarely a simple correction factor/rule.