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| StillTrying:
--- Quote from: rhb on June 22, 2020, 11:06:34 pm ---That in turn implies that the AFE has a *real* BW of 3.5 GHz!!! --- End quote --- To me the <100ps rise times imply timing errors rather than a 3.5GHz AFE in a £400 scope. :o I've estimated :o where the trigger XY point is in comparison to the visible screen. for your 1ns steps capture. |
| rhb:
I was playing with the pulser feeding a tee and coax this morning. There is something odd going on with the vertical settings. Here it is with the trigger on screen. So it is *not* related to the trigger position. I'm going to feed the SD-24 to the AFE and look at the TDR response. That should establish whether it's ringing in the input transmission line or some artifact of the DSO implementation. BTW My calculation was wrong. The pixels are at about 100 ps spacing. So the calculation should have been 0.35/200 ps. My Keysight 33622A has a spec of <1 ps jitter, so if I set up a low amplitude square wave and use the external trigger input I should be able to measure the DSO timebase jitter. Have Fun! Reg |
| RoGeorge:
That wave shape is so weird because of 3 things: - DSP processing on the oscilloscope side - they applied all kind of filters to make the image look good, including non causal filters (e.g. Fourier transform can not be applied to causal signals, therefore it should NOT be used for real time signals, Fourier produces "artifacts" because the resulting spectrum is non causal) - might seem like gibberish talk, but this is how it works. For example, the downwards peak, seen just before the signal starts to raise, is fake. There is no reason for why a signal will first go down before going up, that dip before the signal goes up must be the result of non-causal filters (or other kind of non causal digital processing messing with the samples), that sort of "premonition" artifacts can never to be seen in physical systems, like in an analog oscilloscope. Our physical world is causal. - jitter (phase noise) in the oscilloscope's clock - in Rigol, the triggering moment is detected in software Reducing the delay between the triggering point and the piece of shown waveform on the display helps reducing the jitter in the final image (for a given phase noise, the jitter accumulates in time). LATER EDIT: ------------------ One more thing, the 0.35 factor to determine the ratio between raise time and bandwidth of the oscilloscope, it is NOT true any more for digital oscilloscope. That formula is valid only for analogue oscilloscopes, with analog filters and normal roll of (analog filters are always causal). With DSP filters, waveshape corrections are made non-causal (in anticipation of a signal or an edge that is yet to come), and that 0.35 formula is now unrelated with the raise time shown in the "fake" waveform. The Rigol screen capture speaks for itself how fake that signal is. In this Q&A from Tektronix, the factor is 0.45 and is valid for the kind of DSP they apply, but in general, the coefficient depends of the filters, and since DSP filters can be of any kind, the formula is not valid any more: https://www.tek.com/support/faqs/how-bandwidth-related-rise-time-oscilloscopes In short, a low range digital oscilloscope, like a Rigol DS1202Z-E, can not deal correctly with that kind of signal, and is in no way a match for the performance of a high end 20GHz Tektronix SD-24 analog sampling head. |
| Labrat101:
--- Quote from: StillTrying on June 23, 2020, 11:48:42 am --- --- Quote from: rhb on June 22, 2020, 11:06:34 pm ---That in turn implies that the AFE has a *real* BW of 3.5 GHz!!! --- End quote --- To me the <100ps rise times imply timing errors rather than a 3.5GHz AFE in a £400 scope. :o I've estimated :o where the trigger point is in comparison to the visible screen. (Attachment Link) --- End quote --- I have to agree your trigger is off the screen . and that is an acquisition error or the scope is set up wrong. RNS |
| rhb:
The trigger point is shown in the figure in reply #67 and is on the screen. The use of zero phase, non-causal filters in DSOs is a major complaint of mine. It's not the proper filter. Moreover, the correct interpolation filter is *not* a sinc(t). The correct filter is the minimum phase impulse response of the actual anti-alias filter used. To avoid excessive ringing the corner frequency for the anti-alias filter should be no more than 67% of Nyquist and for really good step response 50% of Nyquist. The "Scope wars" thread will measure jitter among many other things. I'm going to show what they are doing wrong and how it should be done. The failure of a DSO to produce analog scope trace quality is a design failure. It is not inherent in the scope being digital. It's driven by the desire of sales and marketing to advertise a scope as having twice the BW it would have if it had a decent step response. And the naive buyers who don't know enough about the Fourier transform to realize they are being swindled. My LeCroy DDA-125 has a claimed 1.5 GHz BW and has the measured rise time for the 0.35/BW to match that. It also has 20% overshoot on a fast step. My DDA-120 has a 350 ps rise time on a stated 1 GHz BW and 17% overshoot. It's worth noting that Nyquist for the DDA-125 is 1 GHz in 4 channel mode. Can you say "aliasing"? The EE "ideal low pass filter" is only ideal if you don't care about the time domain behavior. If you care about the time domain behavior you must apply a long taper to the BW. If you make the corner 50% of Nyquist, the time domain step response is a minimum phase sinc(t)**2 which suppresses the sidelobes very well. One can do better with a Gaussian or sech(f) taper which produce no ringing, but at the cost of slower rise times. With regard to Tektronix I've been waiting since late April for a step response from the 3 series scopes. I stated I didn't want to bother with a demo unless it had less than 3% overshoot on a <40 ps step. Have Fun! Reg |
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