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| How much noise floor and other things matter in oscilloscope usability |
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| nctnico:
For color grading to be usefull you need a perfectly repetitive signal. After all color grading is a form of averaging. Power supply ripple isn't perfectly repetitive so color grading won't help at all. Just face it: you will want to use an oscilloscope with the least amount of internal noise to look at any signal. After all the purpose of an oscilloscope is to look at the shape of a signal and the less an oscilloscope distorts that signal, the better. It seems Rigol dropped the ball where it comes to noise reduction and at some point cheap doesn't make up for poor performance. |
| Fungus:
--- Quote from: nctnico on December 28, 2021, 12:06:35 pm ---For color grading to be usefull you need a perfectly repetitive signal. After all color grading is a form of averaging. Power supply ripple isn't perfectly repetitive so color grading won't help at all. --- End quote --- I'd still like to see a screenshot of it. |
| Fiorenzo:
I am going to do the photos you asked, maybe this evening or tomorrow |
| David Hess:
--- Quote from: Performa01 on December 28, 2021, 09:28:32 am ---Frequency step is 38.15 Hz, which is equivalent to a RBW of 112 Hz with the Flat-Top window – so noise levels will read slightly higher than in your setup. With a noise level of -134.7 dBm this is very comparable to your RSA 3408A above some 30 kHz. At 1 kHz, the FET input goes up by 17.8 dB to -116.9 dBm, but obviously stops at -114 dBm with this RBW. --- End quote --- If I did my math right, that comes out to 3.9 nV/SqrtHz so similar to a well designed 100 MHz JFET input, and consistent with the specified 15 picofarad input capacitance. (1) For a lower frequency singled ended JFET input instrument, 1 nV/SqrtHz is possible (LSK170) but the input capacitance would be 2 or 3 times higher. So why is the input capacitance low and noise high for such low bandwidth? Given their dynamic range and distortion requirements for 16-bits, a simple FET source follower would have too much distortion; feedback is required to lower the distortion. So they probably used a JFET operational amplifier, and that would be consistent with higher noise, 5 MHz bandwidth, and a 15 picofarad input capacitance. That also places this instrument into a different class than an oscilloscope, although similar to the old Tektronix oscilloscopes which used the 5A22 or 7A22 differential amplifier. (1) There is a close relationship with input capacitance, bandwidth, and input noise. Lower bandwidth FETs have lower noise and higher input capacitance. |
| David Hess:
--- Quote from: Fiorenzo on December 27, 2021, 08:28:48 pm ---I would like to thank you everybody for the many replies. --- End quote --- Hopefully the discussion was of some help. --- Quote ---You have been very important and educative to convince me in the decision that in my work It would be better an oscilloscope with a low noise front end than one with a very fast ADC like the Rigol. --- End quote --- I do not think low noise and high sample rate are mutually exclusive because the digitizer's input noise should be insignificant compared to the noise from earlier stages and especially from the amplified noise of the high impedance input buffer. The instruments in question seem to suffer from higher noise in general rather than because of sample rate. |
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