Electronics > Projects, Designs, and Technical Stuff
Potential DIY Oscilloscope project, screen refresh rate?
David Hess:
--- Quote from: rhb on April 22, 2019, 11:15:19 pm ---I'm sorry, but I've been doing oil industry DSP since 1982. We were doing DSP long before anyone else. Enders Robinson hand digitized seismic data in 1952. He then went on to apply the first Wiener prediction error aka deconvolution aka FIR filter with a desk calculator. If what you are saying is accurate, then the EEs responsible should be shot and dumped in a ditch. Preferably before they reproduce.
--- End quote ---
The requirements for seismology are not the requirements for general purpose time domain instruments.
--- Quote ---Anti-alias filters are an absolute requirement no matter what the sample rate.
--- End quote ---
Sampling instruments lack anti-alias filters. They even generally lack amplifiers before the sampling stage because they would compromise the passband and transition band response. Most instruments intended for use in the time domain lack anti-alias filters; they just get in the way.
--- Quote ---There is no way you can guarantee that you do not have unexpected noise above Nyquist.
--- End quote ---
There is an interesting observation to make about that. For reasons I do not understand, modern DSOs tend to implement bandwidth limiting early in the signal chain instead of after the variable gain amplifier. I wonder if aliased noise contributes to their high noise levels?
--- Quote ---ETS works *if and only if* the signal is band limited.
--- End quote ---
That is only true if triggering occurs after digitization. Modern DSOs which lack analog triggering effectively lack ETS. (1) They mostly make up for it with high real time sample rates.
--- Quote ---I have yet to see *any* DSO with a Gaussian or Bessel step response. The Keysight MSOX3104T and the R&S RTM3104 certainly do not have them.
--- End quote ---
Older Keysight instruments had a Gaussian or Bessel step response. There is a discussion thread here somewhere where people measured and reported the amplitude response of their DSOs and I think every one was a Gaussian or Bessel response. The admittedly old DSOs I have all have a Gaussian response unless bandwidth limiting is engaged in which case it becomes a single pole Bessel response or somewhere in between.
Older and modern higher frequency oscilloscopes and DSOs often have a maximally flat response but this is simply because a Gaussian or Bessel response is not practical and it has advantages in the applications they are more likely to be used for or at least that is what Keysight argues. Modern instrument correct this frequency and phase response in the digital domain but I wonder when that started.
The only example I know of where high order filter was used in a DSO is an odd example from Tektronix. Their last differential comparator was 150 MHz and included 4 pole 100 MHz and 20 MHz bandwidth filters. I suspect now that this was done for noise reduction purposes because the design of a differential comparator gives it almost 10 times the noise of a comparable singled ended input. The preceding instrument may have had a 2 pole 5 MHz bandwidth filter for the same reason.
(1) There are at least two ways, centroid timing and transition midpoint timing, to implement triggering in the digital domain without that limitation however both still involve a comparison in the analog domain. I think some frequency counters use this method for very high measurement rates. I suspect some high end DSOs which support ETS and the fastest waveform acquisition rates may do it.
Scrts:
Just FYI: https://twitter.com/azonenberg check his FREESAMPLE and glscopeclient.
rhb:
--- Quote from: David Hess on April 23, 2019, 02:15:24 am ---
The requirements for seismology are not the requirements for general purpose time domain instruments.
--- End quote ---
Quite correct, seismic is *much* more demanding. Not only must the signal be sampled adequately in time, it must also be sampled adequately in space. The acquisition is 32 bit floating point and the contract specifications are many pages and specify everything short of the phase of the moon. A single marine survey costs upwards of $10 million to acquire and possibly as much to process. For time lapse (4D) work, the new survey is required to place the receivers and sources within a meter or two of the previous shoot. Typical spacing inline is 6.25 m. Crossline is larger to keep the streamers from getting tangled up. The boats tow 6 or more streamers.
The waveform fidelity of the best DSO on the market today would not have met seismic requirements 40 years ago.
--- Quote ---
--- Quote ---Anti-alias filters are an absolute requirement no matter what the sample rate.
--- End quote ---
Sampling instruments lack anti-alias filters. They even generally lack amplifiers before the sampling stage because they would compromise the passband and transition band response. Most instruments intended for use in the time domain lack anti-alias filters; they just get in the way.
--- End quote ---
Indeed. However, sampling scopes are *very* limited in what they can do. But what they can do is very cool. I just bought an SD-32 head yesterday.
--- Quote ---
--- Quote ---There is no way you can guarantee that you do not have unexpected noise above Nyquist.
--- End quote ---
There is an interesting observation to make about that. For reasons I do not understand, modern DSOs tend to implement bandwidth limiting early in the signal chain instead of after the variable gain amplifier. I wonder if aliased noise contributes to their high noise levels?
--- End quote ---
How could that happen? It suppresses the aliasable signal before it is amplified. Are the variable gain amplifiers that noisy? I'd expect that the OEMs are relying on the GBW product of the amplifier for part of the anti-alias filtering roll off.
--- Quote ---
--- Quote ---ETS works *if and only if* the signal is band limited.
--- End quote ---
That is only true if triggering occurs after digitization. Modern DSOs which lack analog triggering effectively lack ETS. (1) They mostly make up for it with high real time sample rates.
--- End quote ---
I am relying on Shannon's analysis in my statement. The EE community has clearly abused Shannon quite a bit more than I would ever dare.
I don't agree with you assertion about triggering for ETS after the ADC, but I'll defer that discussion until I start implementing the DSP chain on the FPGA.
--- Quote ---
--- Quote ---I have yet to see *any* DSO with a Gaussian or Bessel step response. The Keysight MSOX3104T does not.
--- End quote ---
Older Keysight instruments had a Gaussian or Bessel step response. There is a discussion thread here somewhere where people measured and reported the amplitude response of their DSOs and I think every one was a Gaussian or Bessel response. The admittedly old DSOs I have all have a Gaussian response unless bandwidth limiting is engaged in which case it becomes a single pole Bessel response or somewhere in between.
--- End quote ---
I'd like to see the step responses from a reliable fast edge.
My expectations of scope step responses are based on the Tek 465 generation. When I observed a 7% overshoot and 430 ps rise time on the MSOX3104T I went to read the datasheet. Keysight calculated rise time for the lower end models of the MSOX3K as 0.35/BW, but for the 1 GHz model it was 0.45/BW. I was horrified. That is sleazy marketing.
David Hess:
--- Quote from: rhb on April 23, 2019, 02:09:24 pm ---
--- Quote from: David Hess on April 23, 2019, 02:15:24 am ---There is an interesting observation to make about that. For reasons I do not understand, modern DSOs tend to implement bandwidth limiting early in the signal chain instead of after the variable gain amplifier. I wonder if aliased noise contributes to their high noise levels?
--- End quote ---
How could that happen? It suppresses the aliasable signal before it is amplified. Are the variable gain amplifiers that noisy? I'd expect that the OEMs are relying on the GBW product of the amplifier for part of the anti-alias filtering roll off.
--- End quote ---
The amplifiers are not that well controlled and the bandwidth limiting tends to be very early in the signal chain now at least on the instruments I have seen details for. Noise from the full bandwidth of the earlier stages after bandwidth limiting is present and aliases into the measurement.
This might explain why some old DSOs have much lower displayed noise than modern "low noise" instruments despite lacking index graded displays.
Maybe this is done deliberately to support high resolution mode. Applying a clean ramp to the input of those old DSOs and using averaging will clearly display the differential non-linearity of the ADC because the noise is so low. Not only can you see the ADC steps, but they are close to a textbook example when averaging from 8 to 16 bits should result in a smooth line. As you might expect, in sample mode and no input, adjusting the offset/position control clearly shows peak-to-peak noise barely greater than 1 bit.
--- Quote ---
--- Quote ---
--- Quote ---I have yet to see *any* DSO with a Gaussian or Bessel step response. The Keysight MSOX3104T does not.
--- End quote ---
Older Keysight instruments had a Gaussian or Bessel step response. There is a discussion thread here somewhere where people measured and reported the amplitude response of their DSOs and I think every one was a Gaussian or Bessel response. The admittedly old DSOs I have all have a Gaussian response unless bandwidth limiting is engaged in which case it becomes a single pole Bessel response or somewhere in between.
--- End quote ---
I'd like to see the step responses from a reliable fast edge.
--- End quote ---
I could make that measurement but only on an old DSO (because that is all I have at the moment) and it would show the exact response that a 465 would show. I just do not have anything newer for lack of a need and I am as disappointed with modern DSOs as apparently you are. After doing some evaluations, I decided that a 20+ year old DSO was better.
--- Quote ---My expectations of scope step responses are based on the Tek 465 generation. When I observed a 7% overshoot and 430 ps rise time on the MSOX3104T I went to read the datasheet. Keysight calculated rise time for the lower end models of the MSOX3K as 0.35/BW, but for the 1 GHz model it was 0.45/BW. I was horrified. That is sleazy marketing.
--- End quote ---
Mine are also and a Tektronix 2232 delivers exactly that. So did the 100 MHz Rigol DSOs when I tested them just before the DS1000Z series came out. (1)
The results of your 1 GHz Keysight DSO are exactly what I would expect from a maximally flat design which is common somewhere above 500 MHz simply because a single pole roll-off or Gaussian response is not practical. I gather than this bothered enough people that Keysight and Tektronix released application notes discussing it and why it is not a disadvantage.
Older high bandwidth analog oscilloscopes faced this problem as well and in at least one case, the Tektronix 7704A, the instrument was available with a 200 MHz Guassian response or 250 MHz maximally flat response although the 250 MHz response looks pretty good to me (bottom of page).
(1) Knowing what I know now about Rigol's design practices, my testing would include bandwidth and rise time measurements at different signal levels but I am not used to carrying a set of RF attenuators with me.
Navigation
[0] Message Index
[*] Previous page
Go to full version