Author Topic: Oscilloscope specs - which are really important  (Read 14638 times)

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Offline jpb

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Oscilloscope specs - which are really important
« on: January 14, 2013, 07:14:34 pm »
I'm still (after 6 months) trying to determine which oscilloscope to get and how much to spend. I know the theory more or less (I have a degree in Electronics and many years experience of working with microwave integrated circuits but very little practical experience with oscilloscopes, digital circuits etc.) but I'm finding it hard to balance what is marketing and what is really useful. I'm looking to get a good quality 100 or 200 MHz scope for my own use ranging from hobby circuit building to perhaps physics like experiments, which will hopefully last me many years.

Any comments by owners of scopes on the following would be great!

1.) Memory. This seems to vary from a few k (definitely too small) to 56M (the Rigol) with 1M seeming typical.
500k gives a zoom level of a 1000 times while 50M gives a zoom level of a 100,000 times. Is a 1000 times enough or do people often require higher levels? My gut feeling is that memory levels are being pushed higher as a headline spec but I wonder how much importance should be attached to the difference between say 1M and 10M.

2.) sin x/x interpolation. With 1GS/s and 500 pixels of screen resolution interpolation only comes in at faster than 50 ns/div. Even at 2 ns/div there are a couple of sample points per div so the error introduced by linear interpolation is less probably than the dc gain error. Additionally, as sin x/x interpolation assumes a repetitive waveform the same effect could be obtained with equivalent time sampling?

3.) Hi-res mode. The author of the on-line blog article oscilloscope dreams states he wouldn't buy an oscilloscope without hi-res mode but it seems to me that it offers the illusion of increased accuracy which might even be misleading. The dc gain accuracy of the oscilloscopes I've been looking at range from around 2% to 5% or more. 8 bit convertors will have a least significant bit error of less than 0.5%. I can see the advantage of averaging out white noise but to go to 12 bits of precision when the uncertainty of the values is so much more seems to be a bit optimistic. (I remember being in a taxi in Tokyo once and seeing a plate attached to it giving its height to the nearest mm!)

4.) Built in serial decode options. These seem attractive but they are generally quite pricey and have to share rather restricted screen space on the oscilloscope. Stand alone solutions are of a similar price and can use say laptop display space. The main question is how much should a DSO buying decision be based on possible future options being available which I suspect I might not end up wanting to fork out for.

5. General quality. Comparisons between scopes that I can find seem to only consider features such as memory and fps. General quality, precision of timebase and gain don't seem to be mentioned yet I would have thought that these should be at least as important.

I realise that how useful features are depends on what you need to do. My problem is I don't know my future needs in detail. I don't want to buy features that I can live without but don't want to buy a scope and find I'm frustrated a year or two down the road.

What I'm looking for is people to say that feature x is great because they need it for y, I can then decide if I'm likely to also do y and therefore need x!
 

Offline MartinX

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Re: Oscilloscope specs - which are really important
« Reply #1 on: January 14, 2013, 08:02:19 pm »
If you don't know what your needs are now, how can someone else tell you? I understand your desire to try to make an optimal choice now, but I think there is too little information on what you want to do with the oscilloscope. My advice is to by the cheapest scope that satisfies your minimum needs and see how that works out, after using it for a while you will gain experience and then  you will be better prepared to make a better choice if you feel the need for it. Don't invest to heavily into something uncertain, there is always the possibility to sell and get a better scope. A  Rigol DS1102E maybe?
 

Offline madworm

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Re: Oscilloscope specs - which are really important
« Reply #2 on: January 14, 2013, 08:04:50 pm »
What is really important?

Bug-free firmware and support that fixes issues in a timely manner.

If you search this forum for the WaveAce224 threads, you will see what you don't want. I once owned this device, which has multiple nasty issues. I documented and reported all of them. Not a single one has been addressed or fixed so far. The latest firmware update for this device is almost 3 years old. In the end I returned the device and got my money back.

Not getting firmware fixes seems to be especially likely for devices that weren't developed in-house and bought from somewhere else, just to bolster up the low-end range of scopes for some brands.
 

Offline AndyC_772

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Re: Oscilloscope specs - which are really important
« Reply #3 on: January 14, 2013, 08:45:59 pm »
It's really only you that can decide what your requirements are. Long memory depths, for example, are handy if you want to be able to capture a stream of data and then zoom right in to see what the shape of the signal looks like in detail at some point - but then again, if you know what you're looking for, you may well be able to trigger on the event of interest and speed up the time base. The scope I use most often is a fairly old Tektronix with a 10k point memory, and for 99% of the work I do, that's plenty.

Serial decodes are nice to have if you often need to probe a serial bus for some reason, and completely pointless otherwise. I use I2C quite a bit, and on more than a few occasions I've ended up decoding it a bit at a time by hand, so automated decoding would have been nice, and I'll definitely get it on my next scope if the option is available. But if you don't plan on using interfaces like this, don't bother.

I quite like hi-res mode, though I wouldn't regard it as essential. It can give a very clean trace which allows underlying small changes in a signal to be shown which might otherwise be masked by noise. On the other hand, it does limit bandwidth and can hide important signal behaviour too.

Here's an example of how different acquisition modes would show the same signal.

Intensity graded "analogue" display (an essential feature IMHO, standard on any professional grade scope):



Simple digital sampling, typical of an entry level, hobby grade scope:



Peak detect acquisition - tends to exaggerate noise, but doesn't miss important signals like simple sampling can:



Hi-res mode - good for the simple digital signals, not quite so good for the high frequency modulated analogue signal!




Offline CarlG

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Re: Oscilloscope specs - which are really important
« Reply #4 on: January 14, 2013, 10:26:41 pm »
1) Granted, long memory is a useful feature sometimes. But.

I think the memory depth is less important for the zoom level, than it is for the possibility to retain high sample rate at low timebase settings. Unfortunately there's no 1-to-1 correspondence betwen depth and sample rate decrease. (Have a look at my rant here if you like.) I've haven't seen any scope datasheet stating at what timebase the sample rate goes down. You have to test that yourself to get an objective figure.

Having to choose between different memory depth is annoying. Automatic depth selection makes life easier.

Long memory may slow down scope response. Annoying. Check if it does.

"Segmented memory" is a very nice feature and in my opinion more useful than 10x more memory. If it doesn't have it, see to that it's an option.

I think at least 1Mpts/ch, but more than 10Mpts/ch no great advantage. There are pretty cheap protocol analyzers that can do better job for SW debug.

2) not necessary if you have 1GSa/s and only 200 MHz BW

3) Useful somtimes but may play tricks on you, especially when you're not so experienced with scope. But anyways, most scope have it, don't they?

4)  Select a scope that already have, or at least you can upgrade to serial trigger/decode. Some scopes come with serial trigger but decode is optional upgrade. You'll likely to end up with a I2C or SPI interface to a DAC, ADC, etc on some design some day.

5) Naturally is quality important, not so much more to say. Meaning "avoid the chepeast crap".

6) At least 4 channels. You can get away with two channels but it's tedious. Startup behaviour and SPI are two examples. You may even find yourself in a situation where two channels just ain't enough. Option for digital channels will be useful for any design involving CPLD/FPGA(external memories etc.

7) General usability and user interface. Separate knobs for each vertical channel is mandatory (for me). Fast response to knobs and buttons. Frequent used features shall not need you to move your hand over the scope like you're waving away a mosquito. Fast saving of data to USB stick or computer.

8 ) Display resolution: the higher the better. However, how data is processed/displayed is probably more important (with reasonable minimum resolution). Anti-aliasing function avoids confusion.

9) Capture/acquisition rate...the higher the better of course. But the datasheet capture rate doesn't say much; test yourself and see what happens with more channels, different timebase settings, etc.

10) Rise time is more important that bandwidth, but of course they are related.  Logic rise/fall times are down around 1 ns nowadays, so go for 200 MHz (or 100 MHz to start with if you can upgrade to 200 MHz).

//C


 

Offline jpb

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Re: Oscilloscope specs - which are really important
« Reply #5 on: January 14, 2013, 10:38:22 pm »
Thank you for the responses, especially your detailed response CarlG and AndyC.

I realise that my requirements are rather vague even as I was writing my post but it is very useful to have useful features highlighted by users and I suspect that there are others currently bemused by the plethora of different features that the marketeers of oscilloscopes compete on.

I did follow the WaveAce thread a while back, I certainly wouldn't go for a WaveAce but the LeCroy WaveJet seems a rather more well finished and robust, if somewhat out of date instrument.

Rigol and GW-Instek have very good specs for the money but are rather new so the issue raised in terms of firmware and general robustness make me slightly wary of them until they have been out long enough for bugs to be fixed.
 

Offline nctnico

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Re: Oscilloscope specs - which are really important
« Reply #6 on: January 15, 2013, 01:16:04 am »
In my experience peak detection (sometimes called envelope) is the most important feature. Because every DSO I've seen so far uses a variabele sample rate it can miss pulses. With peak-detect the samplers are kept running at the highest rate. The downsampling happens before writing the data to the memory.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Online EEVblog

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Re: Oscilloscope specs - which are really important
« Reply #7 on: January 15, 2013, 02:53:08 am »
I quite like hi-res mode, though I wouldn't regard it as essential. It can give a very clean trace which allows underlying small changes in a signal to be shown which might otherwise be masked by noise. On the other hand, it does limit bandwidth and can hide important signal behaviour too.

Yes, it can be a trap for young players.



Dave.
 

Offline marmad

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Re: Oscilloscope specs - which are really important
« Reply #8 on: January 15, 2013, 05:59:30 am »
Long memory may slow down scope response. Annoying. Check if it does.
Long memory always slows down capture rates - it's a function of time. More info here.

Quote
"Segmented memory" is a very nice feature and in my opinion more useful than 10x more memory. If it doesn't have it, see to that it's an option.
I agree that segmented memory is great, but I'm not sure why you think more memory wouldn't be helpful for segmented memory as well.

Quote
I think at least 1Mpts/ch, but more than 10Mpts/ch no great advantage. There are pretty cheap protocol analyzers that can do better job for SW debug.
Again, the more memory - the more segments. For example, with 56MB, the Rigol DS2000 series can have up to 65000 segments. That means if you use the maximum delay interval of 10 seconds between segments - you can capture a triggered waveform for over a week. The slowest timebase setting for 65000 frames is 50ns - with 700ns being captured with each segment. So that would be ~45 milliseconds of a repeatedly captured event - spread evenly over ~7.5 days. You can then run the segments through the analysis function - and get a histogram of deviation of the waveform over that length of time.
« Last Edit: January 15, 2013, 06:04:34 am by marmad »
 

Online EEVblog

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Re: Oscilloscope specs - which are really important
« Reply #9 on: January 15, 2013, 10:00:59 am »
You would be foolish to buy any scope these days that had under 1M of sample memory, unless you had a good alternative reason for that model (e.g. Agilent 2000X is great, but limited memory).
As for all the features being talked about, pretty much you look at the current benchmark instruments in terms of functionality.
Two years ago the Agilent X series was untouchable, now the Instek seems to have an equivalent, and I'd consider the Rigol 2000 series the current front runner by far in under $1K 2 channel scopes.
As someone else said, 4 channel is handy, and you'll never regret getting a 4 channel.

So unless you are strapped for cash, I'd recommend buying one of the functionality front runners.

Dave.
 

Offline nctnico

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Re: Oscilloscope specs - which are really important
« Reply #10 on: January 15, 2013, 11:35:22 am »
Long memory is nice but the user interface of most scopes doesn't make it easy to sift through the data. I rather have four channels than extremely long memory. With just 32000 points of memory you can already capture very long SPI and I2C messages. If you need more, then a logic analyser may be a better tool.
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Online EEVblog

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Re: Oscilloscope specs - which are really important
« Reply #11 on: January 15, 2013, 12:51:35 pm »
Long memory is nice but the user interface of most scopes doesn't make it easy to sift through the data. I rather have four channels than extremely long memory. With just 32000 points of memory you can already capture very long SPI and I2C messages. If you need more, then a logic analyser may be a better tool.

Yes, I would consider 50-100K to be an absolute minimum for useful work (often halves on dual channel). Although Rigol set the 1M benchmark on sub $1K scopes quite a few years back now, so you'd be hard pressed not to get that much these days.

Dave.
 

Offline CarlG

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Re: Oscilloscope specs - which are really important
« Reply #12 on: January 15, 2013, 08:35:46 pm »
Long memory may slow down scope response. Annoying. Check if it does.
Long memory always slows down capture rates - it's a function of time. More info here.
Who said anything about capture rate? A scope's response time is not defined by its capture rate. Response to user actions may be slow due to a CPU being occupied with processing/decoding (large chunks of) data. Display data processing time may very well be a substantial, maybe even dominating part of the response time. It's something that I wish shouldn't be an issue, but I wouldn't count on it. I admit that I don't know where all the different brands stands today in this respect, but I surely wouldn't buy a scope without testing the "live" feeling of it.

Quote
Quote
"Segmented memory" is a very nice feature and in my opinion more useful than 10x more memory. If it doesn't have it, see to that it's an option.
I agree that segmented memory is great, but I'm not sure why you think more memory wouldn't be helpful for segmented memory as well.
I'm not saying that. What I'm saying is that if my scope has 1Mpts/ch, and I have a choice between 10Mpts/ch upgrade and segmented memory upgrade, I'd go for the latter.

Quote
Quote
I think at least 1Mpts/ch, but more than 10Mpts/ch no great advantage. There are pretty cheap protocol analyzers that can do better job for SW debug.
Again, the more memory - the more segments. For example, with 56MB, the Rigol DS2000 series can have up to 65000 segments. That means if you use the maximum delay interval of 10 seconds between segments - you can capture a triggered waveform for over a week. The slowest timebase setting for 65000 frames is 50ns - with 700ns being captured with each segment. So that would be ~45 milliseconds of a repeatedly captured event - spread evenly over ~7.5 days. You can then run the segments through the analysis function - and get a histogram of deviation of the waveform over that length of time.
True, that might be useful sometimes. I was mostly thinking of serial debugging applications, but of course there are other measurement situations to consider, where a protocol analyzer doesn't help.


But that reminds me of two other features that may be useful sometimes:
a) very slow timebase setting (like 100 - 1000 s/div, or more) and
b) possibility to trigger in roll mode.
I'm thinking of applications e.g. recording battery discharge characteristics, etc

Another (important) parameter/feature I forgot in my list is vertical sensitivity/low noise, and is one of those parameters that might be measleading if you don't read the footnotes. (I'm thinking of Agilent, where 1mV/div only is a digitally magnified 4mV/div setting.)
 

Offline AndyC_772

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Re: Oscilloscope specs - which are really important
« Reply #13 on: January 15, 2013, 09:06:34 pm »
I'd consider the Rigol 2000 series the current front runner by far in under $1K 2 channel scopes.
As someone else said, 4 channel is handy, and you'll never regret getting a 4 channel.
Shame the 2000 series doesn't come in 4 channel. I don't know how anyone manages with just two channels; how do you probe clock, data and chip select, for example? Any kind of stimulus / response uses up two channels straight away, even if both are a single signal. Add anything more complex and you're hosed.

There's the 4000 series, of course, but they're twice the price.

Offline marmad

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Re: Oscilloscope specs - which are really important
« Reply #14 on: January 15, 2013, 09:13:13 pm »
Who said anything about capture rate? A scope's response time is not defined by its capture rate.

Sorry, I thought you were referring to the same thing you mentioned in your 'rant ("Large depth may not be compatible with high capture rate") which I read first. My bad  :)
 

Offline Hydrawerk

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Re: Oscilloscope specs - which are really important
« Reply #15 on: January 15, 2013, 09:58:18 pm »
Long memory (more than 1MB) is good if the scope has proper tools to handle it. There should be posibility to place marks or search automatically. Wave Inspector
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Offline hans

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Re: Oscilloscope specs - which are really important
« Reply #16 on: January 15, 2013, 11:01:43 pm »
I'd consider the Rigol 2000 series the current front runner by far in under $1K 2 channel scopes.
As someone else said, 4 channel is handy, and you'll never regret getting a 4 channel.
Shame the 2000 series doesn't come in 4 channel. I don't know how anyone manages with just two channels; how do you probe clock, data and chip select, for example? Any kind of stimulus / response uses up two channels straight away, even if both are a single signal. Add anything more complex and you're hosed.

There's the 4000 series, of course, but they're twice the price.

The 4000 series does have additional features (although the 2000 seems decent equipped for a hobbyst) and has 2 more channels. So if you calculate the 2 extra channels for 25-50% extra cost, the more sample memory, more wfm/s is the extra cost.

Have to say, I'm also disappointed. 50kwfm/s and 14MPts for me would be more than enough, but I really like to see 4 channels for the reasons you mentioned.

On a 2 channel scope it's hard to measure SPI, I2C is okay. For SPI I would compare ChipSelect vs Clock and then Clock vs Data, but each time you program a firmware update, things change, stuff shifts around, and so assumptions are made, which is the mother of all fuckups. |O
 

alm

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Re: Oscilloscope specs - which are really important
« Reply #17 on: January 15, 2013, 11:13:27 pm »
I tend to use a logic analyzer for protocols like SPI. SPI is usually slow enough so even the cheap 24 MS/s max USB logic analyzers can easily keep up. They are much cheaper than the premium of a four channel scope. I use a scope for the signal integrity stuff, but that doesn't normally require monitoring a lot of channels.
 

Offline nctnico

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Re: Oscilloscope specs - which are really important
« Reply #18 on: January 16, 2013, 02:19:21 am »
There is more to life than SPI. When measuring in circuits I often find myself using 3 channels. Sometimes 4. I actually have two almost identical 4 channel scopes. In one case I stacked the scopes and connected the trigger out on the first to the trigger input on the second to get an 8 channel scope.
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Offline marmad

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Re: Oscilloscope specs - which are really important
« Reply #19 on: January 16, 2013, 02:41:31 am »
There is more to life than SPI. When measuring in circuits I often find myself using 3 channels. Sometimes 4. I actually have two almost identical 4 channel scopes. In one case I stacked the scopes and connected the trigger out on the first to the trigger input on the second to get an 8 channel scope.

Sure, but let's face it - in the long history of oscilloscopes, 2 channels has been the most standard - and most tasks can still be handled with it. Honestly, I remember when an External Trigger input was a premium feature. Of course, the more you have of almost anything (channels, bandwidth, sample speed, memory, update rate, etc) the better, so if you can easily afford 4 channels (or 8 or 12  :) ) get it - but I agree with alm that, for many users, the premium you pay for 4 channels (especially given the tiny sizes of most oscilloscope displays) would be better spent on equipment better suited to looking at and analyzing a larger number of signals.

And I do exactly what you describe with your 2 scopes - but with my scope and a logic analyzer.
« Last Edit: January 16, 2013, 02:44:44 am by marmad »
 

Offline nctnico

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Re: Oscilloscope specs - which are really important
« Reply #20 on: January 16, 2013, 12:13:18 pm »
Who said a scope should be new? There are many excellent oscilloscopes for sale on Ebay.
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Offline Wuerstchenhund

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Re: Oscilloscope specs - which are really important
« Reply #21 on: January 18, 2013, 08:52:47 pm »
Who said a scope should be new? There are many excellent oscilloscopes for sale on Ebay.

I fully agree. Personally I'd rather buy a good second hand scope from one of the major manufacturers than any of the cheap Rigol/Atten/Siglent/Hantek/whatever scopes from China. I had a chance to play with a few of these China scopes, and while the build quality of some of them is quite good and they come with tons of features, for me a stable firmware, a decent user interface and reliability/support are more important with a measurement device. I know I will get shot for saying this and that many are happy with their China scopes but that is my opinion.

Price is another advantage of buying used. Yes, there are lots of commercial dealers that try to flog off 15 or 20 year old scopes for more than the cost of a brand new modern counterpart (and many private sellers seem to take these insane prices as orientation) but if you can wait then there are definitely interesting deals out there.

As to what features are required, well as others said this depends on what you want to do with it. Buying features just because you have been told that this is a 'must-have' no matter what means you're wasting money. Analyze your requirements and make a list of what *you* need and buy something that has what you need and fits your budget. I'd also not put too much value into buying a scope that can be upgraded as when you finally need that new feature you may find that it's more economical to sell your scope and get one which already has the new feature.

Taking myself as an example: large sample memory isn't a concern for me at the moment. My basic scope is an old HP 54510A 250MHz 2Ch 1GSa/s DSO with 8k memory per channel, and for everything more demanding I also have a HP 54542A 500MHz 4Ch 2GSa/s DSO which has 32k memory per channel. Both scopes were cheaper than most 100MHz China scopes. Important for me was that a scope can use it's full sampling rate on all channels and not when using only two or a single channel, a limitation that is often found not only with cheaper China scopes but also with some pretty expensive scopes from well known manufacturers like LeCroy. Should the day come that I need more memory then I'll sell on one of my scope without much loss and just buy a suitable second hand scope which has the memory I need.
« Last Edit: January 18, 2013, 09:03:19 pm by Wuerstchenhund »
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Offline jabramo

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Re: Oscilloscope specs - which are really important
« Reply #22 on: January 19, 2013, 01:30:43 am »
If your really interested in added decoding functionality maybe take a gander at some Tek 2000's and 3000's someone posted here on the forum some links of some guys who figured out how to reverse engineer the modules and make there own with a 0.25$ microchip eeprom.

I used a 2000 series Tek for a while, the one with 4 isolated channels and I generally wasn't a big fan. However it did work. If your big on some specific added functionality it might be worth a look

Here's the link to the information
http://forum.tsebi.com/viewtopic.php?f=4&t=113
 

Offline nctnico

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Re: Oscilloscope specs - which are really important
« Reply #23 on: January 19, 2013, 04:18:35 pm »
AFAIK there is no need to hack because Tektronix has firmware with all options enabled available for download on their website. Be aware that the TDS2000 series scopes have a very short (2.5k points) memory. The TDS3000 series doesn't do much better with only 10k points. Serial protocol decode is pretty much useless with these short memories.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

alm

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Re: Oscilloscope specs - which are really important
« Reply #24 on: January 19, 2013, 04:36:27 pm »
AFAIK there is no need to hack because Tektronix has firmware with all options enabled available for download on their website.
Tektronix released a firmware update for the TDS 3000 series that enabled some of the options (from memory it was advanced triggering, FFT and video triggering). Other options, like advanced video features, advanced analysis, and limit/mask testing still required an expensive plugin EEPROM module.

Be aware that the TDS2000 series scopes have a very short (2.5k points) memory. The TDS3000 series doesn't do much better with only 10k points. Serial protocol decode is pretty much useless with these short memories.
Don't confuse the TDS 2000/3000 series with the much newer DPO or MSO 2000 / 3000 series. Both use plugin I2C EEPROM modules for options, but that's where the similarity ends. The DPO/MSO series have deep memory and other modern features.
 


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