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Does Agilent X3000 series use Linear or sin x/x interpolation?

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kg4arn:
I must be missing something obvious ...

I can't seem to find a statement about the interpolation routine used for the x3000 series scopes, for the displayed data.  The Agilent app notes I read say that, in general, Agilent uses sin x/x and that this is usually user selectable.  I can't find any such option on the scope either.

The scope tracings below look to me like the interpolation is linear.  I was expecting a pretty clean looking sine wave with the sample rate at 200 MSamples/sec, if the interpolation were sin x/x.

The input signal is a 10 MHz sine wave from the scope's WavGen.  There is a minicircuits (BLP 15+) 15 MHz, 50 ohm, low pass filter in line.  The input signal is a very clean sine wave with the 2nd harmonic at -70dBc.  Higher harmonics were undetectable on my spectrum analyzer.


My questions are:
1)  Is the interpolation linear?
2)  Why not sin x/x selectable?

rolycat:
The scope uses sin(x)/x interpolation, and since it never incurs a performance penalty it is engaged automatically when required.

Edited from Agilent's application note 5990-9923EN:

Agilent’s 3000 X-Series oscilloscopes automatically perform sin(x)/x waveform reconstruction filtering. This occurs whenever the scope’s timebase is set to a range such that there would be less than 500 unfiltered sample points across screen.

The scopes perform sin(x)/x waveform reconstruction via DSP hardware, which is part of the scope’s MegaZoom IV ASIC technology. Waveform update rates can be as high as 1,000,000 waveforms/sec, even when the scope automatically engages the sin(x)/x waveform reconstruction filter.

marmad:
For comparison, here is a similar signal on a Rigol using 1.4MPts @ 200MSa/s; first with interpolation on - then with it off:

marmad:
I'm assuming the second image is the exact same sine wave sampled at 4GSa/s? Nothing else has changed? And the Agilent is automatically using a 2MPts record length, correct? If so, that is rather odd.

kg4arn:

--- Quote from: rolycat on April 06, 2013, 07:28:37 pm ---The scope uses sin(x)/x interpolation, and since it never incurs a performance penalty it is engaged automatically when required.

Edited from Agilent's application note 5990-9923EN:

Agilent’s 3000 X-Series oscilloscopes automatically perform sin(x)/x waveform reconstruction filtering. This occurs whenever the scope’s timebase is set to a range such that there would be less than 500 unfiltered sample points across screen.

The scopes perform sin(x)/x waveform reconstruction via DSP hardware, which is part of the scope’s MegaZoom IV ASIC technology. Waveform update rates can be as high as 1,000,000 waveforms/sec, even when the scope automatically engages the sin(x)/x waveform reconstruction filter.

--- End quote ---

Thanks, I missed that in my search.

If I understand this correctly it switches to sin x/x when there is <500 unfiltered sample points, then for:
  1 channel active (4GS/sec) the interpolation is linear on the screen until the horizontal sweep is 12.5 nsec/div or less.
 2 channels active (2GS/sec) the interpolation is linear on the screen until the horizontal sweep is 25 nsec/div or less.

So for the slow sweep rates there are many more than 500 sample points and when you zoom the trace you see the choppier linear interpolation.

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