Author Topic: Quality PC-Based oscilloscope?  (Read 43568 times)

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Offline HiredMind

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Quality PC-Based oscilloscope?
« on: January 19, 2013, 04:19:04 am »
Hi All,

I know most PC-based oscilloscopes are pretty much garbage.  But I'm wondering, does anyone make a pc-based scope with pro-level electronics on par with Tek/Agilent/Rigol, etc?   To give an example: If I can't find an adequate PC-based one, my next choice would probably be a Rigol DS2072)

Since I'm a programmer I have only needed a scope to make sure microcontrollers were doing what they were told, baud rates were set correctly, etc.   But the projects I'm working on are getting faster, and I have some ideas for some projects that will require the ability to see accurate analog waveforms.   

I know what you're saying: Why not just get a regular DSO?   Well as I said I'm a programmer, so I spend most of my working life in front of 4 gigantic monitors and screaming fast development PCs.  And I'm much faster with a mouse than those buttons and knobs on a regular scope.  I'd love to have the quality capture abilities of a high-end scope without paying for the tiny scope display.


« Last Edit: January 19, 2013, 04:23:41 am by HiredMind »
 

Offline mianchen

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Re: Quality PC-Based oscilloscope?
« Reply #1 on: January 19, 2013, 04:28:30 am »
picoscope seems to be everywhere. and smartscope? they both have adverts on eevblog's main page.
 

Offline Bored@Work

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Re: Quality PC-Based oscilloscope?
« Reply #2 on: January 19, 2013, 05:15:53 pm »
Tiny screens? Well, if you pay enough you get oscilloscopes with large screens, too.

But let's stay with the entry level ones. They start with 8 bit DACs. That's 256 possible values. How much vertical resolution do you think you need to get 256 different values all nicely displayed?

But even then, you get entry level oscilloscopes with remote interfaces so you can operate them from a PC. Although the PC software is usually rubbish. You also get some with a monitor output.

And while you might be faster with a mouse, there is a big difference between having to adjust a value with a stupid slider (I hate sliders, even in normal applications), compared to a real knob. And really, when you prod around in some circuit do you think it will work out that you sit in front of your four screen tanning bed, reaching over to the circuit with one hand to probe something, watching your screens and the circuit  at the same time, cruising around with the mouse and typing something with your third hand?
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Offline saturation

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Re: Quality PC-Based oscilloscope?
« Reply #3 on: January 19, 2013, 06:29:59 pm »
As B@W said, you can control modern test instruments that have bus/network options, with a PC or use defacto standard Labview environment as a controller.  National Instruments is a segment leader. 

http://www.ni.com/digitizers/

Agilent has a few instruments here:

http://www.home.agilent.com/en/pc-1418982/oscilloscope?pm=SC&nid=-34492.0&cc=US&lc=eng

When monitoring data status, dedicated instruments provide a secondary portal, if not a control override, to confirm or control the data input shown via PC.  However, if your need includes data collection, clearly some PC approach is optimal, and if your need is fully focused on data collection over much time, a bus based device may save lab space and possibly cost. 

Dedicated instruments are faster to use in tasks were data collection is rarely required, like in hardware design, debug and repair and overall its popularity still shows as you can see from the offering of the big T&M makers like Agilent.


Best Wishes,

 Saturation
 

Offline frenky

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Re: Quality PC-Based oscilloscope?
« Reply #4 on: January 19, 2013, 06:52:02 pm »
Hi.

I bought USB scope Hantek 5200A some years ago and was very happy with it until it stopped working.  :--

You can get it on Aliexpress for 340$ with DHL delivery.

Teardown photos:
https://sites.google.com/site/colorcodeshtml/home/DSO-5200A_top_500K.jpg
https://sites.google.com/site/colorcodeshtml/home/DSO-5200A_bottom_500K.jpg

The specs:
http://www.hantek.com/english/produce_list.asp?unid=66

« Last Edit: January 19, 2013, 07:09:48 pm by frenky »
 

Offline Lightages

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Re: Quality PC-Based oscilloscope?
« Reply #5 on: January 19, 2013, 07:19:55 pm »
HiredMind:

Are you sure what you really want is not a logic analyzer? A scope is still useful for seeing the signal, but if you need to be looking a bus signals and logic......
 

Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #6 on: January 19, 2013, 07:30:19 pm »
Don't bother to buy a scope with more than 100MHz bandwidth if it has no 50 Ohm input impedance setting. Trying to measure a signal over 100MHz with a 1:10 1M Ohm probe is a futile attempt. The capacitance of the probe is way to high.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline zaoka

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Re: Quality PC-Based oscilloscope?
« Reply #7 on: January 19, 2013, 09:57:34 pm »
Picoscope has 5 year warranty.
 

Offline HiredMind

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Re: Quality PC-Based oscilloscope?
« Reply #8 on: January 20, 2013, 12:58:55 am »
Are you sure what you really want is not a logic analyzer? A scope is still useful for seeing the signal, but if you need to be looking a bus signals and logic......

Actually I have one: an MSO-19 from Link Instruments.  It's been fine for my needs so far, as long as I make sure I stay within it's limitations.  But I will be needing a real O-scope soon, and so far my frontrunner is the Rigol DS2072.  But if I can get the same capabilities in a PC-based unit, that's my preference.
 

Offline HiredMind

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Re: Quality PC-Based oscilloscope?
« Reply #9 on: January 20, 2013, 01:12:45 am »
Tiny screens? Well, if you pay enough you get oscilloscopes with large screens, too.

I've never seen any DSO with a 32" monitor.  Have you?

But let's stay with the entry level ones. They start with 8 bit DACs. That's 256 possible values. How much vertical resolution do you think you need to get 256 different values all nicely displayed?

The point of asking the question was to _not_ stick with entry level ones.  I mentioned the Rigol DS2072 - that's not an entry level scope.  It's not the $23,000 Tektronix 24 channel LA that I used at work a few years ago, but it's not entry level. (And I'm not looking to sell my car to buy a scope either  :-DD ).

And while you might be faster with a mouse, there is a big difference between having to adjust a value with a stupid slider (I hate sliders, even in normal applications), compared to a real knob.

Not for me.  After 20 years at a keyboard/mouse, that combination is basically an extension of my hands at this point.

And really, when you prod around in some circuit do you think it will work out that you sit in front of your four screen tanning bed, reaching over to the circuit with one hand to probe something, watching your screens and the circuit  at the same time, cruising around with the mouse and typing something with your third hand?

So, keyboard&mouse/Scope/Circuit is easier to control than keyboard&mouse/Circuit?   Sorry I just don't buy that.  In any case, having worked with both configurations, I prefer the latter. 
 

Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #10 on: January 20, 2013, 01:35:52 am »
IMHO it depends greatly on the user interface whether a mouse/keyboard combo works better than the controls. Because a scope has many buttons with specific functions you have a bunch of hot-keys in a well choosen layout and there is ofcourse the on-screen menu. The more expensive scopes have touchscreens which make live even more easier.

I have played a bit with Picoscope but I still prefer a real scope. The waveform update rate isn't fantastic and I recall they choose the ranges in a weird way. Instead of 1-2-5 V / div they choose 1-2-5V peak-peak which gives you an odd reading when you are used to a normal scope. If you are looking at a 5V signal you need to select 10V peak-peak which throws away half the resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.
« Last Edit: January 20, 2013, 01:44:29 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Lightages

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Re: Quality PC-Based oscilloscope?
« Reply #11 on: January 20, 2013, 01:53:37 am »
I am a computer nerd. One of my best friends is a computer nerd. We both agree there is nothing like a direct hands on control that is where you are looking and "feeling". You can say it isn't so but believe me if you use a full "normal" hardware oscilloscope you will never want to touch a virtual one again for quick and intuitive control.

Get the Rigol DS2072 or similar and don't look back. When you are doing logic analysis it is one thing, but doing things that an oscilloscope can do needs a real interface not a virtual one. If someone made a USB interface with the knobs and buttons to control a virtual scope that would be different.
 

Offline helloworld922

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Re: Quality PC-Based oscilloscope?
« Reply #12 on: January 20, 2013, 02:15:42 am »
Quote
I've never seen any DSO with a 32" monitor.  Have you?

Some O-scopes have video output, or some sort of PC interface so you can hook the o-scope up to your PC and view waveforms on there.

Personally I've also been considering the DS2000 series because it seems like a very good scope for the price. If I had a second pick I might go with a PicoScope PC-based O-scope (maybe a 2207 or a 2208).

Reasons for getting the Rigol:

1. It has a higher max sample rate (2 GS/s single channel, 1 GS/s dual channel compared to 1 GS/s single channel, 500 MS/s double channel)
2. Larger memory depth (14 MPts vs. 32 kPts standard, upgrade-able to 56 MPts).
3. I'm not actually tied to my PC if I need to bring my scope to do field work. Of course, you could just as easily bring a laptop and the Picoscope around.
4. I kind of like the knobs/buttons on the actual scope, but that's a personal preference. With the Rigol you can of course interface with the PC and write your own software if you want to control it from the PC. The Picoscope only has a PC-based interface.
5. Much higher waveforms capture rate on the Rigol
8. Higher voltage input ranges and most likely better over voltage protection. The rigol is CAT II 100Vrms rated, while the Picoscope only states 100V max (AC+DC offset) without any CAT rating.

Reasons to get the Picoscope:

1. Availability of Linux drivers (if that's what you use, I mainly use Windows so it doesn't matter either way for me)
2. Nice and small package, you might be able to tuck it into your laptop bag or a backpack and bring it with you to do field work.
3. Much nicer PC interface without having to write your own code.
4. Comes with a waveform generator (I'm not sure if the 2207 comes with the AWG function, but it looks like it does)
5. Higher bandwidth. Even upgrading to the 200 MHz 2208 is under $1000 USD while the equivalent bandwidth Rigol is ~$1600 USD.
6. Several features are standard such as serial decoding and advanced triggers which on the Rigol are a few hundred bucks extra.
7. Following marmad's experiences with the Rigol DS2000 series so far, it seems like there are quite a few bugs in the Rigol firmware. I'm not sure how many of them are "critical", but it is a newer product than the Picoscope offering. I can't comment on the reliability of the Picoscope software, though since I haven't heard anything about it.
8. For what it's worth, Picotech is a British company with a claimed 20 years experience in the business. Of course Rigol's been around quite a while, too (~15 years) and from what I've been researching about them don't necessarily provide poor quality scopes, even if they are based out of Beijing.

IMO which ever route you go you'll probably end up with a good quality scope. I still have quite a bit of time to save up money before I come to a decision on which route I want to take. It sounds like ultimately it's going to be you who's using the scope, not a co-worker, Dave, or anyone else here. If there's a compelling reason for you to not like one or the other, even if it goes against the grain of what others prefer, so be it. Just be prepared to live with the decision you make (or return the scope if you find you really don't like it).
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #13 on: January 20, 2013, 03:28:52 am »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works. There are also commercial ADC products which use this same technique (adding dither internally) to achieve much higher resolution from low res hardware.

Tektronix was able to achieve a 250MHz BW into a 1M 20pF input all the way back in the 1970's (see Tek 475A), so 15pF is very reasonable. If you really understand scope inputs, then you recognize that the input impedance is not a simple capacitive load. They typically have series resisters that make them look much more like 50 ohms at high frequencies. Scopes with 1M inputs are typically equalized to achieve their specified bandwidth with a 50 ohm source, terminated with an external 50 ohm load at the input. Take a look at a cal procedure.

@HiredMind:

Don't let any jaded old goat steer you away from PC-based instruments. They can be quite good (though not all are). If you already spend much of your time in front of a PC (which describes many modern engineers), they can be very convenient to use. I would just say, look at the software *very* carefully before you choose. I've used many PC-based instruments, and will tell you for sure that some of the support software is truly awful. When evaluating different products, don't just click around in the GUI and assume it will do the job. Go through several real-world setups to get a good idea of how convenient they will be to use. Good luck with your decision.
 

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Re: Quality PC-Based oscilloscope?
« Reply #14 on: January 20, 2013, 04:00:33 am »
I'm sceptical about those 12 bits. Common scopes with 8 bits of vertical resolution might have an ENOB of around 5 at their rated BW. You'd need a lot of oversampling to get this close to 12 bits.

The 50 Ohm generator terminated into 50 Ohm represents a 25 Ohm source impedance. Is the impedance of your DUT 25 Ohm or less? 10x high-Z passive probes will present a similar load. Passive high-Z probing is still feasible at 250 MHz, but loading is starting to become an issue. A 50 Ohm input impedance is a welcome option, although a good feed-through terminator ($50 each) might also work.
 

Offline mazurov

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Re: Quality PC-Based oscilloscope?
« Reply #15 on: January 20, 2013, 04:36:13 am »
Agilent makes 2700 series USB oscilloscopes -> http://www.home.agilent.com/en/pc-1418982/usb-modular-oscilloscope?nid=-34492.0.00&cc=US&lc=eng , they are very nice, but expensive comparing to other PC-based scopes ( but have much better characteristics ).

I do a lot of microcontroller work and the scope is rarely necessary; you'll need one to check your oscillator and to take a look (once) at high-speed signals, if you have any. In both cases, any old 350-500MHz analog instrument will do. The rest of general troubleshooting is done using $30 Open Bench Logic sniffer LA.
 

Offline seattle

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Re: Quality PC-Based oscilloscope?
« Reply #16 on: January 20, 2013, 05:14:46 am »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works. There are also commercial ADC products which use this same technique (adding dither internally) to achieve much higher resolution from low res hardware.


It takes 4X oversampling to pick up an extra bit, so there are some pretty steep severe bandwidth limitations compared to a native 12-bit converter running at 100Msps.
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #17 on: January 20, 2013, 05:36:35 am »
I'm sceptical about those 12 bits. Common scopes with 8 bits of vertical resolution might have an ENOB of around 5 at their rated BW. You'd need a lot of oversampling to get this close to 12 bits.

Sure the ENOB will be lower than the hardware's resolution. A good scope with 200MHz bandwidth would be capable of 6 bits or better. I've seen 7.1 bits claimed by Agilent for their lower bandwidth scopes. But that wasn't the discussion. With an appropriate level of oversampling, an 8 bit converter can give you 12 bits exactly as well as it can give you 8 bits without said oversampling. It's only a matter of how many samples you average for each resulting value. For each factor of 4, you get 1 more bit of resolution.

The 50 Ohm generator terminated into 50 Ohm represents a 25 Ohm source impedance.

Yes, it results in a 25 ohm source whether the termination is internal or external to the scope. We'd like to think that a scope with a selectable internal 50 ohm load would have better response, but it's unlikely for scopes with a bandwidth in the 200MHz range. Why? Because scopes in this range typically just take the existing 1M input and switch a 50 ohm load on top of it. That means the capacitance of the 1M input is still there in parallel with the 50 ohm load, just as it would be if you used an external 50 ohm load. The result is far from a perfect 50 ohm termination. Measure the return loss of your 50 ohm scope input some time if you doubt it. They're typically atrocious at rated bandwidth.
 

Offline free_electron

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Re: Quality PC-Based oscilloscope?
« Reply #18 on: January 20, 2013, 05:46:56 am »
Don't bother to buy a scope with more than 100MHz bandwidth if it has no 50 Ohm input impedance setting. Trying to measure a signal over 100MHz with a 1:10 1M Ohm probe is a futile attempt. The capacitance of the probe is way to high.
You need to go and explain that to Agilent... They still sell scopes with 1:10 passive probes rated for 500MHz ... With 1M input on the scope. The probe is 10Mohm input. n2873a for example. Or N2890. These probes have rise times of 700pS so they are more than capable of doing that...

Even their older discontinued probes like the 116x series or the  :10073 did that 20 years ago.

I agree the chingchangchong probes sold for 20$ on fleabay hardly clamber above 100MHz .. But a real probe doesn't blink at 500MHz ... Then again, these real probes cost more than the base Rigol oscilloscope.... Buy 4 probes, get scope free  ;D
« Last Edit: January 20, 2013, 05:54:26 am by free_electron »
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Offline HiredMind

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Re: Quality PC-Based oscilloscope?
« Reply #19 on: January 20, 2013, 11:09:08 am »
Agilent makes 2700 series USB oscilloscopes -> http://www.home.agilent.com/en/pc-1418982/usb-modular-oscilloscope?nid=-34492.0.00&cc=US&lc=eng , they are very nice, but expensive comparing to other PC-based scopes ( but have much better characteristics ).

Oh wow - I've been trawling the Agilent website off and on for weeks and didn't even notice them.  Thanks Mazurov, that looks very promising, I'll check them out.
 

Online Marco

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Re: Quality PC-Based oscilloscope?
« Reply #20 on: January 20, 2013, 01:02:15 pm »
That means the capacitance of the 1M input is still there in parallel with the 50 ohm load, just as it would be if you used an external 50 ohm load. The result is far from a perfect 50 ohm termination. Measure the return loss of your 50 ohm scope input some time if you doubt it. They're typically atrocious at rated bandwidth.
Would it be possible to bodge some sort of RLC terminator which avoids this?
« Last Edit: January 20, 2013, 01:09:23 pm by Marco »
 

Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #21 on: January 20, 2013, 01:49:40 pm »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works.
I put that to the test on my Tektronix scope and as expected it looks fine for a sine wave but a triangle or sawtooth get distorted due to inperfections of the noise and non-linearities in the ADC. If oversampling where a good option then we would all be using 1 bit AD/converters (comparators) and noise. The reality is that you would need extremely good noise and a comparator with an extremely low input offset and enormous gain. So unless you have perfect noise and use an ADC where the comparators are better then required for the ADCs resolution you have a chance of actually getting more resolution from an ADC.

The problem is that people teaching sampling theory tell nice stories about how you can shape noise into bits totally forget about the practical application of that theory. Some theories just don't work well in real world situations!
Quote
There are also commercial ADC products which use this same technique (adding dither internally) to achieve much higher resolution from low res hardware.
Which are called delta-sigma or pipelined ADCs. And there is also the succesive approximation ADC which basically is a comparator.

Quote
Tektronix was able to achieve a 250MHz BW into a 1M 20pF input all the way back in the 1970's (see Tek 475A), so 15pF is very reasonable. If you really understand scope inputs, then you recognize that the input impedance is not a simple capacitive load. They typically have series resisters that make them look much more like 50 ohms at high frequencies. Scopes with 1M inputs are typically equalized to achieve their specified bandwidth with a 50 ohm source, terminated with an external 50 ohm load at the input. Take a look at a cal procedure.
The people at Picoscope think they can do the same with standard 1:10 1M Ohm probes. Just look at their brochure. 15pf at 250MHz is around 42 Ohms so you may get lucky with the Picoscope and 50 Ohm terminators at the input but if you are into RF then I'd suggest equipment with 50 Ohm inputs. My rule of thumb is to go for 50 Ohm cabling and proper HF probes (active or passive divider) for measuring anything over 100MHz.
« Last Edit: January 20, 2013, 02:27:26 pm by nctnico »
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Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #22 on: January 20, 2013, 02:17:53 pm »
Don't bother to buy a scope with more than 100MHz bandwidth if it has no 50 Ohm input impedance setting. Trying to measure a signal over 100MHz with a 1:10 1M Ohm probe is a futile attempt. The capacitance of the probe is way to high.
You need to go and explain that to Agilent... They still sell scopes with 1:10 passive probes rated for 500MHz ... With 1M input on the scope. The probe is 10Mohm input. n2873a for example. Or N2890. These probes have rise times of 700pS so they are more than capable of doing that...
No, they are not. The probe capacitance has too much influence on the signal. I measured the same signal (a 40MHz square wave from a 50 Ohm source) with a 1:10 probe (top) and a passive divider probe (bottom). I must say that the top signal will look more like a square wave with the probe you mentioned but the fact is the signal will still be distorted due to probe loading. The n2873a has a capacitance of 9.5pf. My passive probe's capacitance is less than 1pf.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline helloworld922

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Re: Quality PC-Based oscilloscope?
« Reply #23 on: January 20, 2013, 07:56:34 pm »
Quote
If oversampling where a good option then we would all be using 1 bit AD/converters (comparators) and noise

The key factor which makes oversampling work is that the noise has to be enough to trigger at least 1 bit digitization changes. If your signal has +/-1mV noise and your minimum ADC resolution is 4mV (what it is for a 1V reference with 8-bit resolution), you won't get increased accuracy no matter how many samples you read.

Also your effective sampling rate drops because you need multiple smaller samples to try and recreate the higher-accuracy "sample". To get a 1 GS/s 8-bit effective resolution using a 1-bit ADC and appropriate noise, you would need to sample at 65.536 Terra samples per second! With a 1 GS/s 1-bit ADC, you only get ~15.259 KS/s at 8-bits effective accuracy..
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #24 on: January 20, 2013, 08:00:13 pm »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works.
I put that to the test on my Tektronix scope and as expected it looks fine for a sine wave but a triangle or sawtooth get distorted due to inperfections of the noise and non-linearities in the ADC.

So did I. Images below for 8 bit normal sampling and 12 bit oversampling. No distortion noted. It just cleans up the noise as you'd expect for any form of averaging.
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #25 on: January 20, 2013, 08:14:58 pm »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.
Tektronix was able to achieve a 250MHz BW into a 1M 20pF input all the way back in the 1970's (see Tek 475A), so 15pF is very reasonable. If you really understand scope inputs, then you recognize that the input impedance is not a simple capacitive load. They typically have series resisters that make them look much more like 50 ohms at high frequencies. Scopes with 1M inputs are typically equalized to achieve their specified bandwidth with a 50 ohm source, terminated with an external 50 ohm load at the input. Take a look at a cal procedure.
The people at Picoscope think they can do the same with standard 1:10 1M Ohm probes. Just look at their brochure. 15pf at 250MHz is around 42 Ohms so you may get lucky with the Picoscope and 50 Ohm terminators at the input but if you are into RF then I'd suggest equipment with 50 Ohm inputs. My rule of thumb is to go for 50 Ohm cabling and proper HF probes (active or passive divider) for measuring anything over 100MHz.

The topic here is about PC-based vs standalone scopes. Picoscope is not claiming anything that Agilent and Tek haven't been claiming for decades. Everyone recognizes that 10:1 1M probes have non-zero input capacitance, and that any capacitance will represent a load to a high speed signal. But it has nothing to do with whether the brand on the box is Tek or Picoscope. It's just a reality. Your "passive probe", if it is 10:1 would have an input impedance of 500 ohms, which would represent severe loading for many circuits. There are also active probes which reduce the tip capacitance to <1pF, but they're expensive and not everyone needs to spend that kind of money to get good results. People have been doing quite a lot with standard probes for many years.
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #26 on: January 20, 2013, 08:26:57 pm »
Quote
If oversampling where a good option then we would all be using 1 bit AD/converters (comparators) and noise
The key factor which makes oversampling work is that the noise has to be enough to trigger at least 1 bit digitization changes. If your signal has +/-1mV noise and your minimum ADC resolution is 4mV (what it is for a 1V reference with 8-bit resolution), you won't get increased accuracy no matter how many samples you read.

In broad-band devices like scopes, there's rarely if ever an issue of not having enough noise. You can always sum extra noise at the ADC input to get the required level of dither, so even a perfectly clean signal could be oversampled and averaged. You can keep the resulting noise level low by summing in a predictable dither pattern, and then subtracting it back out of the digitized result.

Quote
If oversampling where a good option then we would all be using 1 bit AD/converters (comparators) and noise
To get a 1 GS/s 8-bit effective resolution using a 1-bit ADC and appropriate noise, you would need to sample at 65.536 Terra samples per second! With a 1 GS/s 1-bit ADC, you only get ~15.259 KS/s at 8-bits effective accuracy..

Exactly. That's one reason we don't see 1 bit converters used in scopes, but we do see this technique commonly used to increase resolution in scopes.
« Last Edit: January 20, 2013, 08:34:59 pm by JoeyP »
 

Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #27 on: January 20, 2013, 09:10:40 pm »
resolution. On a normal scope you'd choose 1V/div where the signal would fill 6 (out of 8 ) divisions. And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

Of course they can...

Tektronix, Agilent and others have been getting 12bits from 8 bit converters for many years. Just over-sample and average. Input noise provides the needed dithering. That's how their "high res" mode works.
I put that to the test on my Tektronix scope and as expected it looks fine for a sine wave but a triangle or sawtooth get distorted due to inperfections of the noise and non-linearities in the ADC.

So did I. Images below for 8 bit normal sampling and 12 bit oversampling. No distortion noted. It just cleans up the noise as you'd expect for any form of averaging.
Hmm, you expect 12bit (that is 4096 steps) to show in an image about 350 pixels high? The whole extra bits in oversampling mean nothing when a scope's display hasn't substantially more pixels in height than the number of steps of the ADC. So Tek, Agilent, et all can safely claim their high resolution modes add several bits to an 8 bit ADC with about 7 ENOB without customers ever knowing the only thing they get is noise suppression. Just look what happens if I put my Tek scope to the test to see what it does with a signal about 3 LSB in height. The first image is the original signal, the second is the same signal attenuated 100 times and then enlarged 100 times. If the 10 bit high resolution mode would work as promised I'd expect a much better picture.

« Last Edit: January 20, 2013, 09:15:17 pm by nctnico »
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Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #28 on: January 20, 2013, 09:17:31 pm »
Hmm, you expect 12bit (that is 4096 steps) to show in an image about 350 pixels high?

No, I expect to see the distortion that you claim to exist in a practical application for which the scope was designed. I don't.

happens if I put my Tek scope to the test to see what it does with a signal about 3 LSB in height. The first image is the original signal, the second is the same signal attenuated 100 times and then enlarged 100 times.

So this is how you think people normally use scopes? A signal of 3LSBs p-p expanded 100x? Sounds pretty silly to me.
 

Offline tinhead

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Re: Quality PC-Based oscilloscope?
« Reply #29 on: January 20, 2013, 09:27:41 pm »
If the 10 bit high resolution mode would work as promised I'd expect a much better picture.

nah, you forgot something. Change now to "Sample" mode and compare result to your second picture.
And now you can see that Hi-Res is working. If you fair enough, make screenshot of hat i'm talking about and post it here.

EDIT: i did for you, in sample mode as expected 100x times attenuated signal is not readable, in hi-res it is much better.

« Last Edit: January 20, 2013, 09:43:42 pm by tinhead »
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Offline helloworld922

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Re: Quality PC-Based oscilloscope?
« Reply #30 on: January 20, 2013, 10:00:55 pm »

In broad-band devices like scopes, there's rarely if ever an issue of not having enough noise. You can always sum extra noise at the ADC input to get the required level of dither, so even a perfectly clean signal could be oversampled and averaged. You can keep the resulting noise level low by summing in a predictable dither pattern, and then subtracting it back out of the digitized result.


I was just pointing out some potential pitfalls with using over-sampling, and that the required "noise" level will need to be larger for a lower resolution ADC, or a larger reference voltage. Yes, you can compensate for this required gap by either adding artificial noise or amplifying the input signal so any natural noise can be picked up by the ADC (one of the reasons you want a signal's peak to peak to match as closely the peak to peak range of the ADC).
 

Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #31 on: January 20, 2013, 10:08:57 pm »
Hmm, you expect 12bit (that is 4096 steps) to show in an image about 350 pixels high?

No, I expect to see the distortion that you claim to exist in a practical application for which the scope was designed. I don't.

happens if I put my Tek scope to the test to see what it does with a signal about 3 LSB in height. The first image is the original signal, the second is the same signal attenuated 100 times and then enlarged 100 times.

So this is how you think people normally use scopes? A signal of 3LSBs p-p expanded 100x? Sounds pretty silly to me.
Which means you agree with me that claiming to get more bits from an ADC is safe to do for a scope manufacturer because the whole claim is useless to begin with because the screen's resolution isn't high enough to show the errors.  Or you just miss the point entirely... see below.

@tinhead:
I did that but I don't have the screenshot (note the date on the other screenshots, I have posted these pictures before). What you get is a few dots from which you can clearly see the scope's ADC resolution. Hi-res mode does it's best to dream up a signal which could be there but without the actual information the scope's hi-res algorithm's guess is as good as mine. I don't need the scope to guess for me though, I want it to show me what it sees at its inputs. Ofcourse you'd crank the V/div down to get a good view at such a small signal instead of zooming in but that is besides the point that I'm trying to make here and that is that you cannot get more resolution from an ADC than what it has been designed for by just oversampling c.q. averaging.
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Offline tinhead

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Re: Quality PC-Based oscilloscope?
« Reply #32 on: January 20, 2013, 10:30:50 pm »
the point that I'm trying to make here and that is that you cannot get more resolution from an ADC than what it has been designed for by just oversampling c.q. averaging.

of course you can, you can see it even on yuor DSO. When you above the  hi-res limit the signal will be not better
(simply use 10x att. and 10x zoom for better view).
« Last Edit: January 20, 2013, 10:33:01 pm by tinhead »
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Offline tinhead

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Re: Quality PC-Based oscilloscope?
« Reply #33 on: January 20, 2013, 11:36:34 pm »
I agree the chingchangchong probes sold for 20$ on fleabay hardly clamber above 100MHz .. But a real probe doesn't blink at 500MHz ... Then again, these real probes cost more than the base Rigol oscilloscope.... Buy 4 probes, get scope free  ;D

oh yeah, there are some really funny probes on ebay. Sometimes it's simply question of compensation range.
Example below, Hantek P6200 cheap blah blah probe. Rated 200MHz, which is seems to be the truth on a Hantek
DSO (with a +2dBm peak ~150MHz). On a different scope (TDS 7xx series) it's simply useless :

Hantek P6200 200MHz x1/x10 Probe

  1MHz      0dBm
 25MHz   +1.2dBm
 50MHz    +2.0dBm
 75MHz  +2.8dBm
100MHz  +4.3dBm
125MHz  +5.4dBm
150MHz  +5.8dBm
175MHz  +6.4dBm
200MHz  -7.1dBm
225MHz  -7.2dBm
250MHz  -7.1dBm

this is the best result i was able to get. Sure, such probe costs 25USD, but that's already 25USD too much
(except someone need a cheap probe for 'DC').


Some other probes (as well tested on TDS7xx):

TESTEC TT-HF512 500MHz x10 Probe

  1MHz    0dBm
 50MHz   +0.2dBm
100MHz  +0.0dBm
150MHz  -0.6dBm
200MHz  +0.2dBm
250MHz  +1.2dBm
300MHz  +2.9dBm
350MHz  +2.3dBm
400MHz  +1.1dBm
450MHz  -0.8dBm
500MHz  -1.7dBm
550MHz  -3.0dBm



TEXAS 250/II 250MHz x10 Probe

  1MHz      0dBm
 50MHz   -0.2dBm
100MHz  +0.1dBm
150MHz  +1.3dBm
200MHz  +1.5dBm
250MHz  +2.3dBm
300MHz  +1.3dBm
350MHz  +0.2dBm
400MHz  -1.8dBm
450MHz  -3.1dBm
500MHz  -3.8dBm


TESTEC HF512 is about 130USD, so not a cheap one anymore. What not funny, Testec was not able to provide me
any detailed informations about frequency response nor impendance vs. frequency dependency. For an german
manufacturer a real pain in the ass. At least their probe have nice accessory kit included.

Texas costs 50USD, so it's bewteen 'ok, i will use it' and 'no, i will buy used TEK probe'
« Last Edit: January 21, 2013, 01:26:15 am by tinhead »
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Re: Quality PC-Based oscilloscope?
« Reply #34 on: January 21, 2013, 12:11:28 am »
Those responses look pretty terrible, do you have any data for a decent probe (eg. the matching Tek P6139a probe)? Resonance in the pass band is pretty bad, especially for the Hantek and Testec probes. I'm not so worried about behavior beyond the pass band, as in the case of the Texas probe, since you're supposed to use these with <= 250 MHz scopes that would attenuate these signals anyway.

It's very rare to see Bode plots for probes or scopes, so that Testec provide those doesn't surprise me. Impedance vs frequency is standard in any decent probe datasheet, however. I'm quite sure Testec used to produce the Hameg-branded passive probes; I would have expected them to be at least halfway decent.

500 MHz is the upper limit for passive high-Z probes. Probes beyond 200 MHz or so are often tuned to the impedance of a particular scope attenuator, which is why Tek made a whole bunch of ~300 MHz passive probes at some point (P6130, P6131, P6133, P6134, P6136, P6137, P6138, P6139), one for each series of scopes. The recommended probe for a 2465 was the P6131, for the 2465A the P6136 and for the 2465B the P6137. The probes would compensate for bumps in the scopes input impedance. By the time the TDS700 came along, they started standardizing front-ends so the P6139a could be used on all new 250+ MHz designs, but I wouldn't be surprised if a random commodity probe is a bad match for this front-end.
 

Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #35 on: January 21, 2013, 12:51:00 am »
the point that I'm trying to make here and that is that you cannot get more resolution from an ADC than what it has been designed for by just oversampling c.q. averaging.

of course you can, you can see it even on yuor DSO. When you above the  hi-res limit the signal will be not better
(simply use 10x att. and 10x zoom for better view).
Yes, but at those zoom levels the extra bits (and errors) added by the hi-res mode won't be visible. The whole point of having a hi-res mode would be to zoom in at a part of a signal and see the extremely small wiggles.

About the documents you linked: please read the conditions in which oversampling can work very carefully and you'll see I'm not saying anything else then the documents you linked. What I'm also saying is that in a scope those conditions are not met by design so the scope display may not show the actual signal. Again, oversampling sounds great in theory but in practical applications there are a lot of pitfalls.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #36 on: January 21, 2013, 01:30:42 am »
Hmm, you expect 12bit (that is 4096 steps) to show in an image about 350 pixels high?

No, I expect to see the distortion that you claim to exist in a practical application for which the scope was designed. I don't.

happens if I put my Tek scope to the test to see what it does with a signal about 3 LSB in height. The first image is the original signal, the second is the same signal attenuated 100 times and then enlarged 100 times.

So this is how you think people normally use scopes? A signal of 3LSBs p-p expanded 100x? Sounds pretty silly to me.
Which means you agree with me that claiming to get more bits from an ADC is safe to do for a scope manufacturer because the whole claim is useless to begin with because the screen's resolution isn't high enough to show the errors. 

Agree with you? Hardly. Tinhead has it exactly right. His images tell the story, and if you can't tell by comparing the two images that oversampling has greatly improved the resolution, then there really must be something wrong with you. Perhaps you shouldn't be allowed near test equipment at all. You might get hurt!

By multiplying the result up 100X, you are displaying a small 10 bit signal with 16.6 bits resolution - and you're somehow surprised to see distortion? Despite the absurd conditions you've set up, you've actually proved the validity of the technique with your own image. You've just failed to analyze it correctly. I've taken the liberty of finishing the analysis for you:

I drew in black, a more or less ideal representation of the original triangle wave you captured. The height is 290 pixels, and you've stated its amplitude is 3LSBs of the 10 bit result. So each LSB of the 10 bit result is 96.7 pixels.

I then searched and marked the worst case deviation that I could find and drew it in blue. It is 36 pixels.

36 / 96.7 = 0.37, or a deviation (DNL) of just 0.37 LSB referred to the 10 bit result. That is a *very* respectable figure for an ADC. Hi-speed ADCs typically spec DNL on the order of 0.5 LSB or more.

So I'm sure that Tektronix will be relieved to hear that you've validated their product for them. I'll bet they were very worried about your opinion.
 

Offline tinhead

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Re: Quality PC-Based oscilloscope?
« Reply #37 on: January 21, 2013, 01:34:03 am »
Those responses look pretty terrible, do you have any data for a decent probe

yep, i do. And here is the less funny part, all my measurments are bad, the terminator i've used is broken. With different one
terminator and P6243 everything looks flat as in user manual - with the broken one even P6243 was 3dBm out at 400MHz.

I have to sleep, tomorrow i will provide corrected values
« Last Edit: January 21, 2013, 01:35:41 am by tinhead »
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Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #38 on: January 21, 2013, 02:21:32 am »
@JoeyP:
You got the numbers wrong. The signal is about 3 LSB high from the ADC's perspective, not the 10 bit fictional resolution. On top of the 10 bit resolution there is also the scope's sin(x)/x interpolation to make a neat line. Here are two other pictures at lower zoom level settings:
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Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #39 on: January 21, 2013, 02:39:25 am »
@JoeyP:
You got the numbers wrong. The signal is about 3 LSB high from the ADC's perspective, not the 10 bit fictional resolution. On top of the 10 bit resolution there is also the scope's sin(x)/x interpolation to make a neat line. Here are two other pictures at lower zoom level settings:

Actually you got them wrong too. I should have ignored your numbers and done all of the math from the start. Your signal was 1.2Vpp in the first image. You attenuated it 100X for the second image, so it is 12mVpp. 256 x 0.012 / 1.6VFS = 1.92 LSB referred to the 8 bit converter. That's 7.68 LSB referred to the 10 bit result. So the actual DNL in the image is 0.95 LSB referred to the 10 bit result. Still monotonic and happens to be exactly the DNL spec for the TI/National ADC08200, a typical 200MHz 8 bit converter.
« Last Edit: January 21, 2013, 03:10:03 am by JoeyP »
 

Offline tinhead

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Re: Quality PC-Based oscilloscope?
« Reply #40 on: January 21, 2013, 12:39:03 pm »
Those responses look pretty terrible, do you have any data for a decent probe

yep, i do. And here is the less funny part, all my measurments are bad, the terminator i've used is broken. With different one
terminator and P6243 everything looks flat as in user manual - with the broken one even P6243 was 3dBm out at 400MHz.

I have to sleep, tomorrow i will provide corrected values


here we go, both TESTEC and TEXAS are definitely good enough

TESTEC TT-HF512 500MHz x10 Probe

  1MHz    0dBm
 50MHz   -0.1dBm
100MHz  -0.1dBm
150MHz  -0.2dBm
200MHz  +0.2dBm
250MHz  +0.2dBm
300MHz  +0.4dBm
350MHz  -0.3dBm
400MHz  -0.7dBm
450MHz  -1.7dBm
500MHz  -2.2dBm
550MHz  -2.8dBm



TEXAS 250/II 250MHz x10 Probe

  1MHz     0dBm
 50MHz  -0.3dBm
100MHz  -0.5dBm
150MHz  -0.1dBm
200MHz  -0.4dBm
250MHz  -0.1dBm
300MHz   0.0dBm
350MHz  +0.3dBm
400MHz   0.0dBm
450MHz  -0.5dBm
500MHz  -1.1dBm
550MHz  -2.4dBm


The Hantek probe, and i assume all the others almost identical probes on ebay (where only name and color seems to be random),
is still useless on Tek TDS7xx series. Probably with different trim cap insde it would work, but yeah, i don't give a s* on that.


Hantek P6200 200MHz x1/x10 Probe

  1MHz     0dBm
 25MHz     0dBm
 50MHz  +1.7dBm
 75MHz  +2.9dBm
100MHz  +3.8dBm
125MHz  +4.4dBm
150MHz  +4.5dBm
175MHz  +4.7dBm
200MHz  +4.9dBm
I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
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Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #41 on: January 21, 2013, 04:30:27 pm »
@JoeyP:
You got the numbers wrong. The signal is about 3 LSB high from the ADC's perspective, not the 10 bit fictional resolution. On top of the 10 bit resolution there is also the scope's sin(x)/x interpolation to make a neat line. Here are two other pictures at lower zoom level settings:

Actually you got them wrong too. I should have ignored your numbers and done all of the math from the start. Your signal was 1.2Vpp in the first image. You attenuated it 100X for the second image, so it is 12mVpp. 256 x 0.012 / 1.6VFS = 1.92 LSB referred to the 8 bit converter. That's 7.68 LSB referred to the 10 bit result. So the actual DNL in the image is 0.95 LSB referred to the 10 bit result. Still monotonic and happens to be exactly the DNL spec for the TI/National ADC08200, a typical 200MHz 8 bit converter.
You can reason all you want but the last images I posted clearly show that the lack of noise causes the interpolation algorithm not to work properly. In case of an oscilloscope you actually want the input stages to be as low noise as possible which contradicts the noise requirement for oversampling. I'm sure that when I add enough noise to the input signal the hi-res mode will show a triangle waveform with straight lines. I'll leave that as an exercise for the reader though. I think I clearly demonstrated that hi-res modes on scopes can and will produce false readings.
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Offline Wuerstchenhund

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Re: Quality PC-Based oscilloscope?
« Reply #42 on: January 23, 2013, 02:49:33 pm »
I know most PC-based oscilloscopes are pretty much garbage.  But I'm wondering, does anyone make a pc-based scope with pro-level electronics on par with Tek/Agilent/Rigol, etc?   To give an example: If I can't find an adequate PC-based one, my next choice would probably be a Rigol DS2072)

There are lots of external 'scopes' (digitizers), which generally can be divided into two categories: 'low-end/crap' (with often pathetic specs) and good quality ones. Unfortunately, if you look at the latter and take out digitizer modules for VXI, PXI and other embedded systems there is not too much left, and what's left mostly consists of (very expensive) highend stuff like Agilent's DSO90008 series.

Since you mention Rigol I assume the DSO90008 is not what you're looking for.

Quote
I know what you're saying: Why not just get a regular DSO?   Well as I said I'm a programmer, so I spend most of my working life in front of 4 gigantic monitors and screaming fast development PCs.  And I'm much faster with a mouse than those buttons and knobs on a regular scope.  I'd love to have the quality capture abilities of a high-end scope without paying for the tiny scope display.

The problem is that you generally pay excessively for *not* having the tiny scope display. Most of the better external scopes/digitizers are for special applications and come with a price that reflects that. At the end of the day, the most economic way is to just buy a regular DSO which can be remote-controlled, ignore that it has local controls and a screen, and just use it with your PC as you would do with an USB scope.
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Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #43 on: January 26, 2013, 04:59:26 am »
You can reason all you want but the last images I posted clearly show that the lack of noise causes the interpolation algorithm not to work properly.
Your last images may "clearly show" -you- something, but you seem to be working very hard to contradict the obvious.

tinhead has posted a good example of oversampling/hires mode for a Tektronix scope here: https://www.eevblog.com/forum/reviews/quality-pc-based-oscilloscope/msg183740/#msg183740

I've posted an example from an Agilent scope below. The test signal was a 20mV p-p ramp. Each image was captured after a single-shot acquisition using real-time sample mode.

The "Signal x 1" image is the actual signal catpured at 128mV/div. I used the vernier to make the math more intuitive. With 8 divisions, full scale is 1024mV so the 20mV test signal is 2% of full scale. Note the indicated amplitude of 52mV due to internal noise that the scope has added to the test signal. If you magnify the image, you can see that each 8 bit LSB occupies 2 or 3 vertical pixels depending upon rounding error for each position.

The "Normal x 16" image shows the same captured signal zoomed vertically 16x (after the acquisition halted). The 8 bit resolution is quite visible, and as expected 255 x 52mV/1.024V FS yields 13 LSBs p-p.

The "Hires x 16" image shows the same signal captured with hires mode turned on, also at 128mV/div, and then zoomed vertically16x to 8mV/Div. No other changes were made to the scope or the signal. I used 16x because the scope has increased the converter's 8 bit hardwre resolution to a 12 bit oversampled result, so 4096/256=16.

The increase in resolution in the hires image speaks for itself. If you magnify the image, you can see that each LSB of the 12 bit result now occupies 2 or 3 pixels in height, exactly as expected. You can also see that hires mode has cleaned up much of the noise resulting in an accurate amplitude measurement of 20mV p-p. The result is 79 individual steps, which is very close to the theoretical 4095 x 20mV/1.024V FS = 80.

At the very least, the hires/oversampling feature cleans up noise and allows much lower level single-shot signals to be measured. And if you really need the extra resolution, it's there too.

So if nctnico wants to pretend that this feature doesn't work, he is certainly welcome to do so. But while disparaging an entire industry, or a particular manufacturer using a fabricated argument and "polite" language may be easy, it is not fair to those being disparaged nor to those who may be reading in an effort to learn more about a valuable feature they may desire in a scope. It needs to be challenged, I will be here to do so any time I see it happen.
« Last Edit: January 26, 2013, 05:04:41 am by JoeyP »
 

Online nctnico

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Re: Quality PC-Based oscilloscope?
« Reply #44 on: January 26, 2013, 04:18:03 pm »
Let me repeat myself: a scope input has to be low noise to be usefull to begin with. The oversampling used in the high-res mode however requires a certain amount of noise to work. There is a clear contradiction here.  To put that to the test I've demonstrated that the required noise level can actually be insufficient in a normal situation and cause the oscilloscope to show a difference signal than there is in reality without any warning. BTW, for my test I've used a simple RC generator which can be bought for $50 through Ebay nowadays. It most certainly is not some super high end ultra low noise generator.

To put it simple: if there is not enough noise the oversampling simply will not work. You can't change the laws of physics. Oscilloscope makers clearly cut a corner here.
« Last Edit: January 26, 2013, 05:01:30 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline tinhead

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Re: Quality PC-Based oscilloscope?
« Reply #45 on: January 26, 2013, 07:10:25 pm »
To put it simple: if there is not enough noise the oversampling simply will not work.
of course you right here. I've made some other pictures - to finally end up the "discussion" -
and to show a nice Hi-Res trap:

Let's take a 1MHz sine wave (R&S SMX used for that), with amplitude high enough to not add
"any" trigger jitter (which could cange our results):



Tektronix TDS is using 2 vertical pixel (in 8bit mode) for one LSB. One DIV is 50pix high, zoomed 5 times
we have 16mV/40mVper DIV, so 0.4 x 10pix, this makes 4pixel in not zoomed which is equal to max. 2 LSB moise:



The zoomed sine wave looks of course not really clean:



With hi-res it looks better:



But here is the Hi-trap. I have enabled 20MHz BW filter (to have only max. 2LSB noise),
one can think "this will optimize the waveform", well no ... this kills H-Res, the waveform is less smooth:



as here where the full BW is enabled (sure, total amplitude deviation "seems" to be higher, but that't the
result of broader freq. specturm. The hi-res is giving smooth edges and fine steps between values, that's the deal):



The reason is of course the amount of noise, here we have 24mv/40mv per DIV, so 0.6 x 10pix, this makes 6 pixel
which is equal to 3 LSB of the waveform:



Therefore, properly used, hi-res make sense and it is possible.

You can't change the laws of physics. Oscilloscope makers clearly cut a corner here.

Well, normally there will be always some noise available, if not - in most case - it is enough to disable all BW filters,
then you will pickup enough noise to let Hi-res work. In case this is still not enough, apply some random ^^
« Last Edit: January 26, 2013, 07:21:11 pm by tinhead »
I don't want to be human! I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter ...
I want to reach out with something other than these prehensile paws and feel the solar wind of a supernova flowing over me.
 

Offline JoeyP

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Re: Quality PC-Based oscilloscope?
« Reply #46 on: January 26, 2013, 09:14:07 pm »
nctnico then:

... And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

nctnico now:

To put it simple: if there is not enough noise the oversampling simply will not work. You can't change the laws of physics.

I'm glad to see that you now acknowledge that pico, or any other scope vendor can indeed achieve 12 bits resolution from an 8 bit converter through oversampling of the signal+noise typically present in broad band scopes.

You'll be relieved to know that neither I nor Agilent violated any laws of physics in capturing the images posted here:
https://www.eevblog.com/forum/reviews/quality-pc-based-oscilloscope/msg185551/#msg185551

No one is trying or claiming to violate any laws of physics. It is those very laws that explain why noise is proportional to bandwidth. In a broad band device like a scope, lack of noise is rarely if ever an issue. We should all be so lucky as to have scopes with such low noise levels that oversampling were not possible. I would make that trade in a heartbeat, but that option is not available. Scopes have noise. Manufacturers have wisely made good use of that noise to enable a very useful hires mode. But like many scope settings, if you try hard enough you'll find a way to get an invalid measurement.
 


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