New Analog Scopes?

[EEVblog]

typical DSO=TDS220

Yes, 20 YEARS AGO!
A "typical" DSO has advanced just a tad since then...

Dave.

[Gunb]

Hameg make "combi" analog and digital storage scopes. So why not just get a modern digital scope when you need storage, and just a regular analog when you need/want analog?
And why are analog scope superior to digital scope when it comes to triggering?

Dave.

Hameg does not offer combi scopes anymore, think this times are over. They only offer one last analog scope with a bandwidth of 40MHz but announced to cease production soon - no market anymore.

I would also tend to buy a digital scope today but of better quality with sufficient bandwidth and wfm/s if communication electronics is an issue, but then it might become an expensive aquirement.
From a few ham radio guys I know that they often search especially for analog scopes for their purposes, and bandwidth is more important than storage functionality. In this case sometimes you can get very professional scopes as the Tek 465 with various options for less money compared to an equivalent digiscope.

There's no limit caused by the screen resolution with CRT as with TFT displays and the dead time is depending on the beam return, no processing time of data which causes limited wfm/s.

But generally I agree, there have to be special applications for buying an analog scope today.

Only my personal opinion, don't kill me for that, please 


Kind regards
Gunb

[Gunb]

@Hydrawerk: The TDS200 series is so crappy that anyone mentioning it should get banned for a week!

OH YES!

This is the scope that you had to fix with glue on your desk not to follow you like a dog when pulling the probe wire to hard!!! The screen was sooooo slow that you fall asleep when watching it too long.

The reason why I decided to buy my good old Hameg HM407 combi scope in the 90s - fast CRT with digital option, much better than the TDS220 and it's ugly brothers and sisters.
I think, that was also good reason for Agilent to produce digiscopes with CRT instead of using first generation of LCDs/TFTs as my 54622D.

I've never understood why people liked this Tek series and spent much money for this Gameboy design !?!?

[alm]

Don't knock on the TDS-220, it was a decent scope... in the mid-nineties. I believe it was (one of) the first affordable scope with a decent sampling rate (1 GS/s for 100 MHz BW). The competitive Agilent models of that time were stuck in the ~100 MS/s range, although with much more memory.

The TDS-220 was intended to replace the analog scope for low-end service work. The fast sampling rate and peak detect mode made aliasing less likely, and analog scope users don't really need a long memory, 2.5k is plenty for a few screens full of data.

Of course it's not very competitive to modern low-end scopes, just like a computer from that era is not very competitive to a modern computer. DSO technology was still in a fairly early stage back then, so naturally you see much more improvement going from a TDS-220 to a DSOX-2000 than from a Tek 2467 to an inferior modern analog scope (since the 2467 was essentially the end of the line for analog scope development).

There's no limit caused by the screen resolution with CRT as with TFT displays

The screen resolution of a reasonably modern analog scope is quite limited. To reduce the tube length, many later (from the seventies onward) feature an expansion mesh that blurs the trace. Nowhere the infinite resolution you might imagine. For very sharp traces showing tiny details, go for the older, mainly tube-based, analog scope. Nothing of this newfangled transistor stuff . The accuracy from a CRT is also quite limited, maybe 4-6 effective bits over bandwidth if you digitize them (they tried this in the early days of digital scopes).

[EEVblog]

Don't knock on the TDS-220, it was a decent scope... in the mid-nineties. I believe it was (one of) the first affordable scope with a decent sampling rate (1 GS/s for 100 MHz BW). The competitive Agilent models of that time were stuck in the ~100 MS/s range, although with much more memory.

Yes, the TDS-200 series was the industry game changer. Overnight, affordable scopes went from crappy equivalent time sampling, to real time sampling on all timebase settings. And the industry never looked back, real time scopes became the norm. So hats off. But it was almost 20 years ago now!

[GK]

I often find myself switching to an analog scope when the resolution of a DSO proves insufficient. A recent example was a low level burst of RF oscillation over part of the cycle of a repetitive low frequency signal. Completely invisible on the DSO (buried in the "digitization" noise) but clear as bell on the old Tek.

[vk6zgo]

The other thing is trying to look at 50Hz hum on a repetitive pulse train,or similar signal,which is a common problem in Electronic repair.

The classic case I always quote,(ad nauseum),is looking at an analog video signal at field rate in order to find both hum & other artifacts at low frequencies.

 Most 1990s DSOs (& many inexpensive modern ones) are incapable of doing this,as they reduce their sample rate too far at long time/div settings,so that they run into problems with aliasing,due to the high frequency components of the signal.

The last one we were shown at my old work would have worked well for NTSC,but still had problems with aliasing due to the PAL
4.433MHz chroma frequency.

As the last analog 'scope we bought had major problems,we ended up taking the flawed DSO as the best compromise replacement.
Tek took the faulty analog back.

[Gunb]

The screen resolution of a reasonably modern analog scope is quite limited. To reduce the tube length, many later (from the seventies onward) feature an expansion mesh that blurs the trace.

I wrote about former high end scopes like the Tek 465 that you could get for a small budget and of course I'm speaking NOT about scopes where the trace is blurred.
And I can guarantee that even the trace of my HM407 standard scope is still very well, many other owners confirmed that, too.

If I would buy an older scope at ebay I first would try to visit the owner to investigate the device.

Nowhere the infinite resolution you might imagine. For very sharp traces showing tiny details, go for the older, mainly tube-based, analog scope.

Yes, I mainly talk about pure analog tube-based scopes, the Agilent example related only on the faster screen - besides it also has got a tube. And of course the resolution of the CRT is higher, NEALRY limited by the width of the trace only. If we're talking about a 320x whatever pixel display you first have to zoom in if more samples are stored in the memory as displayed. The sample rate limits the physical resolution additionally, and related to this example more than 320 samples can't be displayed at once. Or how for example do you show 10kPts on a screen consisting of 320 pixels horizontally without zooming in??? Easier to see the glitch without zooming on an analog scope, there's no pixel limit. Checked a few times.

If a glitch occurs between two screens you won't see it on a digital scope, but on an analog scope. On a digital scope you need high wfm/s to increase the probability to catch the glitch, on an analog scope set up with high time of persistence you can see it the first time it occurs.

Nothing of this newfangled transistor stuff . The accuracy from a CRT is also quite limited, maybe 4-6 effective bits over bandwidth if you digitize them (they tried this in the early days of digital scopes).

Bits over bandwidtch of an analog scope? Don't know where the bits should come from when talking about analog scopes like the Tek 465.  Strange.


Back to the TDS: didn't say anything about the sample rate or trigger capabilities but the bad slow display and it's cheap impression. Only my personal opinion

[Gunb]

FYI: Attached some useful information. Old but still informative related to the scopes we're talking about.

Kind rgds
Gunb

[nicknails]

Hameg make "combi" analog and digital storage scopes. So why not just get a modern digital scope when you need storage, and just a regular analog when you need/want analog?
And why are analog scope superior to digital scope when it comes to triggering?

Dave.

When I'm using an analog scope, for the most part I don't have to touch any trigger settings (unless I'm in single shot mode).  Whatever my signal is shows up typically without any fiddling.  I can bounce between different points on my board without even touching the scope.

When I'm using the DSO, the trigger settings are incredibly sensitive.  If my waveform changes a little bit, the signal on the screen goes nuts until I adjust the trigger level up or down a hair.  It's just an annoyance.  If I bounce between different points on my board, I have to stop to adjust the trigger levels and settings.

Maybe it's just because I'm using low-end equipment.  I have a Rigol DS1022C and a DS1102E.

[c4757p]

I agree, but I think it is specific to the model/brand. My DS1102D is also insanely touchy, and my Tektronix 2445A triggers on anything without any adjustment at all, but the new Agilents in the school lab feel more like the Tek.

[robrenz]

I often find myself switching to an analog scope when the resolution of a DSO proves insufficient. A recent example was a low level burst of RF oscillation over part of the cycle of a repetitive low frequency signal. Completely invisible on the DSO (buried in the "digitization" noise) but clear as bell on the old Tek.

Yeah, like this:

[alm]

I wrote about former high end scopes like the Tek 465 that you could get for a small budget and of course I'm speaking NOT about scopes where the trace is blurred.

The 465 features an expansion mesh. Customers complained about the blurry trace when it came out. Just compare it to the tube-based 500 series (and I believe some of the early 400 series) to see the difference.

Yes, I mainly talk about pure analog tube-based scopes

The 465 is all transistors, its only tube is the CRT.

If a glitch occurs between two screens you won't see it on a digital scope, but on an analog scope. On a digital scope you need high wfm/s to increase the probability to catch the glitch, on an analog scope set up with high time of persistence you can see it the first time it occurs.

This depends on the edge rate. You can barely see the edges of a fast-rise square wave with a high duty cycle. Good luck catching that low duty cycle glitch. Finding them would require turning off the lights and cranking up the contrast. The exception to this is the MCP CRT that Tektronix only used in the 2467 and 7104. These where specially designed for these kinds of jobs, and had a kind of analog image amplifier. Consisting of tiny channels, kind of like pixels .

Bits over bandwidtch of an analog scope? Don't know where the bits should come from when talking about analog scopes like the Tek 465.  Strange.

It gives an indication of accuracy. You can calculate the accuracy from the ENOB if you feel like it, but ENOB is easy to compare to a digital scope. There was a project where they digitized the image of a CRT, but even after correcting for all of the distortion, the results where still quite horrible compared to a decent ADC. Shows that the CRT has resolution, but not accuracy.

[Gunb]

@alm:

just have a look into my attached file above, that's what I'm talking about. And that's still valid concerning the TDS as well as current cheap digital scopes.

Tek465 was only an example, I know a few people still using that device without any complains - don't have it, so I can only repeat what they are telling me.
I've replied to the question if buying an analog scope still makes sense - and because of the facts in the attached document I would recommend to buy a used Hameg
with CRT or similar in good condition FOR SPECIAL purposes only. In general I would prefer a digital scope.

It gives an indication of accuracy. You can calculate the accuracy from the ENOB if you feel like it, but ENOB is easy to compare to a digital scope. There was a project where they digitized the image of a CRT, but even after correcting for all of the distortion, the results where still quite horrible compared to a decent ADC. Shows that the CRT has resolution, but not accuracy.

Making an analog signal digital - with what? Who made it under which circumstances? Which scope was used for that purpose? Can it be generalised for every input amplifier of any scope manufacturer? Who can guarantee that the used ADC for conversion wasn't the part which made it horrible first?

It's a fact, that an 8 bit ADC has got a resolution of 256 causing quantization noise. It's also a fact that any voltage level between two steps is assigned to the same quantized value. At this point the analog scope can show more resolution than the digital one, implied that the electronic of the scope is better than the required resolution.

By the way I didn't talk about accuracy and vertical resolution, that's a new discussion you're starting now.

I've discussed the horizontal resolution and the fact that the CRT can display more details concerning the x-axis as described in the attached PDF above - seems to be a simple fact.

[alm]

just have a look into my attached file above, that's what I'm talking about. And that's still valid concerning the TDS as well as current cheap digital scopes.

This is an attached ZIP file with presumably a PDF inside. It is served with the wrong content type, forcing me to download it, then unzip it, and read the PDF. As opposed to just clicking on the attached PDF. Probably not your fault but the fault of the forum/server administrator, but I don't feel like jumping through hoops right now.

Making an analog signal digital - with what? Who made it under which circumstances? Which scope was used for that purpose? Can it be generalised for every input amplifier of any scope manufacturer? Who can guarantee that the used ADC for conversion wasn't the part which made it horrible first?

This was a Tektronix project. They attached a digital camera to a CRT to digitize it. Distortion was terrible. Real digital scopes (with a front-end that directly drives an ADC as opposed to a CRT) perform much better. No argument on the resolution, but don't let you fool that into thinking the accuracy is there. It's not.

It's a fact, that an 8 bit ADC has got a resolution of 256 causing quantization noise. It's also a fact that any voltage level between two steps is assigned to the same quantized value. At this point the analog scope can show more resolution than the digital one, implied that the electronic of the scope is better than the required resolution.

The vertical amplifier of an analog is definitely a weak link in the distortion performance. Just because the eye is a terrible judge of distortion does not mean the signal is undistorted, especially as the frequencies go up.

I've discussed the horizontal resolution and the fact that the CRT can display more details concerning the x-axis as described in the attached PDF above - seems to be a simple fact.

Assuming you can't use the zoom function on your DSO. Otherwise any modern DSO with 1Mpoints+ of sample memory will have much more horizontal resolution than an analog scope, but of course not conveniently in one screen.

[nctnico]

If a glitch occurs between two screens you won't see it on a digital scope, but on an analog scope. On a digital scope you need high wfm/s to increase the probability to catch the glitch, on an analog scope set up with high time of persistence you can see it the first time it occurs.

Not quite true. An analog scope also has dead-time because of the horizontal retrace and it needs to find something to trigger on unless its in free run but then you'll see just blurs. Some digital scopes (like the one I have) have a special mode in which it captures waveforms continuously without any dead time.

In my experience a DSO can display a signal better than an analog scope but it takes more effort to set it up properly. Getting a blur to show on an analog scope is easy but that doesn't mean its a meaningful measurement.

[Gunb]

This was a Tektronix project. They attached a digital camera to a CRT to digitize it. Distortion was terrible. Real digital scopes (with a front-end that directly drives an ADC as opposed to a CRT) perform much better. No argument on the resolution, but don't let you fool that into thinking the accuracy is there. It's not.

Who ever said I would?

The vertical amplifier of an analog is definitely a weak link in the distortion performance. Just because the eye is a terrible judge of distortion does not mean the signal is undistorted, especially as the frequencies go up.

Nobody denied that, but it's possible to investigate how good the amplifier is, of any scope. But that's an issue of analog as well as digital scopes.  There's still an analog input stage.

Assuming you can't use the zoom function on your DSO. Otherwise any modern DSO with 1Mpoints+ of sample memory will have much more horizontal resolution than an analog scope, but of course not conveniently in one screen.

Aha! I can't use the zoom?! Really? I mean, when using a scope with an ADC of 1GS/s at max I assume that you understand that the time between two samples is 1ns, right? So you're zoom can't resolve more than showing you samples in a distance of 1ns, what happens between is not displayed, but interpolated if not switched off. An analog scope using 1ns can show any value between because there's no sampling but a continous steering of the beam. Disadvantage: you can't store it. Think about it. 

[Gunb]

Not quite true. An analog scope also has dead-time because of the horizontal retrace ...

Oh man, I've already mentioned that issue a few times before, you should read all my comments before starting from the beginning.   

.... and it needs to find something to trigger on unless its in free run but then you'll see just blurs.

You see just blurs - aha - I can see a baseline in AUTO mode or nothing until a trigger level is reached in NORMAL mode. And your digital scope behaves different in AUTO or NORMAL mode? - strange! So you want to tell me that your digital scope DOES NOT need something to trigger? Funny! All my 4 digital scopes either run free or only if triggered.

Some digital scopes (like the one I have) have a special mode in which it captures waveforms continuously without any dead time.

Ah, and what "magic" scope is that? I mean even the top scopes in the market have limited refresh rates, I'm curious what scope you're using!

In my experience a DSO can display a signal better than an analog scope but it takes more effort to set it up properly. Getting a blur to show on an analog scope is easy but that doesn't mean its a meaningful measurement.

So, when I switch on my analog scope I don't see " a blur" - did you learn that term today? When I set my analog scope to 1mV/DIV and do the same with both digital scopes, the analog one has a nearly noise free beam deflected from left to right, my Rigol DS4012 produces so much noise caused by it's own ADC that 1mV/DIV is a joke! The HMO is very fine, the noise level is nearly perfect and much better than most Agilents because of better ADCs.

I've got a few scopes to compare - you also? And for me it's not a question of what I believe but what I see.

Did you ever work with analog scopes and know what you're talking about? I can't really imagine.

I recommend to read the doc I've attached above and try to understand it!

[nctnico]

I have a TDS744A scope with 'Instavu'. That should display up to 100k wfms/s. Ofcourse Rigol won't compare to an older real high end scope with a low noise input but that is no reason to say every DSO is crap. And yes I have used analog scopes. Actually my previous scopes could be used both analog and digital. But in the end I never really used the analog mode. In my experience the digital mode showed a much clearer image than the analog mode.

About digitizing CRTs: that is done in some high speed oscilloscopes with a scan converter tube.

[EEVblog]

I often find myself switching to an analog scope when the resolution of a DSO proves insufficient. A recent example was a low level burst of RF oscillation over part of the cycle of a repetitive low frequency signal. Completely invisible on the DSO (buried in the "digitization" noise) but clear as bell on the old Tek.

Yeah, like this:

I have a video uploaded and queued that shows how you can easily miss stuff on an analog scope that gets shown easily on a digital scope.
(common mode noise is the example I used)
Whether or not you want that stuff shown is another argument entirely, but it does show that analog scopes have disadvantages when it comes to this sort of stuff too.

[BravoV]

Its just a matter of time that analog scope will be gone one day, that is the undeniable fact.

[Lukas]

Aha! I can't use the zoom?! Really? I mean, when using a scope with an ADC of 1GS/s at max I assume that you understand that the time between two samples is 1ns, right? So you're zoom can't resolve more than showing you samples in a distance of 1ns, what happens between is not displayed, but interpolated if not switched off. An analog scope using 1ns can show any value between because there's no sampling but a continous steering of the beam. Disadvantage: you can't store it. Think about it.

That's only true for single shot signals. Even cheap Chinese are able to determine the timing relation between the sampling clock and the trigger to a resolution of 25ps, which gives you a timing resolution of 25ps when acquiring long enough.

[Gunb]

Its just a matter of time that analog scope will be gone one day, that is the undeniable fact.

Of course, and I wouldn't buy a new analog scope today. But that was not the issue, it's a fact that there are special purposes where a former
expensive analog scope might be better than cheap digital scope of today. Or do you know a new 500MHz scope for a moderate price of less then 1000,- EUROs
that you can use for ham radio?

That was my argumentation from the beginning without lowering the benefits of digital scopes, own myself a few.

[mikeselectricstuff]

Its just a matter of time that analog scope will be gone one day, that is the undeniable fact.

Of course, and I wouldn't buy a new analog scope today. But that was not the issue, it's a fact that there are special purposes where a former
expensive analog scope might be better than cheap digital scope of today. Or do you know a new 500MHz scope for a moderate price of less then 1000,- EUROs
that you can use for ham radio?

That was my argumentation from the beginning without lowering the benefits of digital scopes, own myself a few.

If you're on a limited budget, a cheap DSO plus a cheap old (originally expensive) analogue scope will give you the most functionality for your money.
A DSO needs to have high update rate and an intensity-graded display to fully replace an analogue one - I think the Rigol 2000 is  the cheapest new option unless you get a  very good deal on a used Agilent 5/6/7000 or tek DPO 

[Gunb]

Aha! I can't use the zoom?! Really? I mean, when using a scope with an ADC of 1GS/s at max I assume that you understand that the time between two samples is 1ns, right? So you're zoom can't resolve more than showing you samples in a distance of 1ns, what happens between is not displayed, but interpolated if not switched off. An analog scope using 1ns can show any value between because there's no sampling but a continous steering of the beam. Disadvantage: you can't store it. Think about it.

That's only true for single shot signals. Even cheap Chinese are able to determine the timing relation between the sampling clock and the trigger to a resolution of 25ps, which gives you a timing resolution of 25ps when acquiring long enough.

No it's generally valid even for cheap Chinese scopes and it doesn't depend on single shot.

- ADC with a sample rate of 1GS/s enables 1ns time interval between two samples - as given in my example
- ADC with a sample rate of 4GS/s enables 25ps time interval between two samples - as you mentioned.

But it doesn't influence the fact that a screen of limited pixels for example 800 pixels on the abscissa cannot display more than 800 pixels at once. Either the scope displays only each - lets assume - 10th sample of the sample memory on screen by transferring it to the display buffer or you have to zoom in until you see the samples in the distance of your 25ps, in this case your timebase is very small.

That's how all digital scope work, cheap Chinese ones as well as expensive Agilents, Teks or whatever.

In REFRESH mode digital scopes use double buffering, where one part of the memory is filled up with samples whilst the other part is displayed, which leads to the famous wfm/s limitation. Manufacturers as Agilent use special ASICs for generating high wfm/s with parallel processing where cheap scopes for hobby purposes use simple FPGAs which result in lower wfm/s.

In SINGLE SHOT mode the scope fills up the complete memory with samples and stops. Depending on the chosen timebase the ADC might work with its highest sample rate, in my example I assumed an 1GS/s ADC, so the memory is filled with samples in a distance of 1ns - you can't fill it up with a higher sample rate than the physical limits of the ADC offers - even not with cheap Chinese scopes. And you can easely check that by switching off the interpolation & zoom in until you see the samples. Measure the time interval between two subsequent samples, just that simple. Done a few times.

And we're not talking about random sampling, that's another construction and works for periodical signals only.