| Products > Test Equipment |
| is it true, oscilloscope must reach at least 4x observed freq? |
| << < (5/21) > >> |
| tggzzz:
--- Quote from: coppice on September 12, 2022, 10:56:06 pm --- --- Quote from: tggzzz on September 12, 2022, 10:03:10 pm --- --- Quote from: coppice on September 12, 2022, 08:47:18 pm --- --- Quote from: tggzzz on September 12, 2022, 08:41:01 pm --- --- Quote from: oxy on September 12, 2022, 06:32:08 pm ---Lets take the example of the picoScope Series 6000E: It has several models with bandwidths ranging between 500MHz and 1 GHz, yet all of them with sampling rate of 2.5Gs/s. As I understand the sampling rate sets over Nyquist the max. frequency that I can observe. Thus what hardware specification differentiates those oscilloscopes on the bandwidth? Thanks for ur very nice inputs! :clap: --- End quote --- For digital signals the period is irrelevant; only rise time is important. For a little theory and some practical measurements, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ If you have a repetitive signal, the sampling frequency is separate to the signal frequency. I have a scope that measures 50ps rise times with ~40kSa/s. No, that does not violate Shannon/Nyquist! Various manufacturers have different names for the techniques, e.g. equivalent time sampling. --- End quote --- I think you mean "for repetitive digital signals". --- End quote --- No, for repetitive signals. To put it anthropmorphically, the scope neither knows nor cares how you interpret the voltage waveform. --- End quote --- OK. I have read the article now. What you said is confusing. You replied to something talking about sampling rates, and gave a reply including reference to a slow sample interval. I assumed by "period" you were referring to the sampling interval. From the article you are referring to the period of a waveform to be sampled. Most digital waveforms don't really even have a period. They don't repeat, so they aren't periodic. --- End quote --- When considering sampling signals, it is necessary to be clear what the signal is. If you have an 10kHz audio waveform modulated onto a 10MHz carrier, then to recover the audio waveform you don't have to sample at >20MSa/s; >20MSa/s is sufficient. You do still need a >10MHz front end and a <50ns sampling pulse width, though. If, as in a TDR, the waveform you are observing doesn't change over time, then you can sample arbitrarily infrequently. The slope I referred to has >10 times slower sampling mode that matches the speed of a thermal pen recorder |
| coppice:
--- Quote from: tggzzz on September 12, 2022, 11:48:13 pm ---When considering sampling signals, it is necessary to be clear what the signal is. If you have an 10kHz audio waveform modulated onto a 10MHz carrier, then to recover the audio waveform you don't have to sample at >20MSa/s; >20MSa/s is sufficient. You do still need a >10MHz front end and a <50ns sampling pulse width, though. If, as in a TDR, the waveform you are observing doesn't change over time, then you can sample arbitrarily infrequently. The slope I referred to has >10 times slower sampling mode that matches the speed of a thermal pen recorder --- End quote --- You only need to sample at twice the bandwidth of the signal (or at one times the bandwidth for analytic sampling). However, for that to work well you have to be sure the signal has no content outside the band of interest. Also, the jitter in the sampling aperture needs to be just as low for a 2Msps ADC being used to sample a 1MHz band around 2GHz, as it would for a 5Gsps ADC sampling the entire band up to 2.0005GHz. Scopes don't have any way to clean up a signal, and their sampling jitter is not usually any better than it needs to be for their basic needs. So, it is rarely practical to use them to sample a limited band well above DC. The jitteriness of the sampling aperture can work in your favour in some situations. It makes ETS work a lot better, for example. |
| David Hess:
Bandwidth and sampling rate are completely independent. The sampling rate must meet the Nyquist criteria to accurately reconstruct the waveform, but this has nothing to do with the bandwidth as defined by the -3dB amplitude response, which is why equivalent time sampling works. As pointed out by tggzzz, a bandwidth limited signal can be reconstructed with a sampling rate greater than twice the bandwidth no matter where in the frequency spectrum it is, within the bandwidth of the sampler. The sampling function itself is equivalent to RF mixing, and the circuits can be identical. RF mixers make great microwave samplers when driven with a suitable pulse through their local oscillator port. The sampling part of an analog-to-digital converter can be modeled as a down-conversion mixer. --- Quote from: oxy on September 12, 2022, 04:43:14 pm ---some people say, if I wanna observe 100MHz, I must use an oscilloscope that reaches at least 400MHz. Is it true? --- End quote --- If you want to see anything other than the fundamental sine wave component of the 100 MHz signal, then the oscilloscope bandwidth needs to be much higher to cover the harmonics. Otherwise a 100 MHz oscilloscope viewing a 100 MHz signal will only display the 100 MHz fundamental sine wave component, and with a 3dB amplitude error. A more useful oscilloscope specification is rise time, which in the general case in nanoseconds is 350 divided by the bandwidth in MHz, so 100 MHz yields 3.5 nanoseconds. Oscilloscope rise time needs to be several times faster than the rise time of the signal for an accurate display. |
| tautech:
--- Quote from: David Hess on September 13, 2022, 12:15:14 am --- If you want to see anything other than the fundamental sine wave component of the 100 MHz signal, then the oscilloscope bandwidth needs to be much higher to cover the harmonics. Otherwise a 100 MHz oscilloscope viewing a 100 MHz signal will only display the 100 MHz fundamental sine wave component, and possibly with a 3dB amplitude error. --- End quote --- FTFY. It need be noted label BW can be substantially different from actual -3dB BW as is the case with the 100 MHz rated SDS2104X Plus which the 1st we received I tested with 3 yes 3 signal sources to convince myself the ~185 MHz result was actually real ! Supporting that result is they are also supplied with 200 MHz probes. |
| Fungus:
--- Quote from: coppice on September 12, 2022, 11:56:35 pm ---You only need to sample at twice the bandwidth of the signal (or at one times the bandwidth for analytic sampling). --- End quote --- Completely false. Imagine a sine wave that you're sampling at exactly 2x frequency. a) You might sample the signal exactly on the peaks/troughs in which case you'll be fine. b) OTOH you might sample it exactly on the zero-crossing points, in which case you'll see nothing at all. You can also get every possible value in between (a) and (b), it's just dumb luck. If you sample at 99.99999% of Nyquist you'll drift slowly between (a) and (b) and see the amplitude varying on screen ("AM effect"). 2.5x Nyquist is the minimum to avoid this AM effect. |
| Navigation |
| Message Index |
| Next page |
| Previous page |