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| How much noise floor and other things matter in oscilloscope usability |
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| gf:
--- Quote from: David Hess on December 26, 2021, 11:38:41 am ---High resolution mode is usually or always implemented as a boxcar averaging filter for simplicity since it must operate at the maximum sample rate during decimation, so it should produce something like a sinc response which is what is shown. --- End quote --- In the screeshot, obviously a 8-tap boxcar averaging filter (or similar) was applied, but without down-sampling, otherwise the 3 side-lobes were no longer visible in the spectrum, but already folded down to the first Nyquist zone of the lower sampling rate. So I was just wondering, whether this was really "HiRes" mode (in the sense of LeCroy's definition), or rather a different mode which just applies a post-acquisition filter. Indeed, when it must run in real-time, during acquision, then a boxcar filter has of course the simplicity advantage that it can be implement as CIC filter, not requiring any multiplications. Given the huge memory depths available today, a scope manufacturer may be tempted, though, to renounce capture-time DSP at all, and support only post-acquisition filters, for cost reasons. |
| Performa01:
--- Quote from: David Hess on December 26, 2021, 11:38:41 am --- --- Quote from: Performa01 on December 25, 2021, 10:10:33 pm ---For low frequencies, things are a lot more complex than just a FET buffer, because of the split path design of all contemporary wideband frontend designs. The practical consequence is, that general purpose (wideband) oscilloscopes generally aren’t well suited for low frequency tasks below about 10 kHz regardless of the probes used. There are specialized instruments for this. --- End quote --- Split path high impedance buffers started showing up not long after integrated low input bias current operational amplifiers in the 1970s. The split path actually reduces low frequency noise because even a noisy operational amplifier has lower flicker noise than the RF FET used for the high impedance buffer. Sometimes it is a lot lower. The disadvantage of the split path design is that without careful consideration, overload recovery can be horrible. --- End quote --- Yes, split path input buffer have been invented a long time ago – and it’s all the more baffling that most people don’t seem to be aware of it and make it sound as if an oscilloscope frontend still consists of a cascade of differential amplifiers. Maybe some even think it consists of just a high speed OpAmp… If you actually think the LF noise in a split path design would be reduced, you’re forgetting that the LF path has to be attenuated quite a bit (usually up to 10 times) in order to get the desired input protection and a decent offset compensation range. This has to be compensated for by a corresponding gain in the OpAmp. Together with the high source impedance of the divider (which has to have a total resistance of 1 meg) this can raise the noise floor by more than 20 dB below the crossover frequency. So there is no way around the sad fact, that the usual general purpose DSO isn’t well suited for precision work at low frequencies because of the steeply rising noise floor down there. |
| Performa01:
--- Quote from: gf on December 26, 2021, 11:09:48 am --- --- Quote from: Performa01 on December 25, 2021, 10:10:33 pm ---The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot. --- End quote --- The spectrum reminds me on the typical frequency response of a 8-tap moving average filter (possibly in addition to other filters). Is this the well-known HiRes mode, or yet a different mode? --- End quote --- It is either HiRes or ERES - I'm not quite sure - but in any case it is a true acquisition mode, in the sense of a real time pre-processing. The sample memory gets halved in this mode, because it is expanded to 16 bits width as the captured raw data now consists of 10 bit samples. All the post processing, measurements and math are now using the 10 bit data. The firmware cannot tell the difference between this resolution enhancement (implemented in the FPGA) or a true 10 bit ADC. |
| David Hess:
--- Quote from: gf on December 26, 2021, 12:38:08 pm ---Indeed, when it must run in real-time, during acquision, then a boxcar filter has of course the simplicity advantage that it can be implement as CIC filter, not requiring any multiplications. Given the huge memory depths available today, a scope manufacturer may be tempted, though, to renounce capture-time DSP at all, and support only post-acquisition filters, for cost reasons. --- End quote --- The implementations I have seen all used a power-of-2 number of samples so the filter could be implemented with only adds and shifts, and if promoting 8-bit acquisitions to a 16-bit record, only adds. Modern low end DSOs usually only produce an 8-bit acquisition record but all of the old Tektronix DSOs promoted 8 and 10 bit samples to 16-bits immediately and did all processing in 16-bits. Tektronix was very scrupulous at one time. |
| David Hess:
--- Quote from: Performa01 on December 26, 2021, 12:45:46 pm ---Yes, split path input buffer have been invented a long time ago – and it’s all the more baffling that most people don’t seem to be aware of it and make it sound as if an oscilloscope frontend still consists of a cascade of differential amplifiers. Maybe some even think it consists of just a high speed OpAmp… --- End quote --- Differential amplifiers are still routine and the highest performance digitizers have differential inputs. Usually the first stage after the low impedance attenuators converts from single ended to differential, and this stage is convenient for adding the combined position and offset signal is introduced. The various modern PGAs used in oscilloscopes are differential so they follow the same pattern, but since they replace the low impedance attenuators, position and offset are added after. DSOs with a separate offset control will add it before the PGA. Old designs which do this have to somehow add the offset before some of the attenuation stages which means moving some of the attenuators to the differential part of the signal chain which is relatively expensive. --- Quote ---If you actually think the LF noise in a split path design would be reduced, you’re forgetting that the LF path has to be attenuated quite a bit (usually up to 10 times) in order to get the desired input protection and a decent offset compensation range. This has to be compensated for by a corresponding gain in the OpAmp. Together with the high source impedance of the divider (which has to have a total resistance of 1 meg) this can raise the noise floor by more than 20 dB below the crossover frequency. --- End quote --- That is a good point that I had forgotten, but the noise can still be lower even in old designs. Old designs which have two separate x10 high impedance attenuators limit the input range to the buffer to 1/10th the level of new DSOs, so attenuation on the DC path is also lower. The Tektronix 22xx series only attenuates by 1.33. Luckily for the discussion here, low frequency noise is irrelevant because wideband noise at 20 MHz and higher bandwidths dominates. --- Quote ---So there is no way around the sad fact, that the usual general purpose DSO isn’t well suited for precision work at low frequencies because of the steeply rising noise floor down there. --- End quote --- I agree but if you include older instruments, then some general purposes DSOs are much better than others at low and/or high frequencies. I have not tested enough modern low end DSOs to know if they all have subpar noise performance. Even with older instruments though, I gave up on good low noise performance a long time ago with the exception of anything with the Tektronix 5A22/7A22/AM502. At low frequencies it is relatively easy to make a low noise amplifier, but since oscilloscopes lack the noise marker function for their FFT, I would like to have a low noise dynamic signal analyzer instead. |
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