EEVblog #1235 - Alternate Trigger Trickery On DSOs

[EEVblog]

Most modern DSO's don't have Alternate Triggering, but Dave shows you how to do it anyway!
This allows you to trigger on and view and anaylse two otherwise asynchronous time un-correlated signals on the screen at once.
Also a look at Dual Time Base scopes.

[Tom45]

Dave,

For the 2465 set the vertical mode to Alt and the trigger source to Vert.

Then whenever the Alt is sweeping channel 1, the trigger uses the channel 1 signal. And when sweeping channel 2, the timebase triggers using the channel 2 signal. Most of the later (>= 1968) Tek analog scopes have a Vert (or equivalent) trigger source selection.

And thanks for the video. The various methods you used with DSOs had never occurred to me.

Tom

[EEVblog]

Seems not many people are interested in this kind of video

[EEVblog]

For the 2465 set the vertical mode to Alt and the trigger source to Vert.

Ah, had forgotten that, it's been way too long.

[MikeLud]

Dave,

I found it interesting and gave it a . When are you going to do an entry level scope comparison ($500+- range)?

Thanks,

Mike

[EEVblog]

I found it interesting and gave it a . When are you going to do an entry level scope comparison ($500+- range)?

Been thinking about it...

[Dundarave]

I found it interesting and gave it a . When are you going to do an entry level scope comparison ($500+- range)?

Been thinking about it...

In addition, such a comparison might peripherally be a great way for less-experienced DSO owners (like me) to see how some of the unique-but-lesser-used DSO features (trigger modes, math, decode, etc.) work as you compare the models, but without going into a lot of detail: thumbnail explanations, as it were.  I'd guess that there are many newbies using ~$500ish scopes who might benefit from seeing what their scopes are capable of once one digs into the menus...

[David Hess]

For the 2465 set the vertical mode to Alt and the trigger source to Vert.

Then whenever the Alt is sweeping channel 1, the trigger uses the channel 1 signal. And when sweeping channel 2, the timebase triggers using the channel 2 signal. Most of the later (>= 1968) Tek analog scopes have a Vert (or equivalent) trigger source selection.

Dave, that points to the reason why DSOs do not usually call it "alternate triggering" and why it ever existed in the first place.  It also explains what the vertical source is.

The earliest oscilloscopes took their internal trigger signal from the vertical deflection plates.  When switchable internal trigger sources were added, this became the vertical source.  But when this is done on an oscilloscope which supports alternate switching to display more than one channel, it becomes alternate triggering for essentially free.  Chop mode did not work with this configuration because the trigger would see the chop signal at the vertical deflection plates and trigger off of that instead.

To support higher bandwidths, later oscilloscopes included a separate electronic channel switch for the trigger source which usually did not include the vertical deflection plates as a source.  To preserve the vertical source selection, the channel switch was driven from the alternate signal when the vertical source was selected and you can find this configuration on oscilloscopes into the 1990s.  The big advantage of this configuration, besides higher bandwidth, is that chop mode can now be combined with the vertical trigger source in a useful way by adding the two channels together to produce the trigger signal.  Surprisingly, this actually works in many cases.

So that is why and how many analog oscilloscopes had a vertical or VERT trigger source which allowed alternate triggering.  Early DSOs which copied the architecture of analog oscilloscopes with analog channel switches also often include this as an explicit feature.

Where alternate triggering is really useful is displaying separate waveforms which have no coherent relationship without a second or more expensive oscilloscope.  The switching waveforms from a power supply that has a separate power factor correction circuit is a modern example of this.

[Mr. Scram]

I suspect the ability to only set a single rising edge on pattern triggers has to do with the trigger hardware being physically executed once. In essence it's the same reason dual time base oscilloscopes are rare.

[capt bullshot]

Seems not many people are interested in this kind of video

Most probably I'll watch it somewhen later ... But yes, a teardown or mailbag is more interesting to me, as I'm usually able to figure out such stuff on my own, or just need a slight hint  (written down i a few lines and read in a few seconds) to get the point.
Anyway, I've got enough neat and handy DSOs in my lab so I usually set up two of them when I need to view asynchronous signals. As others pointed out, alternate triggering and dual timebase is well known from analog DSOs.

Edit BTW: Watched your videos about DSO vernier settings and learned something new (some DSOs indeed have variable gain on the analog side, the older ones I'm more familiar with don't). Interesting, thanks.

[Mr. Scram]

Seems not many people are interested in this kind of video

(Attachment Link)

I suspect this is one of those slow but steady climbers over the years.

[thm_w]

I don't see any way do to this on the Rigol MSO5000 that is obvious to me.
The pattern option is there, with the same single rising edge trigger limitation as the other scopes, but there is no OR option. Seems like something simple they've left out.

Might be some sort of workaround with infinite persistence but it wouldn't be great.

Pattern trigger
Identifies a trigger condition by searching for a specified pattern. This pattern is a logical "AND" combination of channels. Each channel can be set to H (high), L (low), or X (don't care). A rising or falling edge (you can only specify a single edge) can be specified for one channel included in the pattern. When an edge is specified, the oscilloscope will trigger at the edge specified if the pattern set for the other channels are true (namely the actual pattern of the channel is the same as the preset pattern). If no edge is specified, the oscilloscope will trigger on the last edge that makes the pattern true. If all the channels in the pattern are set to "X", the oscilloscope will not trigger.

[Tsippaduida]

I found your video useful, me being a somewhat scope noobie. I have Siglent 2000X series scope 2104X, and this series has pattern trigger (OR, AND, NAND and NOR options).

Never tried it haven't needed yet, but at least now I might remember that there's such an option.