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| How much noise floor and other things matter in oscilloscope usability |
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| mawyatt:
--- Quote from: Performa01 on December 27, 2021, 06:41:52 pm --- --- Quote from: mawyatt on December 27, 2021, 05:34:53 pm ---The two tone IMD looks good as one would expect from a "True" 16 bit system. If you don't mind could you do this test at ~1MHz with the Picoscope 4262? --- End quote --- Here you go - this instrument is not sensitive enough - you can barely see the IM3 products at 990 and 1020 kHz. Signal_IMD_40mV_1000-1010kHz --- End quote --- This looks good, but the tone levels are -6dBV below the level used at 20KHz so one would expect the 3rd order IMD to be significantly down from the 20KHz case. Anyway, thanks for the test. Best, |
| G0HZU:
Note also that the other spurious terms are limiting the spurious free dynamic range on the 1MHz IMD test. A really good (old school swept) microwave spectrum analyser can achieve a typical IP3 limited SFDR of about 112dB with a very narrow RBW on the first frequency range up to a few GHz. However, 105dB is more realistic at (say) 10Hz RBW. I wouldn't expect to see those other spurious terms either when using a conventional spectrum analyser. The Pico will have a much faster refresh rate though! On narrow spans the phase noise will slightly limit the SFDR of the swept analyser so usually stuff like this is done at a wider frequency spacing with a conventional analyser. |
| Performa01:
--- Quote from: mawyatt on December 28, 2021, 12:56:48 am ---This looks good, but the tone levels are -6dBV below the level used at 20KHz so one would expect the 3rd order IMD to be significantly down from the 20KHz case. --- End quote --- Sorry, this was not indended – somehow I did not pay attention to the levels. Please find attached the correct measurement. The result is the same as at 20 kHz. Signal_IMD_80mV_1000-1010kHz --- Quote from: G0HZU on December 28, 2021, 01:34:06 am ---Note also that the other spurious terms are limiting the spurious free dynamic range on the 1MHz IMD test. A really good (old school swept) microwave spectrum analyser can achieve a typical IP3 limited SFDR of about 112dB with a very narrow RBW on the first frequency range up to a few GHz. However, 105dB is more realistic at (say) 10Hz RBW. I wouldn't expect to see those other spurious terms either when using a conventional spectrum analyser. The Pico will have a much faster refresh rate though! On narrow spans the phase noise will slightly limit the SFDR of the swept analyser so usually stuff like this is done at a wider frequency spacing with a conventional analyser. --- End quote --- Of course you are right – and just for others to put this into perspective, I would like to add: Swept spectrum analyzers only “see” their resolution bandwidth at any point in time (ok, only true for the last IF), whereas the DSO always works at full bandwidth (5 MHz in this particular case). Under these conditions, some 96 dB dynamic is all you can expect from a 16 bit system – everything beyond that is just a lucky incident based on the specific conditions and the results cannot be trusted any longer. In the previous example with the 6 dB lower level, it has been perfectly possible to measure an IMD of 109.6 dBc, since none of the spurs got in the way of this measurement. According to the textbook theory, it should have been 115 dBc though, so the measurement was flawed anyway. As mentioned before, we cannot expect great accuracy once far outside the first order dynamic range of the acquisition system. |
| G0HZU:
That really is very impressive from the Picoscope in terms of SFDR. I just turned on the RSA3408A to look at the LF noise floor up to 25kHz and I've added a plot below. This analyser is FFT only and can't do a swept measurement at any frequency. Below 40MHz it feeds direct to a 14 bit ADC and the IMD performance isn't that good. It's much worse than the Picoscope in this respect. However, the LF noise floor is quite good considering this isn't a dedicated AF analyser. I've used it to measure the noise figure of AF amplifiers a few times. As long as I provide enough gain to overcome the noise figure of the 3408A it can make fairly good noise figure measurements. I guess not many people make AF amps with 50 ohm ports but this type of amplifier is popular in direct conversion receivers. You can see the noise floor is a fairly flat -154dBm/Hz across the AF band. |
| David Hess:
--- Quote from: G0HZU on December 27, 2021, 08:42:30 pm --- --- Quote ---I wish we had better data on available RF MOSFET noise characteristics. What is available is intended for RF amplifier applications. --- End quote --- Can you measure the noise parameters yourself at audio frequencies? I've done this stuff up at RF and recently measured the s-parameters for the BF998 MOSFET at various bias points across a frequency range of a few MHz up to 3GHz and I also created some noise data for it up at VHF. This noise data gets included in the s-parameter file. I did the same for the old BF981 a few years back with good results when designing amplifiers for low noise figure. I've never tried to do this at audio frequencies though. --- End quote --- Up through audio frequencies would not be sufficient because RF MOSFETs can have a flicker noise corner frequency in the MHz range. I am just not setup to make that kind of measurement easily. I can make spot noise measurements up to 1 MHz but even that would not be high enough. I would have to build something custom and I would prefer a more general solution. |
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