1k sine single fluctuates a bit where 1k square is steady as a rock? - Page 1
Forum section: electronics / beginners

author: LiftedTrace
author_rank: Regular Contributor
timestamp: March 05, 2025, 10:40:20 pm
content: I just got a new rogol DHO804 oscilloscope and a rogol DG1022Z signal gen.The signal gen only came with a dual ended bnc cable and i dont have any cables yet.So to play around, i used my provided scope probes that came with the oscilloscope.Both set to x1, i connected them together and sent a 1k sine wave out of the signal gen and the oscilloscope shows it as a fluctuating 1k signal.Its sublte variation, maybe 1.005k to .995k, but the signal on screen jostles too a bit.When doing a square wave, its dead on 1k, with no movements up or down.Im curious, what would cause the 1k sine to not be rock steady.Is it my poor probes provided with the oscilloscope?Is the signal gen not capable of providing a steady signal?Thanks.

author: ataradov
author_rank: Super Contributor
timestamp: March 06, 2025, 12:17:50 am
content: Show screenshot of the scope setup and the waveform. It may be your triggering setup.

author: radiolistener
author_rank: Super Contributor
timestamp: March 06, 2025, 08:46:38 pm
content: Quote from: LiftedTrace on March 05, 2025, 10:40:20 pmIt's unclear what you mean by "subtle variation" and "the signal on screen jostles a bit."It would be clearer if you could provide a screenshot from the oscilloscope.If you're referring to measured frequency variations, that's normal. Oscilloscopes typically use a short measurement period, which can introduce noticeable measurement errors.If the waveform appears distorted, it could be due to an excessive amplitude setting, causing the generator to struggle to produce a clean signal.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: March 06, 2025, 10:15:11 pm
content: Quote from: radiolistener on March 06, 2025, 08:46:38 pmYeah when you have the measurements on screen, the measured freq fluctuates a bit and the amplitude fluctuates a bit. Im sort of new to this so I am not sure what is normal and what is not.Attached are two screen shots.The fluctuations are quite small I admit, I was expecting something like 1khz being so low to just be a nice firm steady trace like the square wave is.I suppose for a square wave its triggering off a vertical edge as opposed to a continually moving sine wave edge. I can see this being much more challenging to trigger in the same spot each time.The signal generator has trigger outputs, would this be a reason something like those would be used?Thanks.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: March 06, 2025, 10:20:12 pm
content: noise.1kHz is a very slow transition through the trigger point and noise in the system will alter the timing.yes, that's why a logic-level trigger is often provided in sig gens etc.oscilloscope frequency et al measurements are "ok", far from perfect.

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2025, 10:33:19 pm
content: Timing and frequency measurements result from the same system used for triggering.When triggering at zero or thereabouts, a square wave transition passes through the trigger level very, very quickly, so a bit of voltage noise causes very, very little shift of the time at trigger.The speed of the square wave transition is ideally infinite, but real square waves and dsos have a finite rise time and sample rate.A sine wave, however, has an inherent slope (slew rate) going through zero and therefore a small voltage noise will result in more random “jitter” in the timing than for a square wave.

author: LiftedTrace
author_rank: Regular Contributor
timestamp: March 06, 2025, 10:44:25 pm
content: Quote from: TimFox on March 06, 2025, 10:33:19 pmI could test this, but is this saying that the higher the freq the more stable it will be because its passing though much faster and closer to a square wave, and a much slower freq might show even more wildly fluctuating values because its much slower passing through the trigger point?

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2025, 10:56:11 pm
content: Quote from: LiftedTrace on March 06, 2025, 10:44:25 pmIf you increase the frequency, the speed through the trigger point increases, but the period between triggers decreases. Apply that to your question.