4ns jitter is totally absent from sine waves and anything else based on sine waves
this 4 ns IS PRESENT on sine wave, but you don't see it on oscilloscope, because sine wave has too slow slope. You're need to use higher frequency sine wave in order to see it on oscilloscope. Or just use spectrum analyzer, it will show you this jitter (as parasitic spectral components) for sine wave on any frequency.
This picture will explain you why this 4 ns jitter is present on sine wave, but you don't notice it on oscilloscope:
Hi radiolistener,
That's a very nice picture you've provided. However, it does seem to be based on a false premise. I've looked quite closely at the sine wave zero crossing points. Increasing the Y amplifier gain is an effective way to steepen up the flanks of any waveform to unmask any such hidden DAC clock jitter artefacts. At best, all I can see is the usual random jitter noise present on all wave forms.
Only square waves and all others which likewise include a fast edge transient, have exhibited this DAC clock jitter artefact. If you possess one of these Feeltech signal/function/Arbitrary Wave generators (FY66/6800), a simple experiment can demonstrate these observations made by both bdunham7 and myself.
There's no need for a separate 10MHz high precision reference (that just adds another dimension to the observations), nor does the generator have to be upgraded to a better 50MHz clock oscillator. You simply set the frequency to 10MHz exactly and then apply a 10mHz offset which will effectively provide a strobing effect to reveal the 4ns jitter in slow motion. The signal generator's 50MHz clock doesn't even need to be calibrated, the results will be the same regardless.
Having set the frequency, you can now work your way through as many wave forms as you care to examine. You will need to observe each one for at least ten seconds at a time. You will also need to display several cycles worth on the 'scope display. For a ten MHz frequency, this means a setting around the 50ns per division mark in most cases.
If you can run this simple experiment, I'd be quite interested in your own conclusions as to what is actually happening. Mine for the most part are based on the basic theory of digitising analogue wave forms in a bandwidth limited system whose upper frequency limit resides below the Nyquist frequency limit.
All of the wave forms which don't include fast transient edges requiring in theory infinite bandwidth can be conveniently stored as an "Arbitrary Wave form" and clocked out at any frequency without distorting the waveshape since such waves (sines, triangles and other similarly curved waveforms free of any infinitely fast transients) are entirely scalable in their wave shape in both amplitude and their time base.
However, when it comes to the other wave forms possessed of a theoretically perfect infinitely fast transient such as ramps and square/rectangle waves where a less than perfect 7ns rise/fall time compromise has to be accepted, these must be being handled in a completely different way in order that the rise/fall times remain at 7ns regardless of their fundamental frequency (strictly, repetition rate) whether it's one milliHertz or ten MHz.
Since I'm no expert in DDS technology, nor have I studied it in any depth, I can only guess that such sharp edged waveforms which need to preserve fixed rise and fall times regardless of repetition rate (frequency) are handled outside of the bandwidth/Nyquist frequency limited digital processing of the less demanding stored waveforms.
Since the stored arbitrary waveforms which have no such conflicting requirement to generate edges with a fixed rise/fall time regardless of frequency don't suffer from the 4ns jitter, courtesy of the bandwidth limited processing, you might imagine that storing a square wave as an arbitrary wave with a 7ns rise/fall on a 30MHz representation would solve the problem but a moment's consideration will tell you that when such a waveform is clocked out to produce a 3MHz square wave, it will have rise/fall times of 70ns, scaling to 700ns at 300KHz and a whopping 70μs at 3KHz.
Quite obviously, this idea is a non-starter. The method actually being used by Feeltech to preserve a 7ns rise and fall time at all frequencies has come with the less contentious penalty of a 4ns jitter which is a very respectfully low level of jitter compared to what was being accepted in high end professional test equipment costing hundreds of times more only a decade ago.
The jitter issue in such a cheap signal generator, clearly aimed at the hobbyist market, is not a deal breaker. In many cases it'll have little to zero impact on their use. In the rare cases where it does, most resourceful hobbyists will be able to cost effectively apply the work arounds that were once practised by the professionals only a decade or so back when their ten thousand dollar kit fell short of the tasks they were being put to.
At the end of the day, you get what you pay for and these Feeltech products give you a surprisingly large amount of value for your money. As to whether or not Feeltech / Feelelec have applied a firmware update to mitigate the 4ns jitter issue remains to be seen. In view of the lack of any mention of such a radical upgrade, I rather doubt it.
JBG