Hi TomC,
what you mean by external source? The time I made the pics no other noise was present with the Owon disconnected from mains (the terrible noise I had the other day never came back again after it disapeared the third day. Indeed must have been some HAM radio guy or whatever.).
All noise you see in the pics is generated inside the Owon, absolutely no doubt.
Theoretically the added 475 input capacitance of 20pF might reduce the displayed noise a little but I don't think that is significant. And the set up is the same with and without the oil can so why dealing with it?
The question why noise goes down when connecting the 475 to the setup was just btw. The same thing might happen as well if connecting a cubic of solid iron of 1m x 1m x 1m to the setup common ground. Must have something to do with the energy needed to drive a big mass - or antenna as well if you want - with HF.
Only the difference between the pics taken with the Owon in and out of the can counts in my opinion.
The displayed noise of the 475 shows just one snap shot of the noise due to it's more precise triggering whereas the Owon's display is rather unstable, has a tendency to trigger on different spikes each sweep.
But I haven't closely checked it's trigger abilities yet.
I do not know exactly how you have connected Owon+Tek parallel to signal generator but I believe this connection add much more than just pure 20pF Tek input capacitance. So, this connection may reduce much more bandwidth than may expect if think only Tek input 20pF. You can try. Connect fast edge instead of this sinewave and yoy can see high reduction in risetime, and this Network may also peaking with several frequencies die to its possible impedance mismatching.
Just for clariy. If connect 50ohm cable to 1MOhm input and look 50ohm cable free end. There is this cable cpacitance and inductance "Network".
Also I note that you use 2mV/dif in Owon. There is 20MHz BW limit. This noise frequency components have lot of over 20MHz frequency components what are now highly attenuaated and peaks are much lower than they really are.
If you want see around same with Tek oscilloscope and Owon ypu need do impedance matched real splitting Network from Signal gen output parallel to both scopes. Other ways can not see any real data. But of course also this kind of simply test is useful.
Other thing is very big difference if we think how analog oscilloscope show noise what is "random" with scope eyes. This is real thing what have discussed and studied in many places. Owon itself can show much more noise than this Tektronix. It can proof if you look some signal sou´rce what produce some known signal added with some random noise peaks.
Fact, I believe, is of course that this noise in this case is coming from Owon.
Btw, for trigger stability there is adjustment in Owon trigger settings. (HF reject)
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About noise displaying with digital scope or analog scope.
(this is partially for these peoples who do not know and randomly read this topic and look these pictures where Tek analog scope do not show nearly any noise (or only small amount) and Owon screen is full of noise.
Try with 5ns width pulses with 10ms period. What you can see on Tek275 display and what can see on the Owon display?
On the Tek display nearly nothing or nothing and Owon show just clean full pulse without any problem. Same for noise, depending noise characters and what signal is trigged.