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Oscilloscope waveform averaging

Hi all,

I'm using a Tektronix 3034 scope to acquire a 10000 point waveform. I'm sampling at 250MS/sec. The waveform repeats at 5kHz.

This is all fine and good... what confuses me is the behavior of the averaging function. If I set the scope to average 16 waveforms to cut down on random noise, it takes 3-4 seconds for the displayed waveform to settle down to its final values when I change the input signal. Why is this? The Tektronix user manual claims that the scope can acquire 450 10000-point waveforms per second. If this is true, I'd expect the scope to settle on a new waveform in about 1/30th of a second, not 3 or 4 seconds.

Is this just a limitation of the scope's processing power? Or is the scope using a decaying average instead of a simple boxcar?

Is there anything I can do to speed this up on my current scope? I've found that switching to fast-trigger mode greatly speeds up the average's settling time, but I need my 10000 samples. Will a scope like the TDS5000 series help, or perhaps a PC-based scope add-in card?

Thanks in advance for any help or advice!

-Greg

Reply to
Greg Bredthauer
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It is probably doing averaging in the DPO accumulation buffer rather than the acquisition memory itself. (The DPO feature inserts an accumulation buffer between sample storage and video memory; the video buffer itself is built from information appearing in the accumulation buffer over time.)

I imagine that yes, this is one of the features that they give you when you drop the big bucks for a TDS5000. The TDS3000s are nice scopes but they do have their limitations, including the excessive front-end noise that necessitates video averaging in the first place.

-- jm

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Reply to
John Miles

shure it only 5 KHz?

Reduce the sampling rate - for a 5 kHz signal go for 0.1 ms/division timebase. 250MS/s is grossly overkill. Tell us what happens at 0.1 ms/div.

And don't forget that "acquire rate" is not equal to "screen update rate"

450 acquisitions, approx. 30 to 100 updates is not uncommon (all "per second").

A TDS 3xxx should be fine for 5kHz. Even an old TDS420 would do (15 k points; 150MHz; 100 MS/s) for that application.

hth, Andreas

Reply to
TekMan

snipped-for-privacy@bigfoot.com (TekMan) wrote in news: snipped-for-privacy@posting.google.com:

The signal itself is from an ultrasound hyrophone, with interesting signal info in the 0-20MHz range, which is why I'm sampling at 250MS/sec. The ultrasound pulse (and trigger) come at 5kHz rate.

-Greg

Reply to
AliasFan

Greg Bredthauer wrote in news:Xns9574C56A1E249HDTVAF@152.3.101.165:

I've solved my own problem... two things:

  1. The scope captures and processes waveforms much more slowly at 250MS/s than at 100MS/s for an unknown reason.

  1. There is some kind of persistence beyond just averaging (some kind of decaying averaging?) In any case, I can get around it by stopping and restarting acquisition.

Making these 2 changes lets me get 128 averages done in under 1 second. I hope this helps someone else out there :)

-Greg

Reply to
Greg Bredthauer

20 MHz seems incredibly high for mechanical transfer of energy in water. What are you doing, and do you really need this bandwidth? Could you need be related to viewing high-order artifacts of the transducer rather than an actual propagated signal?

Ed wb6wsn

Reply to
Ed Price

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