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| Functional comparison of R&S RTB2000, Siglent SDS2000X and Keysight DSOX1000 |
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| RBBVNL9:
Sinisa, thanks for the detailed feedback. Seems you looked at the video more than just cursory (or you’re a very efficient watcher ;-) Let me react to a couple of things you bring up. Firstly: --- Quote ---Please don't take this as offense. The topic is quite complicated and it is very involved to create representative presentation.. It is obvious a lot of work is put in it. Minor errors are understandable.. --- End quote --- Thanks for noting this. I have benefited a lot from reviews of others when choosing or using measurement instruments, and with this series, I hope to contribute myself to the community. But – needless to say – there inevitably will be mistakes, omissions, and perhaps misunderstandings. Especially on a topic as complex as FFT. --- Quote ---First, there is no RBW in FFT, really --- End quote --- Concerning the use of the term RBW, I guess it’s a bit of a matter of terminology… While there is some divergence in the use of this term, Rohde & Schwarz is certainly not the only one using the term RBW in the context of FFT/DFT. Some examples: * National Instruments (NI) explains the term as follows: “The resolution bandwidth (RBW) determines the fast-Fourier transform (FFT) bin size, or the smallest frequency that can be resolved.” (source). * Tektronix writes: “On traditional SAs, the IF filter bandwidth determines the ability to resolve adjacent signals and is also called the resolution bandwidth (RBW). For example, in order to resolve two signals of equal amplitude and 100 kHz apart in frequency, RBW needs to be less than 100 kHz. For spectrum analyzers based on the DFT technique, the RBW is inversely proportional to the acquisition time. Given the same sampling frequency, more samples are required to achieve a smaller RBW. In addition, windowing also affects the RBW.” ( source). * I also see RBW shown on the screen of some other brands of scopes with FFT, such as the GW INSTEK MDO-2000E (screenshot here). But there is divergence indeed. The SDS shows an “∆f” value, and the DSOX shows an “FFT Resolution” value on the screen... I have been under the impression that all of these basically refer to the same thing, but correct me if I’m wrong… This is what the oscilloscope manuals have to say: * RTB: “The resolution bandwidth (RBW) determines the resolution of the spectrum, that is: the minimum distance between two distinguishable peaks. The higher the resolution (the smaller the ratio), the more peaks are detected, but the longer the measurement requires to finish.” * SDS: “Frequency interval (△f): The frequency interval between two adjacent points in the FFT sequence, which is proportional to the frequency resolution.” (Note: I have to think about this ‘proportional….’. The same? With some factor?) * DSOX: “The FFT resolution is the quotient of the sampling rate and the number of FFT points (fS/N). With a fixed number of FFT points (up to 65,536), the lower the sampling rate, the better the resolution.” --- Quote ---RBW is resolution bandwidth, not realtime bandwidth. --- End quote --- Indeed, during the recordings, I accidentally said ‘real-time bandwidth’ instead of ‘resolution bandwidth’; I thought I corrected that (I did so at 2:01:47, for instance), but I must have overlooked one or more. My bad. --- Quote ---RTB doesn't have regular FFT implementation. What they have is kind of simplified version of spectrum mode from it's "bigger" brethren.. --- End quote --- I don't think I came across this before. Do you have any sources or further information? Trying to look this up, I came across a of how R&S implemented FFT in the RTO oscilloscopes (including downconverting and overlapping blocks), but I have no idea this is also the implementation used in the RTB. There does not seem much to find on this on the internet. --- Quote ---Fact that FFT in RTB2000 takes over control of the scope is annoying as hell to me, personally. --- End quote --- Yes, I see your view. In a FAQ, R&S writes “The timebase is adapted automatically when the frequency parameters of the FFT are changed. This is done to archieve optimized results. You can change the timebase in indirect way, if you change the span. Increase the span --> the timebase becomes faster Decrease the span --> the timebase becomes slower” But this may not be as much flexibility as you wish, as you may want to determine your span on other grounds. You do have the possibility to set the time gate (= extract of the timebase for which the FFT is calculate) on the RTB with the Width and Position parameters. Do play with that! But having that said, indeed, you cannot set time base, sample rate and memory depth yourself as in other instruments. --- Quote ---On SDG2000 (or 6000) you don't need to press amplitude for few seconds to get dBm. If you have it set for 50 Ohm, you simply start typing number 0 and press dBm like you did.. --- End quote --- That is true. But I use the method shown in my video because I like to see first what the current setting in dBm is, before I change it into another dBm value… Matter of preferences, I guess. I am not exactly sure what you mean to say. If it ‘sort on peaks’ does not sort the table, then what does it do? On my device, I see no difference whether I select “Sort to @@” or “Sort to peaks” I get the same table, same marker numbering, etc. --- Quote ---On SDS2000X+, sort peaks (frequency/amplitude) is applied to TABLE view. Peaks markers are always enumerated from left to right in increasing number order. --- End quote --- I looked at it again, and… now I see your point. The “sort by” does not sort the numbers give to the peak markers, but the order in which they are shown in the table. Not sure why I did not see that before. But the fact the SDS manual says "Sort peaks by amplitude or frequency" instead of "Sorts the table by amplitude or frequency" does not help, though. --- Quote ---Noise floor comparisons are valid only if you have exactly same number of data points, time interval bins, averaging and same windowing. Otherwise they will differ.. --- End quote --- Yes, and that is exactly why I added the section starting at 2:01:15 to the video. To point out that all these parameters do matter and affect the outcomes. Since it is impossible to set all devices to exactly the same settings, for most tests I did, I tried to take a users’ perspective and find the best settings I could achieve for each device (and when I do this best-achieved results approach, I say it in the video). For the speed test, however, I tried to test with settings that were as close as possible (and I mention this also in the video). Again, thanks for the feedback, Best, Rudi |
| Performa01:
For the FFT math, the correct term for the frequency spacing is "bin-width". For a 1 Mpts FFT at 1 GSa/s, if you have a mega (1048576) bins, each of them is 953,67 Hz wide. The effective Resolution Bandwidth (RBW) is closely related to the bin-width, but not identical. There is a factor involved, depending on the window function in use. In theory, that factor ranges from 0.89 (Rectangle) up to 2.94 (Flattop), yet when I last measured it, the Flattop window in some Siglent DSO required a factor of 3.8 to determine the true -3 dB bandwidth. Rectangle window is to be avoided except some very special applications like short transients, whereas Flattop is the most accurate window function that is universally used in spectrum analyzers. |
| rf-loop:
--- Quote from: Performa01 on July 15, 2022, 08:03:01 am ---For the FFT math, the correct term for the frequency spacing is "bin-width". For a 1 Mpts FFT at 1 GSa/s, if you have a mega (1048576) bins, each of them is 953,67 Hz wide. The effective Resolution Bandwidth (RBW) is closely related to the bin-width, but not identical. There is a factor involved, depending on the window function in use. In theory, that factor ranges from 0.89 (Rectangle) up to 2.94 (Flattop), yet when I last measured it, the Flattop window in some Siglent DSO required a factor of 3.8 to determine the true -3 dB bandwidth. Rectangle window is to be avoided except some very special applications like short transients, whereas Flattop is the most accurate window function that is universally used in spectrum analyzers. --- End quote --- Just measured 2000X HD. Using center f ~800kHz (freq. fine adjusted to nearest bin) Flattop window. FFT ∆f 5.96Hz (FFT 2.5MSa/s, 2Mpts) -3dB Bandwidth (=RBW) 21.2Hz and width at -60dBc 54Hz. (shape factor ~2.55) (Using 200Hz span, I have measured from linear interpolated lines between bin points (just as FFT draw it)) In this case Flat top window responds RBW 3.56 x FFT ∆f (example Tektronix tell FlatTop window factor is 3.77) |
| pdenisowski:
--- Quote from: RBBVNL9 on July 13, 2022, 07:05:18 pm ---On request, I have decided to do the FFT episode as the next one in the series. --- End quote --- Hi Rudi - Very nice video! Thanks for making these. With regards to the RTB and FFT, two small points: 1) You can change the horizontal scaling of the spectrum using the horizontal knob 2) Autoset sets both the channel parameters and the FFT parameters Thanks again! |
| RBBVNL9:
Performa-01, rf-loop, Very valuable contributions, thank you. They help to understand the difference between the parameters the devices show. With some visual comparison with a 800kHz sine wave as input and a Flat Top window, I indeed see when the RTB is set to 209 Hz, the width of lobe (using cursors) is approximately the same as the lobe on the SDS with FFT ∆f equal to 76 Hz. That would indeed be in line with a factor of approx. 3.5, as you both explain. The DSOX is the same as the SDS in this respect. So, my conclusion in terms of the values these devices show on their screens, would be: * RTB shows (and allows to set) the RBW, which, when using a Flat Top window, is approximately 3.5 times the FFT frequency spacing (a.k.a. the "bin-width"). * SDS shows (as a result of the other settings) the “Frequency interval (△f)” which is the same as the FFT frequency spacing (a.k.a. the "bin-width"). * DSOX shows (as a result of the other settings) the “FFT resolution”, which is, like the SDS, the same as the FFT frequency spacing (a.k.a. the "bin-width"). |
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