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Oscilloscope for SMPS use, a choice between analog & digital?
Oscilloscope for SMPS use, a choice between analog & digital?
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Old 17th October 2008, 09:28 AM   #1
Bootstrapper is offline Bootstrapper  Finland
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Default Oscilloscope for SMPS use, a choice between analog & digital?


I posted this message here, since I'm looking for a new scope for mostly SMPS use (i.e. I intend to use it mostly for observing different SMPS waveforms). I have two candidates available, but I would be grateful for your opinions as well.

These candidates are an analog Tektronix 2246A scope and a digital HP 54501A. Both are 4 channel, 100 MHz devices and the prices are 300 ($400) for the HP 54501A and 350 ($470) for the Tektronix 2246A. Both are used.

I'm looking for a 4 channel scope to be able to perform simultaneous, differential measurements. Both of these fill this requirement. As for the bandwitdh, I believe this 100 Mhz is well enough fo my purposes, I don't do any RF projects involving very high frequencies.

The digital one attracts because of the ability to view very slow waveforms, say, in the order of several hertz. But, I have found out previously that these old HP digital scopes are a bit awkward to use. So the analog one would (I suppose) be much easier to use.

Perhaps some of you have opinions on this? Thanks for commenting!
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Old 17th October 2008, 10:39 AM   #2
mflorin is offline mflorin  Romania
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Analog oscilloscopes have their own charm...
I was looking some time ago for an affordable oscilloscope and I considered both analog and digital types.
My internet search was guided by several criteria like: price, banwidth, features (mathematical functions, copy and paste (waveform portability)...), support, mobility.
My conclusion was that USB based oscilloscope is the way to go. After that I had to filter out all found products and finally I've got a winner: Cleverscope - www.cleverscope.com
Please note that is just my personal opinion. There were plenty other chinese products most of them at half the price but with some important drawbacks.
Cleverscope has a very active community were you can easily ask for help or support if you encounter any problem.

Regarding your need of 2 simultaneously differential measurement - well, Cleversope has only 2 channels but I think the software is capable to operate with 2 oscilloscopes in parallel. However this will require 2 units and the price of one unit is already higher than what you mentioned.
Other option will be 1 or 2 differential probes...
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Old 17th October 2008, 10:56 AM   #3
Bootstrapper is offline Bootstrapper  Finland
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Thanks, I'll check that site!
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Old 17th October 2008, 11:29 AM   #4
Mooly is offline Mooly  United Kingdom
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Oscilloscope for SMPS use, a choice between analog & digital?
Have a look at,
starting around post # 54
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Old 17th October 2008, 12:16 PM   #5
AndrewT is offline AndrewT  Scotland
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a 100MHz analogue scope has a wide enough bandwidth to capture any misbehaviour in an audio amplifier.

What sampling rate would a digital scope need to display similar HF artefacts?
It used to be sampling rate =10times the highest required frequency, but I have been told on this Forum that modern software has reduced this requirement substantially.
It very much depends on how the digital scope has been designed, but expect each channels' sampling rate to be of the order of 200Ms/S to 1000Ms/S. A 4channel machine that can sample at 4Gs/S is going to cost you a fortune.
regards Andrew T.
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Old 17th October 2008, 01:27 PM   #6
x-pro is offline x-pro  United Kingdom
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HP 54501A is next to useless. It has only 10 Msps sampling rate and a single-shot bandwidth only 1 MHz.


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Old 20th October 2008, 07:14 PM   #7
edfed is offline edfed  France
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i have a digital tektronics at work, and frankly, it is the best to capture signals, sometimes, it have some little bugs, like sync complettely impossible to set, but it is mainlly a better tool that analog one.
it is smaller, lighter, faster, beautifuller, very precise, and....

and ..

very usefull to see the transitions, startups, manual switchings, etc etc...

it can show you a lot of measures, like rms voltage, peak to peak, frequency..., and you can add little math plug-in.

i don't see any advantage to buy an analogue scope...
maybe for vintage design??
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Old 21st October 2008, 12:23 PM   #8
Bakmeel is offline Bakmeel  Netherlands
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I have a personal preference for digital oscilloscopes when it comes to SMPS design. At work we use Tektronix TDS3000 and DPO7000 series oscilloscopes. Though probably still more expensive than the two candidated you mention, I recommend at least to have a look at the TDS3000, which is now becoming more reasonably-priced.

I agree with edfed that the main advantage with digital scopes is the ability to freeze the image or to run single shot measurements. But if you want performance on digital sampling scopes, have a look at the following specs:

- Sampling speed: the number of samples taken per second, is not the number of samples shown on your screen. Divide samples per second by the timescale (us per division) to get an idea of the horizontal resolution you will get. In the HP 54501A case, the screen resolution is even limited to 501 samples.

- Memory Depth: especially useful if you want to investigate start-up transients and transient duty-cycle behaviour. Think of the number of samples per switching cycle times the number of switching cycles to complete the transient. A memory depth of at least several Msamples is needed.

Cycle Averaging: with 8 bits resolution, you have 256 steps to divide over the measurement range. especially for differential measurments this can be critical. Cycle averaging can help to improve your measurements, because it enhances (virtually) the resolution of the waveform measured. Note this only applies to repetetive waveforms.

You should go for a digital oscilloscope, as it is paticularly useful in SMPS design, but the HP scope offers only minimal digital "advantage" w.r.t. analogue scopes. It certainly should cost more...

More Power Igor! More Power!
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Old 21st October 2008, 02:13 PM   #9
star882 is offline star882  United States
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Are there any PC-based oscilloscopes that get good sample rate and don't cost a lot? Perhaps a DIY oscilloscope is in order?
A FPGA with an ADC on one end, some cheap buffer memory, and a 1394 interface on the other end should be a good start.
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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