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10 MHz on Sampling Scope w/40 MHz Trigger, Not As Expected
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TheWaterbug:
Hmmm. I swapped out the CSA8200 sampling scope for a TDS5104 digital phosphor scope. Triggering on the 40 MHz edge with 2 seconds of display persistence:



Now the 40 MHz signal looks more like what I'd expected--if I trigger on the 40 MHz rising edge, I get a single 40 MHz rising edge, and the 40 MHz falling edge shows my width jitter:



(hard to see on the scope capture, but there are 4 distinct falling edges in there).

If I trigger on the 40 MHz falling edge, I get a single 40 MHz falling edge, and the 40 MHz rising edge now shows 4 distinct traces:



And in all cases, the 10 MHz edge refuses to jitter :-), and looks too clean to be true. There's no jitter measurement available on this scope, but it looks as clean as my trigger edge:



If I were to guess, I'd say that the CSA8200 trigger circuit was trying to find some inherent periodicity in the somewhat unstable 40 MHz signal, and somehow locked onto the 10 MHz, since that's the original source of the 40 MHz pulses.

Whereas the TDS5104 is just dutifully doing what it's told, and triggering on the rising (or falling) edges.

But none of that explains why the 10 MHz is so clean relative to a 40 MHz trigger that has both period and width jitter. I don't believe it.  ;D

p.s. anyone know how to un-invert the colors on TekVisa Toolbar screen captures? I've tried File: Page Setup: Ink Saver Off and On, and it's always the same over TekVISA.
2N3055:
It is simple.

You are always seeing only one edge from 10MHz synchronized with first edge from 40MHz signal.
Other 3 trigger events from 40Mhz signal will not show any edges from 10 MHz one, but will show low or high (hence the solid line on top and bottom).
40Mhz signal is actually 4 periodic events repeated at 10Mhz period in perfect synch with original 10Mhz.
It is not jittery 40Mhz but asymmetrical pattern repeated at 10 Mhz.

This can be simply demonstrated on a scope with segmented memory, where all 4 consecutive trigger events are captured separately and can be observed sequentially.
TheWaterbug:

--- Quote from: 2N3055 on January 25, 2024, 12:05:57 am ---It is simple.

You are always seeing only one edge from 10MHz synchronized with first edge from 40MHz signal.
Other 3 trigger events from 40Mhz signal will not show any edges from 10 MHz one, but will show low or high (hence the solid line on top and bottom).
40Mhz signal is actually 4 periodic events repeated at 10Mhz period in perfect synch with original 10Mhz.
It is not jittery 40Mhz but asymmetrical pattern repeated at 10 Mhz.

This can be simply demonstrated on a scope with segmented memory, where all 4 consecutive trigger events are captured separately and can be observed sequentially.

--- End quote ---

Ah, now this is finally making sense! It's exactly the inverse of the ABCD diagram, above, when triggering on the 10 MHz:



Each time I trigger on a 40 MHz edge it'll be associated with one of an A, B, C, or D edge from the 10 MHz, but they'll always be in those positions relative to my trigger edge, so the A edge never jitters, and the B edge never jitters, but the interval from A to B is different from the interval from B to C, etc.

Thank you!! This has been driving me crazy for a week.
points2:
hi 2N3055,

--- Quote from: 2N3055 on January 25, 2024, 12:05:57 am ---This can be simply demonstrated on a scope with segmented memory, where all 4 consecutive trigger events are captured separately and can be observed sequentially.
--- End quote ---
Waouh ! Interesting !
Do you have in mind a scope that can do that ?

My "noob-level" comment (if I'm wrong, don't hesitate to tell me, I have some many thing to learn...  ;D ):
- I use my scope to get a rough view of the wave (eg a 10MHz osc, or even an Ethernet signal @ 125MHz). But I don't trust its figures.
- To get trusty results => I use a spectrum analyzer, to see/measure the spectral density around the frequency corresponding to the rising edge seen on the scope! As simple & efficient as that ! (I'm correct or... not ? comments from experts is requested  :-DD )

So, to me :
- an eye-diagram is a marketing stuff ! Because it is nice to watch... But at which cost, and to get sometimes untrusty/weird/false results ?! Waouh ! => time-domain => watch out ! A scope isn't a fine & precise device (hobbyist scopes).
- the spectral density (mirror of an eye-D in the frequency domain) using a SA is less fun to watch... of course.. => but these devices are way cheaper, fixtures/etc are way simplier, and you get trusty & very very precise results (eg : a 10MHz sinwave signal, using a basic SA, you can look at the spectral density with a RBW=1Hz ! Converted in second => crazy precision ! )

My conclusion :
- I have a 500MHz BW scope, good&quite unexpensive (SDS2104X+ converted into 2504X+) => cool => it's a keeper
- last weeks, I've been looking for a more advanced scope to see more clearly down to the ns & sub-ns level => no go ! Bad idea !
- to see clear & deep => I already have a SA (SSA3021X+) => it is a keeper.
- if I change a measurement device in the near future => I'll get a more advance SA => a RTSA (Aaronia is releasing a new RTSA for hobbyist with crazy low price/perf ratio vs any other RTSA for hobbyist, and way cheaper than high-end scope for hobbyist when the goal is to watch a eye-diag. ; but I wait for feedbacks from early adopters...)
2N3055:

--- Quote from: points2 on January 25, 2024, 03:17:07 am ---hi 2N3055,

--- Quote from: 2N3055 on January 25, 2024, 12:05:57 am ---This can be simply demonstrated on a scope with segmented memory, where all 4 consecutive trigger events are captured separately and can be observed sequentially.
--- End quote ---
Waouh ! Interesting !
Do you have in mind a scope that can do that ?

My "noob-level" comment (if I'm wrong, don't hesitate to tell me, I have some many thing to learn...  ;D ):
- I use my scope to get a rough view of the wave (eg a 10MHz osc, or even an Ethernet signal @ 125MHz). But I don't trust its figures.
- To get trusty results => I use a spectrum analyzer, to see/measure the spectral density around the frequency corresponding to the rising edge seen on the scope! As simple & efficient as that ! (I'm correct or... not ? comments from experts is requested  :-DD )

So, to me :
- an eye-diagram is a marketing stuff ! Because it is nice to watch... But at which cost, and to get sometimes untrusty/weird/false results ?! Waouh ! => time-domain => watch out ! A scope isn't a fine & precise device (hobbyist scopes).
- the spectral density (mirror of an eye-D in the frequency domain) using a SA is less fun to watch... of course.. => but these devices are way cheaper, fixtures/etc are way simplier, and you get trusty & very very precise results (eg : a 10MHz sinwave signal, using a basic SA, you can look at the spectral density with a RBW=1Hz ! Converted in second => crazy precision ! )

My conclusion :
- I have a 500MHz BW scope, good&quite unexpensive (SDS2104X+ converted into 2504X+) => cool => it's a keeper
- last weeks, I've been looking for a more advanced scope to see more clearly down to the ns & sub-ns level => no go ! Bad idea !
- to see clear & deep => I already have a SA (SSA3021X+) => it is a keeper.
- if I change a measurement device in the near future => I'll get a more advance SA => a RTSA (Aaronia is releasing a new RTSA for hobbyist with crazy low price/perf ratio vs any other RTSA for hobbyist, and way cheaper than high-end scope for hobbyist when the goal is to watch a eye-diag. ; but I wait for feedbacks from early adopters...)

--- End quote ---

Fast scopes are expensive..
Eye diagram is a diagram. It is useful for certain type of signals and usage.

Many scopes have segmented mode. If you have SDS2000X+ then you already have one that has it.
Problem is that retrigger time on scopes is much slower that <25ns period you have here..
So it won't be able to catch every consecutive period as trigger sequentially.  But by simply using history mode you can "walk" and see what they are..
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