I have tested the FFT on SDS1000X-E, SDS2000X Plus and SDS5000X extensively and the results have always been correct and accurate. Siglent FFT provides class leading 2 Mpts FFT length (number of FFT points), except for SDS1000X-E, which is "only" 1 Mpts. This enables low resolution bandwidths and low noise, but keep in mind it's still an 8 bit DSO, hence don't trust results below 48 dBFS blindingly. In most cases, the usable dynamic range with decent accuracy can be up to 70 dB though.

Some hints for proper setup of the FFT on Siglent DSOs:

FFT-Bandwidth and RBWThis is quite different to a real SA. There is no menu for the resolution bandwidth and also no direct setting for the FFT-bandwidth, even though we have a menu item for the horizontal scale in Hz/div, which ultimately specifies the visible span. But this is just for zooming into a longer FFT trace; for best speed and lowest RBW we need to make sure that no high zoom factor is required to get the display we want. The following rules apply:

• The analysis bandwidth (FFT-BW) is always half the FFT sample rate (FFT-SR).

• The frequency step (Δf or df) is the sample rate divided by the number of FFT points.

• The resolution bandwidth (RBW) is the frequency step multiplied by a factor specific for the

window function in use.

• The maximum number of FFT points depends on the record length, which in turn increases with slower timebase settings, but is ultimately limited by the maximum memory set in the Acquire menu and of course also the specified maximum possible FFT length. Apart from that, the max. number of FFT points can be further limited by the specific setting in the FFT menu.

SR (Sampe rate) = RBW * k, where k is the 3 dB bandwidth factor in bins, depending on the window function: Rectangle 0.99, Blackman 1.74, Hanning 1.62, Hamming 1.64, Flattop 3.73.

Blackman and especially Flattop are the most universal and useful window functions in practice, whereas Rectangle is rather specialized and should be avoided unless you absolutely know that you actually need it (e.g. for short transients).

Thus: df = RBW / 4 (rounded) in case of the flattop window.

To get the proper settings for any given FFT-BW and RBW pair, proceed as follows:

Determine the FFT samplerate: SR = FFT-BW * 2 [Sa/s];

Determine the number of FFT points: FFT-Pts >= SR / df [-];

Determine the timebase: TB >= FFT-Pts / SR / 10 [s/div];

Setting up an FFT MeasurementEven from the best FFT implementation, we can only expect good results as long as the scope has been set up properly for that specific task. How many so called “reviews” have we seen where FFT has been engaged and some scope settings randomly altered just to get some halfway plausible but actually rather meaningless FFT graph, which was then either praised or criticized?

Of course we can get away with some quick & dirty setup if our requirements are low, but we should never ignore the most important parameters like FFT bandwidth. We won't see anything meaningful, i.e. just some aliasing artifacts, if, for instance, we try to get the spectrum of a 33 MHz signal with just 25 MHz FFT bandwidth. Furthermore, for optimal speed, frequency resolution and dynamic range, we need to put a little more effort into a proper setup, which has quite different requirements compared to the usual Y-t view. Below there is a complete checklist how to properly set up the DSO for analysis in the frequency domain (most of these topics should be obvious, but still listed for completeness):

- Set acquisition mode to normal. Use average only for a good reason and stay away from ERES. Avoid Peak Detect under all circumstances and without any exception!
- Use edge trigger in auto mode to make sure signal acquisition doesn’t stop even when the signal amplitude drops below the trigger sensitivity. FFT doesn’t require a stable trigger by the way.
- Determine the lower bandwidth limit for the FFT analysis. If it is >10 Hz, use AC-coupling for the input channel to ensure maximum dynamic range even with large DC offsets and/or high input sensitivities. If DC-coupling has to be used, use the vertical position control to compensate for the DC offset, so you can get maximum sensitivity, hence highest dynamic range.
- Determine the upper bandwidth limit for the FFT analysis. In order to avoid aliasing artifacts, this should not only cover the desired analysis bandwidth, but include the highest expected input frequency. In general, it’s best to start with a higher upper bandwidth limit and reduce it only after it has been confirmed that there is no significant signal content above the desired final limit.
- Choose the frequency step size according to the explanations given earlier in this article, which would be about one quarter of the required resolution bandwidth when using the Flattop window.
- Find an appropriate set of horizontal timebase setting and the number of FFT points; refer to the explanations given earlier in this article. You should watch the displayed FFT parameters and double check that the chosen timebase together with the selected FFT length (number of points) matches your expectations. Be aware that the desired resolution bandwidth might not be achievable due to the limited choice of sample rates and FFT lengths and/or the maximum specified FFT length of your specific instrument.
- Engage FFT mode, select the correct source channel and start with Split Screen mode.
- Set the vertical gain so that the peak amplitude of the input signal is between ±2 to ±4 divisions.
- Set the FFT center frequency to the arithmetic mean between lower and upper bandwidth limit.
- Set the FFT frequency scale so that the desired analysis bandwidth is displayed on the screen.
- Set the desired level units and make sure the external load impedance matches reality whenever working with power levels, i.e. dBm.
- Set the reference level and vertical scale so that the FFT amplitude range of interest makes best use of the available screen space.
- Setup automatic peak-peak (and maybe RMS) measurement for the input channel, as well as Max for the math channel. During frequency domain analysis, especially in Exclusive mode, keep an eye on the Vpp measurement for the input channel to make sure no overload occurs.
- Select an appropriate window function; refer to the hints earlier in this document.

Hint: stay in Split Screen mode until the amplitude setup is finished and the levels are reasonably stable,

then switch to Exclusive mode. By keeping an eye on the peak to peak measurement of the input signal,

you can still detect an overload condition instantly; the scope indicates that by displaying > instead of = in

front of the measurement value, e.g.

**Pk-Pk[4]>796.00mV** instead of

**Pk-Pk[4]=640.00mV**.