Products > Test Equipment
Rigol HDO1000 and HDO4000 12bit oscilloscopes launched in China
mawyatt:
--- Quote from: tv84 on January 30, 2023, 05:00:19 pm ---
--- Quote from: gf on January 30, 2023, 02:41:06 pm ---The FFT trace looks horizontally mirrored at 100kHz, even the noise seems mirrored. Strange.
--- End quote ---
I would bet it is mirrored! That should say something to those who know this stuff.
--- End quote ---
Yes the display is mirrored around 100KHz, which implies a 200KSPS rate.
If one is viewing a center frequency of 100KHz, then the FFT should create a sample rate that supports a reasonable result around 100KHz, which 200KSPS is not. As mentioned this sample rate should not be allowed to be this low and the DSO should produce a much higher sample rate which evidently it does not, unless user intervention to "force" the lower sample rate and incur Nyquist sub-sampling artifacts, but hopefully knowing such artifacts will occur.
This is a serious FFT bug, that could easily fool a user into "thinking" these sub-sampled artifacts are real when in fact they are not.
scander36 has done a great service in showing these FFT results, which initially were to show the channel and ADC linearity by means of a Two Tone IMD signal and subsequent FFT, but this low sampling rate issue was uncovered.
Best,
Fungus:
--- Quote from: tv84 on January 30, 2023, 05:17:19 pm ---Shouldn't the FFT stop at 100kHz if you have the 200kSa/s ?
--- End quote ---
You mean simply display nothing on screen above that frequency?
That's a good question. Logic says "yes"... but it would look weird and I'm sure people would complain just as loudly. :popcorn:
I don't know what other 'scopes do in this situation - I don't own them all. It seems suspicious that they claim an "FFT frequency" at double the sample rate so I'd guess "2x upsampling followed by a low pass filter" to try and minimize the effects of mirroring. If this is the case then there's a bug somewhere...
(yes, I know "2x upsampling" already involves a low pass filter so it can be a single step in practice...)
mawyatt:
--- Quote from: Fungus on January 30, 2023, 05:48:45 pm ---
--- Quote from: tv84 on January 30, 2023, 05:17:19 pm ---Shouldn't the FFT stop at 100kHz if you have the 200kSa/s ?
--- End quote ---
You mean simply display nothing on screen above that frequency?
That's a good question. Logic says "yes"... but it would look weird and I'm sure people would complain just as loudly. :popcorn:
I don't know what other 'scopes do in this situation - I don't own them all. It seems suspicious that they claim an "FFT frequency" at double the sample rate so I'd guess "2x upsampling followed by a low pass filter" to try and minimize the effects of mirroring. If this is the case then there's a bug somewhere...
(yes, I know "2x upsampling" already involves a low pass filter so it can be a single step in practice...)
--- End quote ---
Here's as close as we can get to the Rigol HD at 100KHz with 200KSPS and 10K memory depth with the SDS2000X+ at 125KHz with 250KSPS and 16K memory depth.
Note, it does not display anything past 125KHz, immediately conveying to the user that this is "forbidden Nyquist frequency zone", and FFT display is not to be trusted!!
Edit: This makes one wonder if the Rigol HD uses the lower sampling rates to show a quicker user FFT response? Also, this shows that a significant amount of effort has been expended in the SDS2000X+, likely by some folks on here, to produce a quality FFT function & display that is user friendly and reliable.
After all, what good is a DSO function that can't be relied upon to produce reliable results?
Best,
skander36:
--- Quote from: Fungus on January 30, 2023, 04:44:29 pm ---Question: What happens if the input signal is 99.5kHz?
--- End quote ---
Same thing.
It is more a memory lack problem.
This is happened only at 10kpts and lower. If you pay attention to the memory and you will intervene to alocate more memory (100kpts and over) this ghost will disapear.
skander36:
--- Quote from: mawyatt on January 30, 2023, 06:11:13 pm ---
--- Quote from: Fungus on January 30, 2023, 05:48:45 pm ---
--- Quote from: tv84 on January 30, 2023, 05:17:19 pm ---Shouldn't the FFT stop at 100kHz if you have the 200kSa/s ?
--- End quote ---
You mean simply display nothing on screen above that frequency?
That's a good question. Logic says "yes"... but it would look weird and I'm sure people would complain just as loudly. :popcorn:
I don't know what other 'scopes do in this situation - I don't own them all. It seems suspicious that they claim an "FFT frequency" at double the sample rate so I'd guess "2x upsampling followed by a low pass filter" to try and minimize the effects of mirroring. If this is the case then there's a bug somewhere...
(yes, I know "2x upsampling" already involves a low pass filter so it can be a single step in practice...)
--- End quote ---
Here's as close as we can get to the Rigol HD at 100KHz with 200KSPS and 10K memory depth with the SDS2000X+ at 125KHz with 250KSPS and 16K memory depth.
Note, it does not display anything past 125KHz, immediately conveying to the user that this is "forbidden Nyquist frequency zone", and FFT display is not to be trusted!!
Best,
--- End quote ---
Same on Rigol at even lower samplerate.
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