Author Topic: Why so many 4 Channel scopes dont have an Ext. Trigger?  (Read 14963 times)

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Offline Trax

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Why so many 4 Channel scopes dont have an Ext. Trigger?
« on: July 25, 2015, 05:44:19 pm »
I'm wondering why so many (not only cheep) 4 Channel scopes dont have an Extern Trigger input?

A plausible explanation might be really enlightening ;)
 

Offline electr_peter

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #1 on: July 25, 2015, 06:10:45 pm »
Reason are pretty obvious - space and price constraints. 4 ch scopes are typically very cramped with LCD, BNCs and buttons. Also, manufacturer have to dedicate "fifth" dummy channel for triggering which adds to the complexity of the scope significantly.

Lack of external trigger means you can either use 4 ch without external trigger or 3 ch with external trigger. Cases when you need 4 ch + ext trigger are rare. If you are desperate to have 4 ch + ext, either buy scope that allows that or rig up two scopes to have >=4 ch.
 

Offline tautech

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #2 on: July 25, 2015, 07:54:50 pm »
4 ch scopes are typically very cramped with LCD, BNCs and buttons. Also, manufacturer have to dedicate "fifth" dummy channel for triggering which adds to the complexity of the scope significantly.
:o
This is an unusual POV.
These days the EXT BNC is usually on the rear and therefore has no effect on the valuable front panel real estate.

Unless a 4 ch scope has external trigger, it can't be called well featured.
The only reason to omit it is cost.

Nearly all scopes have a line trigger, how hard is it to add an external trigger , additional signal path, BNC connector and/or some  :-/O to FW and the UI.
These things are easy when compared to the overall complexity of scope design.

All scopes should have it IMO, even entry level models.

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Offline Trax

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #3 on: July 25, 2015, 08:14:37 pm »
How realistic do you think would it be to tap into the line trigger circuit and re-purpose it as external trigger.
Would it be as accurate as a real one?
 

Offline tautech

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #4 on: July 25, 2015, 08:19:11 pm »
How realistic do you think would it be to tap into the line trigger circuit and re-purpose it as external trigger.
Foolish.
Line trigger is an important feature, don't muck with it.

Share with us your external trigger needs and there may be another method to achieve the result you need.
« Last Edit: July 25, 2015, 08:21:06 pm by tautech »
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Offline SeanB

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #5 on: July 25, 2015, 08:20:51 pm »
To be a decent trigger it would have to have the analogue front end of the other channels, so using a channel as a trigger is going to work in the majority of cases, and as a bonus it also will have a useful function when not being used as a trigger input. Line trigger in most cases has only limited bandwidth, and will have a fixed trigger level in most cases as it will, in most modern scopes, be a digital signal out of the power supply.
 

Offline rollatorwieltje

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #6 on: July 25, 2015, 08:52:58 pm »
Isn't it a bit of a relic of the analog age? I mean, I only really found myself using the trigger channel on analog scopes as a crude "pre-trigger", to see what happens just before the interesting part. Digital scopes store stuff before the trigger point as well, so it's good enough to trigger on the actual data.
On my DS1052E I do use it, but that's just a 2 channel scope.
 

Offline electr_peter

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #7 on: July 25, 2015, 09:00:14 pm »
4 ch scopes are typically very cramped with LCD, BNCs and buttons. Also, manufacturer have to dedicate "fifth" dummy channel for triggering which adds to the complexity of the scope significantly.
:o
This is an unusual POV.
These days the EXT BNC is usually on the rear and therefore has no effect on the valuable front panel real estate.
Well, modern scope typically has all the inputs in a neat single row on the front panel. Rear panel may have some BNCs and auxiliary connections, but it is considered not as accessible as the front panel. Such design limits designer options to include additional inputs/outputs.
Contrast this with older CROs where front panel BNCs are almost in random places and there may be myriad of connections on the back. My current 2 ch CRO has 2 external trigger inputs on the front and 4 additional BNCs on the back - and I still consider it to lack some connectivity ;)

As noted in my previous post, I agree that main driving force behind missing ext trigger on 4 ch scopes is cost/complexity. Manufacturers are gambling a bit - they know that most users in most cases won't require 4 ch + ext trigger setup, so they offer "more affordable" compromise.
 

Offline Wuerstchenhund

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #8 on: July 25, 2015, 10:32:49 pm »
I'm wondering why so many (not only cheep) 4 Channel scopes dont have an Extern Trigger input?

A plausible explanation might be really enlightening ;)

I'm not sure what you're talking about, because aside from maybe some cheap bottom-of-the-barrel and some entry-level scopes an external trigger input is still pretty much standard on most modern scopes.
 

Offline alsetalokin4017

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #9 on: July 26, 2015, 01:17:44 am »
How often does someone actually need 4 channels + an external trigger off of a fifth signal?

My Rigol DS1054Z is a three-channel scope with external trigger, or a 4 channel scope without it. That is plenty for me.

I do find the external trigger on my 2 channel analog scopes to be useful from time to time, and the delayed external trigger on the old HP180a with 1821a timebase has been useful at times as well.

I agree, though about the "Line" trigger (or Source AC on the Rigol). This is an extremely useful and necessary function, more so than a 5th trigger channel on a 4 ch scope would be.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline Trax

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #10 on: July 26, 2015, 07:26:47 am »
Line trigger is an important feature, don't muck with it.

Share with us your external trigger needs and there may be another method to achieve the result you need.
Why? Who needs Line trigger nowadays?

I don't have any particular needs at the moment, that would require 4 CH + Ext, I'm just course.

To be a decent trigger it would have to have the analogue front end of the other channels, so using a channel as a trigger is going to work in the majority of cases, and as a bonus it also will have a useful function when not being used as a trigger input. Line trigger in most cases has only limited bandwidth, and will have a fixed trigger level in most cases as it will, in most modern scopes, be a digital signal out of the power supply.
Yes i know thats just a wire from the scopes PSU, but the question is can you attach the wire to some self made front end and use it. You could even add some Schmitt trigger circuit to be able even to set the level.


I'm not sure what you're talking about, because aside from maybe some cheap bottom-of-the-barrel and some entry-level scopes an external trigger input is still pretty much standard on most modern scopes.
Hameg HMO2024 -> 2k$
Keysight DSOX3000 -> 4k$
thay are not so cheep or so entry and thay dont have a ext trigger
 

Offline rs20

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #11 on: July 26, 2015, 07:41:13 am »
The only reason to omit it is cost.
This is a bit of a tautology, the reason for omitting any feature is cost. I think the more complete answer to the OP's question is "diminishing returns": the value to the end user of that extra trigger channel is lower on a 4ch scope than a 2ch scope, because circumstances where you need to view/trigger a total of 3 channels are much more common than the circumstances when you need to view/trigger a total of 5. So it's that the cost/benefit tradeoff doesn't make the cut, not because the manufacturers are just completely arbitrarily cutting costs.

Put another way, if you have 4 channels to begin with, you can just dedicate one of those channels to triggering and have a 3 channel scope; saying the same for a 2ch scope (dedicate one for triggering and you're left with 1 ch) is obviously far more damaging to the usability of the instrument.

Put yet another way, think of a 2ch scope with ext trigger as a "3 channel scope with a crippled, trigger-only third channel". Now a non-crippled, 4 channel scope doesn't sound so unpalatable.

I think my TDS 754C has an external trigger on the back, but I don't really know because I've never used it  :)
 

Offline EEVblog

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #12 on: July 26, 2015, 07:53:23 am »
An external trigger is actually a compromise, always has been and still is.
Full channels are better, as you have a full attenuator and you can view the signal.
It's pretty much essential on a 2CH scope, but when you go to the expense of a 4CH, that need diminishes greatly, so not much need to spend any extra on the ext trigger. Been that way since the days of analog scopes.
Take the Tek 2465, best analog scope ever made, spared no expense, no external trigger.
 

Offline mikron

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #13 on: July 26, 2015, 07:57:48 am »
Hameg HMO2024 -> 2k$
Keysight DSOX3000 -> 4k$
thay are not so cheep or so entry and thay dont have a ext trigger

Both have EXT TRIG IN and TRIG OUT on the back.
 

Offline tautech

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #14 on: July 26, 2015, 08:01:02 am »
Line trigger is an important feature, don't muck with it.

Share with us your external trigger needs and there may be another method to achieve the result you need.
Why? Who needs Line trigger nowadays?
Anybody working with mains frequency referenced equipment.
Anybody examining ripple in a linear PSU.

Line Trigger FORCES triggering at line frequency, very useful on occasions.

You may or may not be aware many scopes these days can operate from supplies ranging from ~45 to 400 Hz, wouldn't Line triggering be useful over that range.
Fact is it's all internal, cost little to implement and arguably more useful than Ext trigger, however IMHO both are a necessity in a decent scope.

You might ask the same question as why various video trigger modes are included in scopes these days with the prevalence of new formats like HDMI.  :-//
The only reason to omit it is cost.
This is a bit of a tautology,
:-DD  :-+
Quote
Put yet another way, think of a 2ch scope with ext trigger as a "3 channel scope with a crippled, trigger-only third channel". Now a non-crippled, 4 channel scope doesn't sound so unpalatable.
Isn't all this JUST another selection criteria one must apply to a prospective new scope.  :-//

Years ago before LA's a 4ch scope with Ext triggering WAS your LA and for those that can't justify a separate LA unit these days, a scope with Ext trigger is an attactive option.
Used that senario several times myself.  ;)
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Offline SeanB

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #15 on: July 26, 2015, 08:23:35 am »

You may or may not be aware many scopes these days can operate from supplies ranging from ~45 to 400 Hz, wouldn't Line triggering be useful over that range.
Fact is it's all internal, cost little to implement and arguably more useful than Ext trigger, however IMHO both are a necessity in a decent scope.

Most Tek scopes came with the option of a wide input frequency range, which in most cases was very easy to implement, they changed the input mains filter to one with a lower value X capacitor so it would not overheat at 400Hz. The mains transformers were good enough in most cases to not saturate at 45Hz, and still not have excessive core loss at 440Hz. You design it for low copper loss and are very conservative in the power rating for the core.

When they started to use SMPS designs you also got some with a very wide input range, 65VAC to 260VAC, 40 to 480Hz and also capable of operation from 28VDC to 300VDC. No configuration required.
 

Offline dom0

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #16 on: July 26, 2015, 08:27:27 am »
Isn't it a bit of a relic of the analog age? I mean, I only really found myself using the trigger channel on analog scopes as a crude "pre-trigger", to see what happens just before the interesting part. Digital scopes store stuff before the trigger point as well, so it's good enough to trigger on the actual data.
On my DS1052E I do use it, but that's just a 2 channel scope.

No, external trigger is /extremely/ handy when tracing signals through complex circuits, because you only need to adjust V/div and maybe offset, while the trigger just runs off the SYNC output of the generator and doesn't need any adjustment. Without external trigger you'd need to adjust the trigger as well.

One can also use another input channel for this, but whether you can hide it on screen and still trigger on it depends on the scope. And it obviously costs you a trace.
« Last Edit: July 26, 2015, 08:30:15 am by dom0 »
,
 

Offline Howardlong

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #17 on: July 26, 2015, 09:15:59 am »
Why only one external trigger when you can have two?

 

Offline Wuerstchenhund

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #18 on: July 26, 2015, 10:16:23 am »
I'm not sure what you're talking about, because aside from maybe some cheap bottom-of-the-barrel and some entry-level scopes an external trigger input is still pretty much standard on most modern scopes.
Hameg HMO2024 -> 2k$
Keysight DSOX3000 -> 4k$
thay are not so cheep or so entry and thay dont have a ext trigger

Are you kidding? Both have external Trigger inputs (see HMO202x manual p.13 and p42 of the DSOX3k manual).

And frankly, if you think $4k is expensive for a scope then you haven't seen much.
« Last Edit: July 26, 2015, 10:24:54 am by Wuerstchenhund »
 

Offline Zero999

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #19 on: July 26, 2015, 10:43:22 am »
When they started to use SMPS designs you also got some with a very wide input range, 65VAC to 260VAC, 40 to 480Hz and also capable of operation from 28VDC to 300VDC. No configuration required.
Hmm, I wonder what the minimum operating voltage of the Rigol DS1054Z is. It's probably lower than what it says on the datasheet. It would be good if it would work from 48VDC so could be powered from some lead acid batteries.
 

Offline dom0

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #20 on: July 26, 2015, 10:55:37 am »
Most world-wide SMPS can't sustain output regulation or shut down below 80-90 Vdc.
,
 

Offline SeanB

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #21 on: July 26, 2015, 11:15:51 am »
I was surprised as well when I got a Tektronix at work. Just had to power it on the 28VDC supply bus, and it worked perfectly, as well as working on the 115VAC 400HZ supply from the wall. Either they ordered it with all the options or it was standard for the range delivered to my employer at the time.

I had a call from the original group that had placed the order and then neglected to collect it for 2 years in the stores group. I referred them to my boss, and his reply was to go sit on something sharp. They had to order the 2 scopes again on the budget...........
 

Offline tautech

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #22 on: July 26, 2015, 11:27:25 am »
Had a BWD CRO that could be powered by 30V DC as part of diagnosing the pre_regulator, it even derived the CRT HV from it.  :o
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Offline Zero999

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #23 on: July 26, 2015, 06:46:06 pm »
I've just tested my Rigol DS1054Z on DC. It works from 4 lead acid batteries to give 47VDC (one was a bit flat). I tried with three batteries but it didn't work.

I might build myself a battery pack for it.
 

Offline dom0

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Re: Why so many 4 Channel scopes dont have an Ext. Trigger?
« Reply #24 on: July 26, 2015, 06:54:03 pm »
Tried it on my Hantek DSO5102B:
Doesn't do anything until 100 Vdc.
But if it's running, it only turns off after going below ~45 Vdc.
Crashes if continually kept around 46-48 Vdc.

Advertised as 100-240 Vrms, 45 - 440 Hz. So that's totally fine.
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