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Old 26th May 2011, 03:50 PM   #1
Salas is offline Salas  Greece
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Default DSO useless or not for measuring low noise?

Hello guys

I was looking around for a low end DSO the other day, Rigol DS1052E and the like. The various displays shown are striking me as ''noisy'' compared to my trusty 20MHZ analogue. OK, low BW brings less noise but I have used American, Japanese and European analogue ones with much more BW and still looked much smoother. I fancy the portability, small size, the auto measurements and configs and usb stick .bmp saves etc. But, hey, are those little computers any good for evaluating a PSU's or high gain circuit's total output noise roughly at all? I have seen Agilent's arguments and techniques (Hint#4 see link below) about working around that somehow (in much more expensive DSO Agilent example units that is). What is your experience in low end DSOs? Noisy as hell, or there are same ways around noise also with those? Results? Do they equal a classic analogue scope of medium BW after application of such tricks?

http://cp.literature.agilent.com/lit...989-7894EN.pdf
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Old 26th May 2011, 05:01 PM   #2
Elvee is online now Elvee  Belgium
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Hi,

DSO's are great when you need math, FFT, memory, capturing single events, exporting waveforms, making eye-diagrams etc but, if youd need to check the smoothness of a sinewave, detect a minuscule crossover distortion or low level noise or oscillations, they are pretty worthless.

I use a Combiscope from Fluke, this gives the best of both worlds, but it stays approx. 95% of the time in analog mode.
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Old 26th May 2011, 05:07 PM   #3
Salas is offline Salas  Greece
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Was afraid so... thanks for your info.
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Old 26th May 2011, 09:43 PM   #4
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You need a good analog scope, like Tek 7603 with some vertical plug-ins e.g. 7A22, 7A13 (single channel differential amplifiers) for low and high frequency differential measurements and a dual time base 7B53A. The combination has affordable prices, for a well preserved machine. It is easily repairable and full of free documentation (service and operators manuals). And you can buy additional plug-in to cover your future needs. Then you need a cheep digital scope like Rigol. You can use one input of the Rigol, connected to the analog output of the Tek 7603 (it has the appropriate output BNC in rear panel). This way you can digitize your signal, with the better or more convenient front end of the old analog scope. The total cost (both analog and digital) will be less than 800 euro. After 10 years.... you can throw away the Rigol and.....go on with the Tek 7603 and a new cheep (maybe better) digital scope.
The other solution is to buy a relative expensive digital scope (Agilent. LeCroy, Tek). For the same features you must pay a lot of money and you must take in account the additional cost of differential probes and the any options you need (like software). The high performance scopes can have everything from both worlds, analog and digital but unfortunately in a high cost.
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Old 21st April 2012, 01:06 AM   #5
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I recently bought a Chinese DSO with FFT to measure distortion from a power amp.
It shows about -60db at 1kHz, but I get about the same result from my signal source itself. I'm wondering if the problem is with my signal source or the DSO. I found this item on ebay, but hesitate to buy if the DSO is not up to the task. Any guidance much appreciated

Ultra low distortion (<0.00005%) 1kHz sine generator assembled and tested PCB | eBay
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Old 21st April 2012, 04:32 AM   #6
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IMO, low end DSOs are useless for analog work and I can't afford the high end ones. For starters, 8 bit vertical resolution is completely inadequate for seeing noise or producing a useful FFT. They're nice for troubleshooting common problems and especially digital and control stuff, but I'd take a boat anchor analog scope for analog design any day.
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Old 21st April 2012, 05:05 AM   #7
Enzo is offline Enzo  United States
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Oh my, I feel silly. Around here, DSO means Detroit Symphony Orchestra. Imagine my confusion.
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Old 21st April 2012, 06:08 AM   #8
benb is offline benb  United States
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Part of the problem with digital scopes IS the bandwidth. Most good scopes, such as the Teks, both the older analog and the more recent DSO's, have bandwidth limit switches. Setting the bandwidth to 20MHz can substantially reduce the noise shown on the trace for both analog and digital scopes.

Also, what exactly are you measuring? If it's a noise floor with no signal present, the limitation may be the scope's analog preamplifier, regardless of whether the scope is analog or digital. You might make a 20dB preamp with a low noise opamp to go between what you're measuring and the oscilloscope.

If you want to measure the noise floor with a signal present, I don't see where either an analog or digital scope would work (unless you make a high rejection notch filter for a single sine wave signal). A better option would be an audio interface for a computer. An oscilloscope is still a good idea for checking for RF oscillations in the MHz or higher range, frequencies beyond what the audio interface picks up.

Quote:
Originally Posted by polyphaze View Post
I recently bought a Chinese DSO with FFT to measure distortion from a power amp.
It shows about -60db at 1kHz,
How many bits resolution does it have? Most DSO's only do 8 bits, giving a theoretical (without dither) 48dB dynamic range.
Quote:
but I get about the same result from my signal source itself. I'm wondering if the problem is with my signal source or the DSO. I found this item on ebay, but hesitate to buy if the DSO is not up to the task. Any guidance much appreciated

Ultra low distortion (<0.00005%) 1kHz sine generator assembled and tested PCB | eBay
Knowing more about your model of DSO might help, but
Quote:
Originally Posted by Enzo View Post
Oh my, I feel silly. Around here, DSO means Detroit Symphony Orchestra. Imagine my confusion.
Take comfort that the Detroit Symphony Orchestra surely has much greater resolution than the other DSO's being discussed in this thread.
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Old 21st April 2012, 02:31 PM   #9
Salas is offline Salas  Greece
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Quote:
Originally Posted by benb View Post
Part of the problem with digital scopes IS the bandwidth. Most good scopes, such as the Teks, both the older analog and the more recent DSO's, have bandwidth limit switches. Setting the bandwidth to 20MHz can substantially reduce the noise shown on the trace for both analog and digital scopes.
Hello zombie thread. Should post a conclusion I guess.
Got one 1052E in last year's summer anyway, and this on the pics is what it can show with probe X1 BW20MHz and 32X averaging. BW limit at 20MHZ does little compared to averaging, its only a (hacked) 100MHZ DSO. At X10 probe setting, hopeless at 4mV RMS trace noise with all the above aids. Otherwise it has good construction and parts, is very portable, used to be Agilent's OEM 1000 series. The screen is low analysis which I can't do anything about it, looks like a Sinclair ZX Spectrum video game, the other thing is its fan's annoying noise level, which I did something about. Very handy for many things due to modern automation and measuring, looks better screen wise if fed to the PC, I kept my old German CRT scope anyway.
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Old 21st April 2012, 10:49 PM   #10
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Enzo: uis funny. Next time we are in Motown we make sure we will get tickets. They better be good! E
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