Practical external trigger question - Page 1
Forum section: electronics / testgear

author: Slappy_g
author_rank: Regular Contributor
timestamp: September 01, 2014, 01:53:06 pm
content: Bear with me, this may sound a bit newb-ish...I know that on many traditional scopes, the external trigger input was essentially a (latched?) basic comparator. But with new scopes like my MSO2202A, with all of its advanced trigger options, a question occurs.To support these advanced trigger types, doesn't the external trigger have to essentially have as much capability as the other input channels, if perhaps a lower bandwidth? If so, then why not allow that signal to be plotted with the others?Seems like a quick feature win for a mid level scope.Sent from my SM-N900T using Tapatalk

author: nctnico
author_rank: Super Contributor
timestamp: September 01, 2014, 02:43:48 pm
content: Back in the old days there where somes scopes where the trigger input could be used as a third channel. Then again the use is limited because usually there is very limited attenuator and offset control.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2014, 03:23:26 pm
content: I have a 4-channel scope, I never need to use the trigger input...

author: HighVoltage
author_rank: Super Contributor
timestamp: September 01, 2014, 06:41:19 pm
content: I have some older Philips / Fluke PM3390B 2 Channel scopes with a separate BNC for an external Trigger input.This Trigger signal can be turned ON with "View Trigger Signal"But only with 2 settings for attenuation, high and low.The same series scope with 4 Channels PM3394B does not have an external trigger, just 4 channels.Sometimes it would be helpful to have a trigger view on a more modern scope.But may be the manufactures to want to spoil us.

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2014, 06:49:04 pm
content: Quote from: Slappy_g on September 01, 2014, 01:53:06 pmI do not think the external trigger usually does support all of the same trigger capabilities as the vertical channels with modern cheap oscilloscopes which may be one of the reasons they lack equivalent time sampling support. Digital triggering while supporting equivalent time sampling would require the external trigger inputs to be digitized just like the vertical inputs.Alternatively if the external trigger does support all of the same capabilities, then the trigger input is processed the same way as any other vertical channel but lacks a path to the acquisition record memory so the trigger waveform may be processed but not saved.This may explain some of the older Tektronix TDS series oscilloscopes which have 2+2 channels and can display any 2 of the 4 input channels while using the others as external trigger inputs.Quote from: nctnico on September 01, 2014, 02:43:48 pmThis got taken to the extreme with oscilloscopes like the Tektronix 2465 and 2246 series which include two external trigger channels which behave like a third and forth vertical input with position controls but limited attenuator settings and fixed DC coupling.To be fair to these designs however, the external trigger inputs on these oscilloscopes work great for logic level signals which were becoming more common when they were produced so the lack of coupling and a full attenuator was often not a problem.

author: David Hess
author_rank: Super Contributor
timestamp: September 01, 2014, 06:51:55 pm
content: Quote from: HighVoltage on September 01, 2014, 06:41:19 pmI think the problem here is that once you have promoted the external trigger to an additional channel on a DSO, you have added most of the cost of producing a full 3 or 4 channel oscilloscope.

author: Slappy_g
author_rank: Regular Contributor
timestamp: September 02, 2014, 01:32:55 pm
content: Good points, all. I appreciate your insight.I bet the connection to acquisition memory is the main issue, and the related costs.Though, it does make me wonder how the advanced trigger types such as Nth-edge work without some form of storage...Sent from my SM-N900T using Tapatalk

author: alex.forencich
author_rank: Frequent Contributor
timestamp: September 02, 2014, 06:00:02 pm
content: Quote from: Slappy_g on September 02, 2014, 01:32:55 pmMy Agilent MSO7104A only supports basic edge triggering on the external trigger input, as far as I can tell. None of the other trigger types allow 'external' to be selected as the source. Generally that's all you would need to use it for. It's not designed to put a scope probe on, it's designed to receive a trigger pulse from another piece of test equipment. Have you checked on your scope to see where you can actually use the ext trigger input?

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2014, 07:15:04 pm
content: Quote from: Slappy_g on September 02, 2014, 01:32:55 pmIt has gotten better but for a long time, the interconnect between the digitizer and memory was the most expensive part. Now we have fast and narrow memories so you do not need a 32 bit wide memory bus and associated wide array of memory chips for each simultaneous acquisition channel.In many older DSOs including most of the first ones, the sample rate halves when you double the number of channels because of the memory interleaving and not because of anything to do with the ADCs.QuoteIf the triggers are processed in real time which is a requirement for digital triggering anyway and the external trigger is digitized like the other vertical inputs, then this does not present a problem. In the case of triggering on the Nth edge, even without digitizing the external trigger, trigger events can just be counted.The ancient 2440 series of DSOs can count trigger events. Most or all analog and old DSOs which have delayed timebase support can either delay by events or trigger after delay which results in almost the same thing.

author: Slappy_g
author_rank: Regular Contributor
timestamp: September 03, 2014, 01:19:47 am
content: @David Hess:Clearly, you're more knowledgeable about scope history than me.The Nth-edge was a bad example, as I realized shortly after I typed it that a counter would do the job.@alex.forencich:I'll check what the options are after I grab a late dinner here...Sent from my SM-N900T using Tapatalk

author: Slappy_g
author_rank: Regular Contributor
timestamp: September 03, 2014, 10:54:59 am
content: For the record, you guys were right. I had thought that more trigger types were available on the external input, but it turns out that only edge triggering is supported.Sent from my SM-N900T using Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2014, 11:24:55 am
content: Quote from: Slappy_g on September 03, 2014, 10:54:59 amMy guess is that the external edge trigger just latches the sample count (position within the acquisition record) at the sampling frequency or some sub-multiple of the sampling frequency which limits its timing resolution.This would be an interesting thing to test for but it might be difficult to see. Connect a vertical input and external trigger input to the same signal with a fast edge, set the oscilloscope for external triggering, and see if the displayed edge shows signs of a bimodal or other distribution.