@2N3055
You were rigtht. I looked at the wrong values, not stdev but stdev(stdev).
I repeated the measurment after a self-calibration of the scope.
This time I have two points.
First the same a before +
CH1 mean(stdev) 5.268 mv
CH2 mean(stdev) 5.301 mv
CH3 mean(stdev) 5.701 mv
CH4 mean(stdev) 5.815 mv
I assume CH1 and CH2 share an A/D-converter and CH3 and CH4 the second one.
That is a diff 0.8dB and ok, but something one has to keep at back of his
mind.
Second is the difference between RMS and stdev.
The scope was warmed up half an hour before self-calibration was started.
I owned a Siglent SDS2354X Plus before and I remember that past
a self-calibration the differences between rms and stdev were
remarkable mininal.
So I assume the self-calibration routine could need some improvement.
So I am still interested in the values others see repeating
the same measurement at same conditions at their scopes.
Another observation is that were the rms differences to stdev
values indicate an dc offset (past input condenser, I measured AC)
the stdev(stdev) is high.
Has anybody idea what could cause this?
At 0.5mV the lowest noise is at CH1 with 109.4 uV ant 50 Ohm.
This result in a noise power of P = 239.37pW or 10xlog(P x 1000) = -66.2 dBm
at full bandwith.
Divided by the bandwidth of 750MHz (datasheet) this gives -154.95 dBm/Hz
If the bandwith is ca. 1.2 GHz (meassured) this gives -157 dBm/Hz.
Both are very good noisefigures of 19 and 17.
I deleted my previous post and with it the fanboy statement.
Just came across this as we are seriously looking at the SDS3000X HD.
Shouldn't the Noise Power Density be computed as the Total Noise Power measured divided by the "Equivalent Noise Bandwidth" and not the 3dB Bandwidth? For example, the ENB of a 1st order LP Filter is pi/2 times the 3dB BW, and the ENB approaches the 3dB BW with higher order LP Filters.
If so, then the DSO Noise Figure is even better than 17dB, as the ENB is always greater than the 3dB BW, and since the DSO doesn't have a "steep rolloff" effective input low pass filter, maybe a couple dB better NF?
Does anyone know the ENB of these scopes?
Best
As usual, correct. RMS will calculate across full spectrum it has in buffer. Nominal BW is only -3dB. There will be significant energy from that point to actual stopband.
Apart from the contribution from analog front end, there is ADC noise contribution that has more bandwidth. These ADC have larger than 1GHz BW at their sampling inputs.
1GHz scope I have has 120µV RMS noise at full BW, one channel enabled, interleaved at 4GS/s.
By looking at the FFT of the noise floor, I would "guesstimate" 1,4-1,5 GHz ENB, by simply visually "folding" the tail up and back on top of the slope to get rectangular window.
That makes it cca -157 dBm/Hz, which makes Kladit's second estimate plausible order of magnitude.
But it is not that simple. When non interleaved, scope drops to 2 GS/s and enables digital filtering of anything above 1GHz. RMS noise drops to 100µV RMS and with ENB of 1GHz (again we can see it in FFT), we get, again consistent -157 dBm/Hz
So I would say that would be good order of magnitude figure.