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Products => Test Equipment => Topic started by: bmdaly on June 09, 2019, 01:26:21 pm

Title: Siglent SDS1104X-E - FFT frequency centering
Post by: bmdaly on June 09, 2019, 01:26:21 pm
Hi,

Is there any means of auto-centering the frequency in FFT mode, so that the left hand side of the frequency scale always starts with 0Hz, regardless of horizontal scale?
This would be useful where one wants to zoom out briefly, while still using the full screen horizontal range.

In the attached examples, I needed to make 2 adjustments to keep the screen full - otherwise, I would get an unused negative frequency range to the left of the screen.

For reference, I've also attached a screenshot from Picoscope's FFT mode - you set an upper frequency, rather than Center frequency and Hz/div on horizontal scale.
Title: Re: Siglent SDS1104X-E - FFT frequency centering
Post by: rf-loop on June 09, 2019, 03:52:57 pm
At this time (not know future) there is no. (there is free places for function button after Center and Hz/div)

There is so many users and needs or wishes. Also if used different machines then may miss that other machine works like machine what have previously used.

So example me, I think I'm interested some point in freq axis and so I want mostly set center to this signal frequency what I'm interested and span how then need as is also very usually (my) practice with spectrum analyzers. But also some times need define start and stop and so on.

Example here is 455kHz carrier and I want look it because I suspect there is some problem. So I want  check how it is near carrier.

First more wide display and after then zoom in to look near carrier things ... oups.. and there I can find 50Hz sidebands..looks like ~1% AM modulation.. with -46dBc sidebands) so after I see this in signal what need be good I know what to do next for example repair... (here mains freq is 50Hz and same case in example US show 60Hz sidebands)
If FFT horizontal is zero freq oriented I will throw it out from window or try find other tool.

But. Perhaps, due to very different users/needs. To wishlist: Add feature where User can define horizontal start and span (and/or stop) These are also normal in usual SA's.
Title: Re: Siglent SDS1104X-E - FFT frequency centering
Post by: Performa01 on June 09, 2019, 04:10:21 pm
Hi,

Is there any means of auto-centering the frequency in FFT mode, so that the left hand side of the frequency scale always starts with 0Hz, regardless of horizontal scale?
This would be useful where one wants to zoom out briefly, while still using the full screen horizontal range.

In the attached examples, I needed to make 2 adjustments to keep the screen full - otherwise, I would get an unused negative frequency range to the left of the screen.

For reference, I've also attached a screenshot from Picoscope's FFT mode - you set an upper frequency, rather than Center frequency and Hz/div on horizontal scale.
The short answer is “no”, however here comes the long one: ;)

The Picoscopes have an independent FFT module with a nice and simple user interface. The scope is set up automatically to give you the best possible FFT result. You just need to be careful to not run into aliasing issues – always set the upper frequency high enough so that all frequency components of the input signal are within that span. Use the zoom feature if you want to look at a particular subrange.

The FFT in the SDS1004X-E doesn’t work like this, you need to set it up manually.
Looking at your screenshots, I don’t think this is an optimal FFT setup. You’re using 1Mpts FFT length, yet the resolution is poor at 476.84Hz. With that many points, FFT is not very fast on top of that.

Have a look at the attached screenshot, where I’ve tried to replicate a similar scenario, but with an alternative setup.

I’ve limited FFT length to 32k and set the timebase to 10ms/div. By this I get a sample rate of 500kSa/s, hence an FFT bandwidth of 250kHz where the frequency resolution is much finer at 15.26Hz and the FFT is faster updating too.

I’ve also set a vertical scale of 10dB/div, there’s little point in having more, as the max. dynamic range of the 8-bit FFT in this scope cannot exceed 80dB anyway. Finally I’ve set the reference level in order to make good use of the available screen height.