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| How much noise floor and other things matter in oscilloscope usability |
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| Performa01:
--- 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 |
| David Hess:
--- Quote from: Performa01 on December 27, 2021, 06:19:21 pm --- --- Quote from: David Hess on December 27, 2021, 03:27:38 pm ---The designs Steve Roach shows (attached below) include automated trimming of the gain of the low frequency path. He briefly mentions noise on page 70 where he discusses the shortcomings of RF MOSFETs. --- End quote --- Yes, this well known article is brilliant indeed! Yet we can see its age by looking at the first schematic, figure 7-1: the 50 ohms termination is accomplished by just a resistor, that is connected in parallel to the ordinary high impedance input with its high shunt capacitance – a solution that is barely suitable for scopes with a bandwidth exceeding some 100 MHz. --- End quote --- It works well with my 150 MHz 2445 and 300 MHz 2440 but they use hybrid construction so the parasitic elements are much less than with a surface mount printed circuit board. I think the later TDS series did it up to 500 MHz but maybe not because the 1 GHz models obviously could not have. As mentioned earlier, the old Tektronix 485 with printed board construction did *not* use a switchable termination but instead an RF relay to direct the input to either the high impedance buffer or a separate 50 ohm input, and the specifications reflect it with lower bandwidth in high impedance mode. At the time I do not think they had a faster JFET high impedance buffer or they would have used it. I consider the 485 to be a "heroic" engineering effort. Based on context, I think Steve Roach worked on the 500 MHz and 1 GHz TDS series of oscilloscopes so his article gives an idea about what was going on in the late 1990s and early 2000s. I believe this makes it particularly useful for emulation in modern amateur designs. --- Quote ---Consider a high impedance (100 Mohm), x100 high voltage probe connected to 2kV. If you now switch to AC coupling by accident, the input DC-block capacitor will charge up. Current is limited by the probe resistance, but after 10 seconds the capacitor might be charged to about 1.9 kV and this is equivalent to some 60 mJ of energy. So if the (supposedly) 400 volts rated capacitor doesn’t break down (and suffers damage or at least permanent degradation), it will send a potentially destructive pulse of electric energy into the frontend as soon as someone connects a low impedance source to the input after that incident. --- End quote --- Tektronix made high voltage 10x and 100x probes with a built in parallel resistance to avoid that problem. They can be identified by having a lower than expected input resistance. Probes like this are still made but they are difficult to find and come with a premium price. --- Quote ---Figure 7-3 in your document shows the usual approach where to feed V_offset. In your circuit diagram of the Tek 22xx the offset voltage (delivered from an OpAmp with close to zero output impedance within the LF frequency range) would have to be fed into the lower leg of R98 (after disconnecting it from ground, that is). But with the low division ratio, which clearly is an attempt to keep the LF noise down, there is almost nothing gained, so I can completely understand why it’s done differently in this particular case. EDIT: Sorry, only now i've checked what you mean. Of course, with the transistor output stage the original approach for offset compensation cannot be used. --- End quote --- The 22xx series also did not need a different method because it is based on a traditional analog design where that was a solved problem. It just represents the last fully documented oscilloscope design along with the 24xx series of analog and digital storage models. Even so, I consider the 2232 to be the first "modern" DSO design with a recognizable user interface. It's predecessor, the 2230, has an archaic albeit interesting user interface and really bridges the gap between analog and digital designs. I do not recommend duplicating the 22xx design, but a lot can still be learned from it. I wish we had better data on available RF MOSFET noise characteristics. What is available is intended for RF amplifier applications. |
| Fiorenzo:
I would like to thank you everybody for the many replies. 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. I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have. Thank you again. |
| G0HZU:
--- 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. |
| G0HZU:
--- Quote from: Fiorenzo on December 27, 2021, 08:28:48 pm ---I would like to thank you everybody for the many replies. 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. I am receiving an sds2104x plus in the next two days so I will do a limited comparison with the Rigol mso5000 that I still have. Thank you again. --- End quote --- Sounds good! My first digital scope (Tektronix) was noisy and it spoiled the experience a bit. It is still possible to do good work with a noisy scope but I don't think I'd want to buy another one. Especially if it as noisy as that Rigol. |
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