FFT scope measurements - Page 1
Forum section: electronics / beginners

author: pough
author_rank: Newbie
timestamp: August 15, 2025, 10:39:46 am
content: I'm newbie at metrology, so please correct me if i'm wrong.The base freq for Pico PIO is 125MHz.When I devide that freq. by the int32 scope should show the clean sq. waveform. But for some of the freq. FFT is not as should be in math. calcs.A good example is 25MHz.On SDS1202X-E scope set with ref.Level at 36,4 dB I'm reading:1st odd harm. 25MHz (24 MHz reading is OK since the scope jumps every 2MHz),3rd odd harm. 75MHz (scope gives me HIGH 74MHz), but erarlier I read strong signal of 51MHz,5th odd harm. 125MHz (scope gives me LOW -weak ~-13dB for that freq.) while I get a HIGH strong ~+10dB for 102MHz and another HIGH strong ~+3dB for 149MHz. The next 7th odd harm is about to be OK 175MHz with HIGH scope reading.Why 125MHz (3rd harm.) has so bad reading on FFT.Enclosed the scope screen with 125MHz in the meadle for FFT (SDS00007.png).For other freq. ie: 31.250 MHz the reading are almost acurate (32MHz, 94MHz, 156MHz, 220MHz and 284MHz) - SDS00008.png

author: pough
author_rank: Newbie
timestamp: August 15, 2025, 04:06:53 pm
content: Just noticed a mistake...The question soppouse to be:Why 125MHz (5thharmonic) has so bad reading on FFT?Why there are stronger signals on the side of the some odd peaks? for some frequencies?

author: radiolistener
author_rank: Super Contributor
timestamp: August 15, 2025, 04:24:52 pm
content: It is important to clarify how exactly you generate 25 MHz from the 125 MHz base clock?If you use a simple integer divider by 5, you cannot achieve an exact 50% duty cycle - the output will be high for 2 base cycles and low for 3 cycles (or vice versa). This asymmetry introduces strong even harmonics (50, 100, 150 MHz, etc.) and alters the amplitude of some odd harmonics. In your FFT, the suppression of the 125 MHz component is expected for this duty cycle, while the peaks near 51, 102, and 149 MHz are the even-harmonic products of the non-symmetric waveform.The small frequency offsets you observe are due to limited FFT resolution. Your oscilloscope (200 MHz BW, 1 GS/s) is also at its limits for analysing waveform details at these frequencies - both frequency resolution and amplitude accuracy in FFT mode will be limited. Such an oscilloscope is good for analyzing signal details up to 10-20 MHz, but for higher frequencies its resolution is not sufficient to see the details.When dividing by an even number such as 4, a simple divider can provide a 50% duty cycle, which naturally suppresses even harmonics and gives a "cleaner" spectrum. However, even integer division is not perfect: any asymmetry in rise/fall times will still slightly distort the spectrum. Most digital divider circuits cannot provide an exact 50% duty cycle on the output even for even dividers, there will be some minor error due to asymmetry between 1->0 and 0->1 transitions.For applications where spectral purity matters, frequency synthesis is usually done with PLL-based clock generators (for example si5351, ADF4351, etc). Such devices first multiply the reference to a high internal frequency (usually to 1-10 GHz), then divide down with phase-balanced logic, maintaining accurate duty cycle and reducing unwanted harmonic content - even for odd and fractional division ratios.For the same reason, when designing FPGA-based systems, simple logic dividers should not be used to generate clock signals.The duty cycle accuracy and phase delay of the clock directly affect the stability of the logic being driven and the maximum achievable operating frequency. Therefore, dedicated PLL blocks must be used for clock generation. A PLL ensures precise duty cycle control and a defined phase relationship between the input and output clocks - something that cannot be reliably achieved with a divider implemented in ordinary digital logic.

author: pough
author_rank: Newbie
timestamp: August 15, 2025, 05:28:48 pm
content: 50% Duty Cycle - Something that I did not consider, up until now.Will have to read more about it, and how it affects the mesurement.@radiolistener: Thank you for your response. I appreciate it a lot.I think I got it.Divider of 2xwill help to get the clean 50% duty cycle. Other dividers (also even) could not...This is prooved by mesurements, as far as I remeber.Have to check it on the scope

author: pough
author_rank: Newbie
timestamp: August 15, 2025, 06:12:58 pm
content: OK. I've checked that on the scope. The +Duty and -Duty is available on the scope Mesure function.Every time, when the divider is even, the duty cycle splits evenly (50% Duty Cycle is achived) and therefore the FFT mesurement is correct with the math calc.