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Oscilloscope input noise comparison
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ci11:

--- Quote from: JPortici on November 16, 2018, 06:12:33 am ---My first guess is that it's correlated with the sampling clock (2.5GS/s -> 4 interleaved ADCs at 625MS/s)
that wouldn't be so strange but geez.. i would have expected better.
Shall i power up my ancient boat anchor and try to get some data, just for kicks :)?

--- End quote ---

That thought did not come to mind since this HDO is supposed to use a single TI ADC12D1600 per channel, for dual 1.6 GS/s or interleaved 3.2 GS/s. 625MHz is not an easy guess. But perhaps you're right - only LeCroy knows.
JPortici:
mine is just a guess :) a peak at a hint right of 600MHz suggest 625 which is a divider of 1250 and 2500, the latter being a very common sampling frequency for oscilloscopes.
I don't know about the HDO internals unfortunately

EDIT: I looked at the HDO4032 datasheet: Sample Rate (Single-shot) 2.5 GS/s
:)
_Wim_:
Today I was measuring a high frequency spectrum with my Picoscope 5442B, and started wondering the following:

According to the datasheet the 5443B and 5444B have a better time base accuracy than my 5442B (50ppm for the 5442B, and only 2ppm for the higher end models). When I run a high bandwidth FFT, I always see some spurs around 33Mhz (see attached screenshots with shorted input). These are the worst in 12bit mode, and get better in 15 and 16 bit mode (due to paralleling of the ADC's?). I was wondering if these are simular on a 5443/5444 model or if these are related to the lower quality clock?


Performa01:
FFT spurs are not related to time base accuracy.
The close-in phase noise is related to time base stability, but certainly not visible in a wideband spectrum plot without a signal.

The reason for spurs with a signal applied are non-linearities of the ADC. But without any signal, the non-linearities are pretty much irrelevant and all spurs can be attributed to internal interference signals.

Since the Pico 5000 series use some tricky configurations of the ADC cores in order to get the various resolutions, the 12-bit mode obviously generates more interference than other modes.

By the way, here you can see what a true 16 bit ADC in a Pico 4262 (which is optimized for low noise and low distortion) can do with regard to noise floor and spur level (with the same 50ppm time base tolerance!):


Pico_4262_FFT_Noise_50Ohm_term
_Wim_:

--- Quote from: Performa01 on December 23, 2018, 10:06:01 pm ---FFT spurs are not related to time base accuracy.
The close-in phase noise is related to time base stability, but certainly not visible in a wideband spectrum plot without a signal.

The reason for spurs with a signal applied are non-linearities of the ADC. But without any signal, the non-linearities are pretty much irrelevant and all spurs can be attributed to internal interference signals.

Since the Pico 5000 series use some tricky configurations of the ADC cores in order to get the various resolutions, the 12-bit mode obviously generates more interference than other modes.

By the way, here you can see what a true 16 bit ADC in a Pico 4262 (which is oiptimized for low noise and low distortion) can do with regard to noise floor and spur level (with the same 50ppm time base tolerance!):


--- End quote ---

Thanks for this plot and the explanation. If only the 4262 would have had more bandwidth! Sometimes I do which I had bought that one, but the additional bandwidth made me choose the 5442B.
I agree this is related to the trickery, but do not understand why the 12bit mode is worse, because that sample rate is supported directly by the ADC without trickery (unless the ADC does some internal trickery off course).
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