Timeline for Strange extra frequency in crystal oscillator
Current License: CC BY-SA 3.0
12 events
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| Jul 27, 2016 at 17:17 | comment | added | Jared Goguen | Is it possible for sample aliasing to result in an artifact frequency that is greater than the actual frequency? That seems terribly counter-intuitive to me. For example, with a sampling frequency of 25 MHz, the alias frequency would be the 12.28 MHz peak, not the 12.72 MHz peak... right? | |
| Jul 27, 2016 at 15:02 | comment | added | Olin Lathrop | @Dmitry: Yes, that sounds like something to at least consider when you see that kind of artifact. | |
| Jul 27, 2016 at 14:48 | comment | added | Dmitry Grigoryev | @OlinLathrop BTW, would an unexpected FFT symmetry be a good indicator of a sampling problem here? | |
| Jul 27, 2016 at 13:26 | comment | added | stefandz | @SredniVashtar this is fantastic - I shall give this a try! Looks like a great piece of software (and a pretty awful oversight on behalf of Rigol not to implement FFT in a more flexible manner) - thank you! | |
| Jul 27, 2016 at 13:21 | comment | added | Sredni Vashtar | @stefandz Rigol scopes (at least the 1000Z series) have this habit of outputting measurements based on the displayed data only. I believe that even with a 1Ts/s sampling rate you will be limited by what is shown on the screen. Try to acquire the raw data and compute the FFT on a PC to see if you get the same result. Something like this rheslip.blogspot.com/2015/09/… might help. | |
| Jul 27, 2016 at 12:52 | history | edited | Olin Lathrop | CC BY-SA 3.0 |
added 490 characters in body
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| Jul 27, 2016 at 12:46 | comment | added | Olin Lathrop | @stef: First, that impressive Gsamples/s is probably only on the fastest time scales. Second, even with a lot of sampling, it is still aliasing when showing you the result on probably a 256 or 360 pixel screen. | |
| Jul 27, 2016 at 11:15 | comment | added | stefandz | @pipe perhaps time for me to RTM! | |
| Jul 27, 2016 at 11:14 | comment | added | pipe | @stefandz This is why I hinted at perhaps operator error - I wouldn't be so sure that the FFT in Rigol works with all those 1 Gs/s when you're zoomed-out. Maybe it has different modes, dropping samples down to a fixed number to gain speed. Maybe you can turn that off. | |
| Jul 27, 2016 at 11:13 | comment | added | stefandz | BTW, which part of Nyquist-Shannon theory am I missing out on here? The DSO sample rate is 1 Gs/s with a front-end anti-aliasing filter. Although I realise I would get a more faithful reproduction with a whole number of cycles (which I can't do due to a scope limitation - but I partially allow for using a Blackman window) what else can I do to better use the FFT? | |
| Jul 27, 2016 at 11:10 | comment | added | stefandz | Thanks for this, Olin. Adjusting the capture window to include just one or two cycles per division helps (at the cost of frequency domain resolution). Also, viewing using an analogue scope shows a nice, jitter-free clock. I guess the mistrustful part of me finds it hard to reconcile whether the adjustments I made result in a more faithful representation of the original signal or whether I have just dialled out what I didn't want to see, if that makes sense. Sadly I can't adjust the capture window to be a full number of cycles which would also help. | |
| Jul 27, 2016 at 10:51 | history | answered | Olin Lathrop | CC BY-SA 3.0 |