Noise DensityIn my previous test
https://www.eevblog.com/forum/testgear/sds800x-hd-review-demonstration-thread/msg5293744/#msg5293744I used a constant sample rate of 100 MSa/s, which allowed a 2 Mpts FFT with an effective FFT-sample rate of 50 MSa/s and Δf = 23.84 Hz. This was required, since the FFT in the SDS800X HD is limited to 2 Mpts max. and I wanted to measure the 1/f noise down to at least 100 Hz. As a consequence, I had to set up the FFT in a way that I get a frequency step (Δf) well below that.
Of course, if we want any accuracy in the spectrum plot, the Flattop window has to be used, and the RBW is Δf * 3.73 in case of Siglent’s version of the Flattop Window.
Because of the low sample rate of just 100 MSa/s for the acquisition, there will inevitably be aliasing, folding back all the noise above 50 MHz to the first Nyquist zone. Then there will be even more aliasing because the FFT introduces one more decimation step, from 100 to 50 MSa/s. The latter could be countered by a digital filter, but it doesn't make that much of a difference anymore.
All this does not matter much as long as we are mainly interested in the 1/f noise below about 300 kHz, because it is much stronger than the high frequency noise anyway.
Now we want to see the real noise density up to 10 MHz without any aliasing spoiling our measurements. For this we can activate all channels, thus reducing the input bandwidth to a well defined 200 MHz and engage the 20 MHz bandwidth limiter on top of that, so that we can be absolutely sure that there will be no aliasing products of any significance affecting the measurement at 10 MHz.
SDS824X HD_ND_1mV_20MHz_500MSa
Calculation for 10 MHz: -144.58 dBV = 59 nVrms.
The noise density at this point is 59 nV / √889.3 Hz = 59 nV / 29.8 = 1.98 nV/√Hz;
Here is the complete table:
10 MHz: -144.58 dBV 2.0 nV/√Hz
3 MHz: -142.48 dBV 2.5 nV/√Hz
1 MHz: -141.88 dBV 2.7 nV/√Hz
300 kHz: -141.55 dBV 2.8 nV/√Hz
100 kHz: -131.63 dBV 8.8 nV/√Hz
30 kHz: -125.49 dBV 17.8 nV/√Hz
10 kHz: -113.34 dBV 72.2 nV/√Hz
1 kHz: -101.97 dBV 267.2 nV/√Hz
With a noise density below 2 nV/√Hz, the Siglent SDS824 X HD beats most of the competition at higher frequencies, whereas the 1/f noise is nothing to write home about, but that has to do with the special split path input buffer design with its enormous offset compensation capability (±8 V starting at only 10.2 mV/div!).
Attached is the binary data file for this measurement.
SDS824X_HD_Binary_C4_2.7z
Channel 4, 1 mV/div, 50 ohms terminated;
500 µs/div, 2.5 Mpts, 500 MSa/s;
Bandwidth limit = ~20 MHz to get rid of any remaining aliasing from >250 MHz;
EDIT: Of course, the above measurement was flawed, because the 20 MHz bandwidth limiter affects the 10 MHz measurement. The actual noise density, measured without bandwidth limit at 10 MHz is 2.4 nV/√Hz, just as it was stated in the first test.
SDS824X HD_ND_1mV_200MHz_500MSa
Here is the updated noise density table:
10 MHz: -144.58 dBV 2.4 nV/√Hz
3 MHz: -142.48 dBV 2.5 nV/√Hz
1 MHz: -141.88 dBV 2.6 nV/√Hz
300 kHz: -141.55 dBV 2.9 nV/√Hz
100 kHz: -131.63 dBV 6.0 nV/√Hz
30 kHz: -125.49 dBV 16.0 nV/√Hz
10 kHz: -113.34 dBV 68.8 nV/√Hz
1 kHz: -101.97 dBV 247.0 nV/√Hz
A noise density of <2.4 nV/√Hz is still one of the best in the industry.
Attached is the binary data file for this measurement.
SDS824X_HD_Binary_C4_3.7z
Channel 4, 1 mV/div, 50 ohms terminated;
500 µs/div, 2.5 Mpts, 500 MSa/s;
Full Bandwidth;