Makes a "low f" FFT (10 Hz....100 kHz) with the SDS2000X+ sense??
I’ve said it before and I’ll repeat it again that a general-purpose oscilloscope is neither a dynamic signal analyzer nor an audio analyzer, even when it can provide more than 8 bits of resolution. The guaranteed dynamic range of an 8-bit acquisition system is limited to about 49 dB and below 1 MHz the strong 1/f noise might cause further restrictions. Most importantly, the linearity of a 570 MHz wideband system cannot be the same as an audio-analyzer whose bandwidth only needs to be far less than one megahertz.
Nevertheless, the SDS2000X Plus and its FFT can be quite useful even at very low frequencies. Look at the first two screenshots, where I’ve measured a 10 Hz sine to 1.07% accuracy and a 100 kHz sine to 0.72% with the very same settings.
SDS2354X Plus_Meas_Sine_10Hz
SDS2354X Plus_Meas_Sine_100kHz
The accuracy of level measurements in this frequency range is generally quite good. The following screenshot demonstrates the FFT measurement of a 200 mVrms (= -14 dBV) sine signal in the range of 0-100 kHz. As can be seen, the measurement is spot-on around 1 kHz to 20 kHz and the error is barely exceeding 1% otherwise.
SDS2354X Plus_FFT_FR_100kHz
The next three screenshots show some distortion measurements for 33 Hz, 440 Hz and 20 kHz. The source is a low distortion (-96 dBc) sine wave, and we get about -60 dBc for the strongest harmonic at these frequencies. That’s certainly better than what we could normally expect from an 8-bit system, yet not nearly enough to characterize a high-quality audio system.
SDS2354X Plus_FFT_THD_33Hz_VBW1kHz
SDS2354X Plus_FFT_THD_440Hz_VBW20kHz
SDS2354X Plus_FFT_THD_20kHz_VBW100kHz
After buying a SSA I'm out of the "SDS-FFT business" for a while but the SSA starts at 100 kHz and is not useable for low f spectra's.
See my attached try. I'm missing a logarithmic scale for "f"
Yes, we don’t have a logarithmic X-axis. It certainly could be implemented, yet the bin-width in any FFT is constant over the entire frequency span, hence a linear X-axis is the natural fit. Consider a range from 1 Hz to 100 kHz. For the lowest decade of 1-10 Hz we might want to have a bin-width of 0.1 Hz, whereas for the highest decade of 10-100 kHz a bin-width of 1 kHz should be plenty. The FFT has to be setup for the narrowest bin-width, so we had to use at least 200 kSa/s / 0.1 Hz = 2 Mpts FFT-length, which happens to be the maximum the SDS2000X Plus can deliver. So yes, it’s possible, but not very common.
Sorry, I’m not quite sure what you’re trying to demonstrate with your screenshot.
Is this the noise floor and some spurious signals of your instrument? Markers without corresponding table don’t make much sense…
A noise floor around -140 dBV looks rather impressive – that’s about 100 nVrms. What are you intending to do? Connecting a moving coil pickup system to the scope directly? Even then you should get a couple of hundred microvolts, hence see signals around -80 dBV.
I have checked the same frequency span up to 2 kHz on my SDS2354X Plus with slightly different settings and got a similar spectrum with the strongest spur of about -107 dBV at the mains frequency of 50 Hz.
SDS2354X Plus_FFT_NF_VBW2kHz
EDIT: description of the noise spectrum corrected.