Analog Oscilloscope advantages?

[member_xyz]

I am looking for opinions about Old Analog Scope  advantages over Modern Digital storage Scope. Do old analog scopes have any superior technical or practically useful advantages over modern digital storage scopes?

For example, if I have or can afford to have a modern digital storage scope worth $2000-$5000, would there be any REAL useful benefit in having an old Tektronix 2465 or Tektronix 7000 analog scope. These old Tek scopes are arguably the best analog scopes ever made.

I am not interested in negative arguments about old analog scope such as reliability, size, weight or negative or lack of performance issues. I would only like to know about positive advantages if there are any.

Cheers.

[c4757p]

Keep in mind my digital scope is $600, not $5000, and I don't spend much time with the more expensive ones my school has, so I don't know how much better they are. However, one massive advantage that my 2445A has over my DS1102D is a decent X-Y mode. It's quite useful in analog-land. The Rigol's X-Y mode really sucks.

[c4757p]

I also find the signals much easier to look at on the screen - the lack of antialiasing combined with the low resolution makes the Rigol's screen very, very ugly - but I notice that the more expensive Agilents and such usually look a lot better. I find myself missing small peaks and whatnot on the digital screen if I don't zoom in because they blend in with the rest of the pixels.

[Conrad Hoffman]

IMO, if you do low noise analog stuff nothing can beat an old boat anchor Tek 545B and the 1A7A plug-in that goes down to (10?) uV per division with LP and HP filters. There's a similar plug-in for the 5000 and 7000 series.

Where the digital scopes shine is when you work with low frequencies, when you need data acquisition and when you need math or other functions. They're the only way to deal with non-repetitive inputs. The trouble I have with digital scopes is that the affordable ones aren't very good. Lousy vertical resolution and not enough memory. The top end (>$7k) leave analog in the dust for almost everything, but I can't afford 'em.

[w2aew]

In addition to very nice XY mode, you also get z-axis modulation as well.
No aliasing whatsoever.
Trace intensity gives good indication of frequent vs infrequent events.
Delayed triggerable dual timebase can be more flexible that a simple digital zoom in some cases.

It's tough to make too many arguments where an analog scope is superior, but it is very nice to have them around.  In my case, I use my analog scopes first, until I need the capability offered by a digital scope.  As a result, I typically use the analog scope more often than the digital scope.

[FenderBender]

They also look cooler....

....if that counts...

[w2aew]

They also look cooler....

....if that counts...

...and, nearly zero boot time...
...and you wouldn't be able to make the image like my avatar on a digital scope.  My avatar was made on a 30yr old Tektronix 485 analog scope in XYZ mode using a simple NTSC video conversion circuit.

[c4757p]

Trace intensity gives good indication of frequent vs infrequent events.

This makes it infinitely more intuitive in my opinion - what you see on the screen is more like what's actually happening in the circuit. I'm currently playing around with a flyback converter in LTspice, and because every dot on the graph is shown with the same intensity, the output voltage with ripple appears to average at around 19V - until you zoom in and see that it's mostly a steady 22V with just some negative pulses lasting a few nanoseconds (diode recovery time). A more "analog-style" display would show a line around 22V with a "fog" underneath it, which conveys the waveform much better without requiring me to play around with the zoom.

(Now, why none of the affordable/free SPICE simulators have a plot feature like this I don't know. It would be really easy to implement in software - much easier than in, for instance, a digital oscilloscope.)

Delayed triggerable dual timebase can be more flexible that a simple digital zoom in some cases.

This feature was completely new to me when I got my 2445A, having come from a crappy little 20 MHz piece of garbage with nothing anybody would ever call a "feature". Fell in love with the scope immediately.

As a result, I typically use the analog scope more often than the digital scope.

Yeah. I'd say that the digital scope is "objectively" superior, in that it has many useful features that no analog scope has, but when I don't need those features I feel no compulsion to use it. I'd survive with just a digital scope, but I find the analog one much more comfortable to use and I rarely need to leave it for the digital one.

[w2aew]

@c4657p
Well put, could agree more.

Another good application for analog scopes is the observation of the envelope of modulated RF signals.  Not nearly as clear or easy on a digital scope due to undersampling and aliasing.

[vk6zgo]

@c4657p
Well put, could agree more.

Another good application for analog scopes is the observation of the envelope of modulated RF signals.  Not nearly as clear or easy on a digital scope due to undersampling and aliasing.

Or looking at field rate on analog video signals!

Another neat feature on many analog 'scopes is the availability of "vert amp output".
This allows you to use the vertical amp as a variable gain buffer amplifier to drive various other instruments ,such as frequency counters from very low level or non standard impedance signals.

[BravoV]

They're the only way to deal with non-repetitive inputs.

This, the most important matter, if you can not live with that, then digital scope is the only choice and forget about analog.

For enthusiast/hobbyist perspective, ideally buy both, a good "working" old analog and a cheap Chinese digital one, then you are almost fully covered, just my own personal opinion. 

[robrenz]

IMO, if you do low noise analog stuff nothing can beat an old boat anchor Tek 545B and the 1A7A plug-in that goes down to (10?) uV per division with LP and HP filters. There's a similar plug-in for the 5000 and 7000 series.

I highly agree. HERE reply #142 is an example. Keep in mind that this scope has a signal out that can feed your digital scope. Where are you going to get a $185.00 differential amplifier that goes down to 10µV/div with a 5µV noise floor at 3kHz upper BW limit? Get your signal set up on the analog scope and then capture it on the digital scope with the digital scopes triggering if you need to capture a single shot event.

[GK]

Where are you going to get a $185.00 differential amplifier that goes down to 10µV/div with a 5µV noise floor at 3kHz upper BW limit? Get your signal set up on the analog scope and then capture it on the digital scope with the digital scopes triggering if you need to capture a single shot event.

5uV rms in a 3kHz noise bandwidth is an abysmal input-referred noise of 91.2nV Sqrt/Hz (a little less worse in a 3kHz -3dB bandwidth). You could better that by > an order of magnitude with a $15 instrumentation amplifier chip and a few bucks worth of passives as a preamplifier (using x1 probes only!) for your DSO.

[BravoV]

Where are you going to get a $185.00 differential amplifier that goes down to 10µV/div with a 5µV noise floor at 3kHz upper BW limit?

I'm afraid to say this statement is not true anymore, looking at current market price for used old analog scope, especially a good one like yours, for a complete set with the main frame might cost arm & leg.

[robrenz]

Where are you going to get a $185.00 differential amplifier that goes down to 10µV/div with a 5µV noise floor at 3kHz upper BW limit?

I'm afraid to say this statement is not true anymore, looking at current market price for used old analog scope, especially a good one like yours, for a complete set with the main frame might cost arm & leg.

I just bought those two on Oct 26 2012 for those prices. the mainframe included a 7A18 vertical amp and a 7B53A timebase all working for $150.00.  The 7A22 was also in perfect working order for $35.00.  If you are not in the US then I understand completely.

[robrenz]

5uV rms in a 3kHz noise bandwidth is an abysmal input-referred noise of 91.2nV Sqrt/Hz (a little less worse in a 3kHz -3dB bandwidth). You could better that by > an order of magnitude with a $15 instrumentation amplifier chip and a few bucks worth of passives as a preamplifier (using x1 probes only!) for your DSO.

It is a little better at 1MHz 16nV Sqrt/Hz.  If you duplicate all the features of the 7A22: adjustable upper and lower BW filters, 100000:1 CMRR, DC offset, infinite input impedance mode, 25V/div thru 10µV/div, 500V input capable, etc etc. you are going to spend more in parts than my whole setup cost. 

I would be interested in seeing your design for a simple differential preamp for a scope. If it was isolated and had 1X, 100X and 1000X amplification I think many would be interested in a kit or board.  From reading some of your posts you seem more than capable of doing this.

[JoannaK]

I would say that Analog scope usually has less features and less options per knob so it's easier to understand what's happening. Ok, digital ones have these auto-modes, but IMHO the user shoudl at least try to understand what's going on behind the scenes and learn to eyeball basic measurements and not just trust the digital scope tricks.

I'd say it's a bit like taking photos, automatic systems are quite ok most of the time. But in the end it should be the photographer who *knows* the light and makes the decisions, even though most of the time he/she can let go and tryst the camera to do Focus, apertue etc..

[c4757p]

Ok, digital ones have these auto-modes

Has anyone ever found these useful? The only thing I use it for on my Rigol is to align two channels to the quarter and three-quarter lines, since there's no detent like for the center line. Otherwise I almost always find that the signal is visible enough as soon as I plug in the cable to be just a knob-twirl away from complete visibility.

[JuiceKing]

Not mentioned but also so very useful is the way you can easily capture a screenshot off a DSO for your own records or to share. Of course, you can photograph an analog scope's screen but it's not nearly as convenient. Like the others I find a combination of a cheap DSO and 200-350MHz analog scope to cover the bases at the lowest cost.

[vk6zgo]

Where are you going to get a $185.00 differential amplifier that goes down to 10µV/div with a 5µV noise floor at 3kHz upper BW limit?

I'm afraid to say this statement is not true anymore, looking at current market price for used old analog scope, especially a good one like yours, for a complete set with the main frame might cost arm & leg.

I just bought those two on Oct 26 2012 for those prices. the mainframe included a 7A18 vertical amp and a 7B53A timebase all working for $150.00.  The 7A22 was also in perfect working order for $35.00.  If you are not in the US then I understand completely.

Or Australia!
Apart from the "dreamers" on eBay,the prices of 7000 series Tektronix 'scopes are pretty much in the doldrums in this country.
I bought my 7613 a few years back for $A135,& I saw a fully loaded higher spec 7000 series unit at the local Hamfest this year for $ A150.
Many people don't like them because they are large & old,but in terms of "bang for the buck" they are hard to go past.

[tinhead]

I am looking for opinions about Old Analog Scope  advantages over Modern Digital storage Scope. Do old analog scopes have any superior technical or practically useful advantages over modern digital storage scopes?

a 15yrs old Tek TDS7xx series DPO is better than all these analog scopes together. A modern DSO, which is for sure not
an cheap Rigol with 320x240 resolution, have no issues with 99% of all "the magic things" you can do with analog one.
Of course you have to spend at least ~1k USD, but you will get warranty and extra features which are non existing
on analog gears.

I've finally sold my analog scopes (oh well, one left - as i repaired it i will sell it as well), i don't see any reason to
have "magic low noise" gears in use where the whole environment everywhere is overloaded with so much digital
broadband crap that all the analog low noise gears are not working anymore peoperly.

[GK]

5uV rms in a 3kHz noise bandwidth is an abysmal input-referred noise of 91.2nV Sqrt/Hz (a little less worse in a 3kHz -3dB bandwidth). You could better that by > an order of magnitude with a $15 instrumentation amplifier chip and a few bucks worth of passives as a preamplifier (using x1 probes only!) for your DSO.

It is a little better at 1MHz 16nV Sqrt/Hz.  If you duplicate all the features of the 7A22: adjustable upper and lower BW filters, 100000:1 CMRR, DC offset, infinite input impedance mode, 25V/div thru 10µV/div, 500V input capable, etc etc. you are going to spend more in parts than my whole setup cost. 

I would be interested in seeing your design for a simple differential preamp for a scope. If it was isolated and had 1X, 100X and 1000X amplification I think many would be interested in a kit or board.  From reading some of your posts you seem more than capable of doing this.

If you are just making a low noise, high gain pre-amp for a CRO or DSO the switched gain / frequency compensated input attenuator facility is superfluous. For high voltage signals you just use the CRO/DSO as normal. A differential pre-amp with specs along the lines you describe (but with much better noise than the old 7A22) is something I'd like on my bench as well. ~1nV sqrt/Hz input-referred noise with bootstrapped differential jfet inputs wouldn't be too difficult to achieve. I'd make a hybrid discrete/IC differential input stage with parallel BF682 jfets. These are RF parts made for AM radios but also have a very low (<1kHz) 1/f noise corner frequency. Despite what many would say, 100dB gain with, say 10 or 20MHz ac bandwidth is relatively trivial on a spacious, well layed out PCB with the gain distributed over several stages. Unfortunately right now I really need another project like a hole in the head, but I'll think about it a bit. 

[robrenz]

I hereby stake my claim to purchase the first board/kit when ready 

[alm]

I highly agree. HERE reply #142 is an example. Keep in mind that this scope has a signal out that can feed your digital scope.

These signal outs were usually designed for driving things like counters, and not optimized for high-fidelity low-noise. I believe they convert the (internal) differential signal to single-ended by just throwing away half of it, for example. Did you actually try it and find the performance satisfactory?

If you are just making a low noise, high gain pre-amp for a CRO or DSO the switched gain / frequency compensated input attenuator facility is superfluous. For high voltage signals you just use the CRO/DSO as normal.

The differential signal may be in the mV/uV range, but the common mode voltage may be much larger. That's why the CMRR was so high. The 7A22 also serves as (a low-bandwidth) general-purpose differential plugin with excellent CMRR (either 10,000:1 or 100,000:1).

The other interesting plugin in my opinion is the 7A13 (similar to the later Preamble differential amp). This was a 100 MHz differential plugin with normal sensitivity, but with excellent overload recovery specs, which is useful for measuring parameters like transistor saturation. It could connect one of the channels to a variable DC voltage, allowing huge DC offsets (eg. 10 V at 1 mV/div) without using DC coupling.

[member_xyz]

Thanks Guys,

Some interesting responses.

I can definitely see the advantage of Teks 7A22 diff. amp low voltage input plug in module.

I have done a quick Google search for similar stand alone generic oscilloscope preamp and I can't see many around, they cost big $$$$. About the lowest cost is OSP-1 from Advanced Research Instruments for about $1K and they include 2 probes (I assume matched).
link http://www.aricorp.com/OSP-1.html.

Does anyone know of any lower cost stand alone scope differential preamps with adjustable gain and a variable filter, would require input down to at least 10uV, reasonably high CMR, bandwidth could be lower down to 100Khz?

Cheers.