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Rigol DHO804 Test and Compare Thread

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Martin72:
That sounds sensible, the longer the time base, the lower the lower cut-off frequency, the higher the noise.
You know what, I'm trying it out now, from the living room (scope is on the router in the study). ;)

mawyatt:

--- Quote from: gf on December 26, 2023, 09:42:49 pm ---
--- Quote from: Performa01 on December 26, 2023, 08:48:00 pm ---The differences come from the very pronounced 1/f noise characteristic of scope frontends.

--- End quote ---

Oh sh*t, I didn't expect it to be that much :phew:
Your explanation makes sense, of course.

--- End quote ---

If the Rigol front end chip is implemented in pure CMOS then likely a relatively high 1/f corner. Scope front ends don't/can't take advantage of signal processing like CAZ and Chopping which help reduce 1/f noise as well as offsets.

Some utilize bipolar front ends, often SiGe or SiGe BiCMOS, which have good BW and low 1/f corners, but tend to be more expensive, with fewer chip foundry sources available.

Best,

Martin72:
So...
The scope is now the only thing that was on in the room, even the light was off, so the noise values are correspondingly lower compared to before.
Input open, 1Mohm, full bandwidth, auto-memory.
Starting at 1µs/div.
The values increase up to 1ms/div, after that no more.

pakakezu:
Maybe a bit unrelated. But i could not find the file format for the ARB waveform for the DHO924S. Looked in the programming and user manual. Can select csv from the menu but shows load fail if i load a saved trace.

I expected the arb file to be selectable and even uploadable from SCPI but only the arb mode can be seleceted without specifics.

If someone managed to output an arb trace can please post how the csv should look like?

Performa01:

--- Quote from: mawyatt on December 26, 2023, 10:22:55 pm ---If the Rigol front end chip is implemented in pure CMOS then likely a relatively high 1/f corner. Scope front ends don't/can't take advantage of signal processing like CAZ and Chopping which help reduce 1/f noise as well as offsets.

--- End quote ---
This is only the harmless part of the story.

The major contribution to the extremely pronounced 1/f noise is the LF path of the split path input buffer.

The HF path is not bad at all and its noise can be well below 3 nV/sqrt(Hz) at e.g. 10 MHz.

The LF path on the other hand has a noise figure of up to 20 dB by design - in addition to the 1/f noise of the FET amplifier utilized here. Bipolar wouldn't be an option since high input impedance is required, and it is almost irrelevant because of the high NF by design mentioned before.

The crossover frequency between LF and HF path is usually only a few kHz, yet the LF path affects the noise of the HF path up to at least 100 kHz.

In practice we get the expected 1/f noise characteristics down to about 100 kHz and  see an excessive increase in noise below 10 kHz. The reason why even lower bandwidth limits than 100 Hz would not make much of a difference anymore might be the small portion of the total bandwidth.

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