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Measuring Distortions with the Scope:What you see is not what you really have..
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_Wim_:

--- Quote from: JeremyC on January 08, 2023, 02:11:17 am ---Sorry, I should mention in my posts that I disagree with W2AEW about the averaging/hires modes with FFT. In my opinion W2AEW is excellent (if not the best) information source and I’m glad he’s sharing his knowledge and experience with the public. But in this topic I would disagree with his opinion about averaging combined with FFT, sorry.

--- End quote ---

No need to say sorry. As seen from the replies above, it is clearly something were different opinions exist. I agree about W2AEW, excellent channel, have watched all of his video's, even the HAM ones as a non-HAM.
_Wim_:

--- Quote from: Performa01 on January 08, 2023, 06:20:23 am ---No doubt that the real deal – a true higher resolution ADC – is the superior solution. Yet there is nothing wrong with a proper ERES/HiRes implementation as long as the user thinks a little. And that should not be too much asked, since there are so many situations in T&M where we cannot trust blindingly and just copy some values from an instrument to the lab protocol without a second thought.

For instance we might run into troubles when we try to measure a -60 dBm signal using FFT when the corresponding input channel is set to 100 mV/div (800 mVpp = +2 dBm full scale). This is clearly outside the genuine 8 bit dynamic range.

SDS2354X Plus_LVL_10MHz_100mV_-60dBm_8bit

The measured amplitude is -57.5 dBm, hence 2.5 dB off – just remember, we try to measure a 632 µVpp signal at an 800 mVpp full scale input range. In a low noise DSO, the so called "process gain" doesn't work that well, fair enough.

But then again, not many will attempt to measure such a weak signal by using a particular insensitive input range of the scope – we would rather choose 1mV/div instead:

SDS2354X Plus_LVL_10MHz_1mV_-60dBm_8bit

Unsurprisingly, the measured amplitude is correct now; -59.9 dBm means that the error is just 0.1 dB.

For a fair comparison, now that we used a 40 dB more sensitive input range, we should also try to measure a 40 dB lower signal, i.e. -100 dBm instead of -60 dBm:

SDS2354X Plus_LVL_10MHz_1mV_-100dBm_8bit

All of a sudden even the signals outside the genuine dynamic range are measured accurately: -99.9 dBm is only 0.1 dB off again. This is because at a high sensitivity of 1 mV/div we have sufficient inherent noise to make the resolution enhancement inherent to a longer FFT fully work.

What if there are any stronger signals present, so we cannot select a higher sensitivity without overdriving the scope input? All the better, then even the lower sensitivities will work, because now we don't need noise as dither anymore, but have those stronger signals instead.


Of course, a 12 bit SDS2000X HD doesn't have any troubles measuring -60 dBm even in the insensitive 100 mV/div (+2 dBm FS) range. After all, this is still within its genuine dynamic range of ~72 dB.

SDS2504X HD_LVL_10MHz_100mV_-60dBm_Normal

The last screenshot demonstrates how we can still measure -100 dBm (6.3 µVpp, far outside the genuine dynamic range) at 100 mV/div (800 mVpp) with only 0.74 dB error:

SDS2504X HD_LVL_10MHz_100mV_-100dBm_Normal

--- End quote ---

Thanks for this. Quite impressed by the performance of the SDS2354X!

@nctnico, do you now of a signal that can be generated easily with an AWG where artifacts would appear within the effective dynamic range. Would be nice to do a side by side comparison, and also useful to see what to watch out for...
tautech:

--- Quote from: _Wim_ on January 08, 2023, 06:47:20 am ---Thanks for this. Quite impressed by the performance of the SDS2354X!

--- End quote ---
Do note Performa01 used 2 DSO's, SDS2354X Plus and SDS2354X HD.  ;)
_Wim_:

--- Quote from: tautech on January 08, 2023, 07:00:32 am ---Do note Performa01 used 2 DSO's, SDS2354X Plus and SDS2354X HD.  ;)

--- End quote ---

Ah, missed that one. Thanks for pointing that out! Still, remains very good results with the HD. Would be a excellent replacement for my pico5000...
gf:

--- Quote from: JeremyC on January 08, 2023, 02:11:17 am ---
--- Quote from: _Wim_ on January 07, 2023, 06:36:23 am ---I think you need to re-watch the referenced video by W2AEW (#65: Basics of using FFT on an oscilloscope), because it perfectly makes sense to use HI-RES or ERES to increase the effective number of bits. This way an 8-bit scope can have a larger effective number of bits at a much lower bandwidth.

--- End quote ---

I watched this video many times and I read many FFT related publications from the “A” vendors in the last few years.
Sorry, I should mention in my posts that I disagree with W2AEW about the averaging/hires modes with FFT. In my opinion W2AEW is excellent (if not the best) information source and I’m glad he’s sharing his knowledge and experience with the public. But in this topic I would disagree with his opinion about averaging combined with FFT, sorry.

--- End quote ---

HiRes/Eres provides additional processing gain for a subsequent FFT (assuming that the FFT size is already maximum and cannot be increased any more) if the particular HiRes/Eres implementation does decimate the data after filtering, so that the FFT is applied to the lower sample rate.

OTOH, if the HiRes/Eres implementation only applies a lowpass filter (w/o decimation) then it does not increase processing gain of a subseqent FFT, but only distorts the frequency response. No need to filter, the FFT acts as filter bank anyway.

And even if the HiRes implementation does decimate, the next question is whether its filter is an appropriate anti-aliasing filter for the decimation. The traditional boxcar averaging filter certainly isn't -- not at all. For the high dynamic range we are after, we would need an anti-aliasing filter with very high stopband attenuation in order that aliases folded into the 1st Nyquist region drown in the FFT noise floor.
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