‘scope query: instantaneous , one-shot trigger and hold ?

[Al (astral highway)]

Hey folks , I have a list of peak current measurements to do and have built a some high quality current transformers to help me.

What’d be fantastic is if my ‘scope had a one-shot trigger and save option... does that make sense ?

So, for example, if we had a current sense resistor or current transformer and we set a circuit up to produce a one-shot peak current, it would trigger the scope but the scope would then hold the voltage and waveform of the pulse over say 10uS to 10mS. Clearly, the triggering event would now be in the past .

The waveform would not be recurrent so it would just be a one shot event. Obviously I know how to set trigger volts on the scope but have no idea what this functipn would be called, if indeed it exists.

Any input much appreciated ! (Pun intended )

[julie_m]

I would have thought that capturing a single event and maintaining at least half a screenful of sample data from before the triggering event (there is usually at least two screenfuls' worth of memory) ought to be a fairly standard feature on any DSO. What are the menu options for "trigger mode" (or similar) on yours?

[Al (astral highway)]

Hi Julie,

You know what? I’d never investigated the trigger ‘menu’ before.. I did know that I can select ‘pulse’ or ‘edge’, and some noise rejection options .

But relevant to my Q... Under ‘pulse’ I can select ‘when’ and <,>,= 20nS. So looks like I’ve answered my own question, but prompted by your hint on where to look ! So thank you !

I also have channels 1, 2 (obviously )and external .

Not clear when to use ‘external’ , tbh.

Thanks again, Julie

[Vicboduk]

Hi Al what type of oscilloscope do you have, analogue or digital? As Julie states this is a standard feature for a digital scope.
Typically, you would set the Trigger mode to “Normal or Single”

Normal mode triggers on each trigger event and holds the display until the next event when the display will be up dated.

Single mode requires the operator to re-arm the trigger after each event before the next event can be recorded.
This can be advantageous if the signal is noisy and contains spurious trigger events.

If you only have an analogue oscilloscope, then you will need a digital adaptor to capture the event. As it happens I may have one available should you need one.

Vic

Edit; having read your reply I see you have a digital scope - so no need for an adaptor

[Al (astral highway)]

Hey Vic,

I do have ‘single’ on my ‘scope . It doesn’t respond on the sceeen you say that it’s accepted that mode but now I see that it has; it allows me to set trigger level down to 4mV as the first increment above zero:that’s a great improvement on the normal 10mV lower limit.


Thanks for the clarification.

[dave cox]

With a typical modern DSO it doesn't really matter what the trigger level is set to, as long as it triggers 'sometime' during the event you are interested in. When waiting for the trigger event it is sampling the input waveform at full throttle, when the event occurs it will continue sampling for another half 'sample buffer' so when you look at the sample data the trigger point is in the middle !

dc

[MotorBikeLes]

Now is the time to dig out that late '60's Telequipment D53S or DM53A, with its JD y amps. It has single shot, trigger delay (not really needed) delay lines in the Y-amps, and a decent GEC manufactured storage tube. Capture your trace and keep it for weeks.
Or you could get something more modern.
My HP1703A would do the job.
Les.

[Bazz4CQJ]

Something like a Picoscope (but preferably, not an actual Picoscope) would do nicely. I used to use one to take readings on a combustion process, triggered when ignition was detected by a UV flame sensor.

[cmjones01]

Every digital and analogue storage scope I've ever used, and most of the analogue non-storage ones, has this feature. Usually it's one of the triggering modes, labelled usually 'single sweep' associated with a 'reset' button to arm it for the next sweep. On digital scopes it's sometimes tucked away in an acquisition menu: on an HP one I have on the bench here, there's a 'run' button which brings up a menu showing 'single' or 'repetitive'.

Chris

[Al (astral highway)]

Update:

As soon as I found the relevant function, another inquiry was quickly solved!
Here's the write-up to the calibration of my beefy home-made current transformer.

https://www.vintage-radio.net/forum/...58#post1036758

Thanks all for your comments. I'm intrigued to see that this was a feature even of analogue scopes, though, Chris (cmjones01) I wonder how that was done?

[GrimJosef]

Google is your friend https://en.wikipedia.org/wiki/Oscill...e_oscilloscope.

Cheers,

GJ

[kalee20]

Almost all scopes have this feature, digital and traditional. With a trad, purely analogue scope, it's really only useful if you have a scope camera to record the single sweep (open the shutter, press the 'arm' button, wait for the event to happen, close the shutter, develop the film).

With a DSO it's much better, as you can have the screen display continuously its last sweepful of data.

I once had to capture an abnormal event - a switching regulator's output protection circuitry for if the switching device failed short-circuit. Making sure the trigger controls were properly set was crucial, the idea of closing a bypass switch across the main switching transistor (blowing up £100 of components in the process) and failing to capture what happened to the output was not to be thought of! Fortunately the scope did its stuff - and as the sparks flashed from the circuit board, the trace showed the output voltage serenely climbing to the over-voltage trip point then falling to zero. Phew!!

[Argus25]

Originally Posted by astral highway

I'm intrigued to see that this was a feature even of analogue scopes, though, Chris (cmjones01) I wonder how that was done?

Al,

All that happens is the sweep timebase is held off until it receives the trigger pulse, rather than be allowed to free run (oscillate), so it is obvious in this mode as no trace is visible prior to the trigger. Mostly this function though is only useful in analog storage scopes for non repetitive events, because the single trigger event vanishes from the screen quickly with the usual short persistence analog CRT.

It is often more difficult to capture single events (even with the theory on how the scope's trigger should work) because the leading edge of the event has happened before the trace or data acquisition occurs (at least in analog storage scopes) Digital scopes can get around this.

One method I found for slow random events (over hundreds of milli-seconds duration) that had unpredictable onset, I simply left the storage scope in free run mode with a slow trace and when captured the event in a good position on the trace, pressed the store button manually.

One other trick though if you have some event you want to study that is only occasional, set up a test circuit to drive it repetitively with regular timing from an oscillator, eg 555 is fine (taking issues of dissipation into account if required). This way you can get a nice stable image of the event, on practically any scope and not have to worry about one shot triggering & storage.

[MotorBikeLes]

Hugo, if you read my tongue in cheek D53s/DM53A message (not really tongue in cheek though, some of them still out there), you will note I specified the JD amps, the "D" part telling of a 200ns delay line in the Y amp before the trigger take off. The pulse start is not missed.
Les.

[Argus25]

Les,

Yes that (small) delay helps for fast rising waves, however for slow rising waveforms I have never found that delay adequate, because the trigger is delayed much much longer into the initial part of the wave than this and still something is missed at the start.

For slow events it would be lovely to have a 2ms delay at least which would be a very long delay line. With the digital scope it can be recording data prior to the trigger event and display the voltages prior to the leading edge of the event which is handy. I do not have one though, only analog scopes and analog storage scopes.

[G0HZU_JMR]

For your 'capture and save' application it might be worth finding out if your scope lets you dump out the raw sample data to a PC once it has done the single shot.

Then you could post process the saved data in some freebie software or simply overlay the raw data for several single shot events in an Excel chart to see how repeatable your results are. Plus it would be easy to record your results in a document. There might even be some software specifically written for your scope that can do all this.

[Al (astral highway)]

Hello , Les, Kallee20, Hugo and Jeremy thanks all for your more detailed explanation of the capacities and workings of a DSO in relation to my questions.

Please can I invite any of you to follow my link in post #10 up there where I illustrate the results of my pulse experiments successfully recorded in single shot mode?

I feel really happy with this outcome and would love to share the process and the role of the scope, either in this thread or the one with the link. Even if you just take s look at the waveform in the capture, I’d be really happy to share that.

The icing on the cake would be if I had known how to record this data permanently and indeed, even export it as Jeremy suggests!

I really do have a much more competent DSO than I’d imagined.

[cmjones01]

Originally Posted by astral highway

I'm intrigued to see that this was a feature even of analogue scopes, though, Chris (cmjones01) I wonder how that was done?

The technical breakthrough that allows single-sweep operation is the triggered timebase, whose invention is attributed to Howard Vollum, one of the founders of Tektronix. The innovation was to change the timebase so that instead of being an oscillator synchronised to the input signal, it was a state machine which cycles through the states "armed", "sweeping", "retrace" and "holdoff". The trigger event starts the sweep, and the state machine ensures that the sweep, retrace and any holdoff period are completed before another trigger can take place. That's what leads to the stable traces we're used to today. This system was first used in the Tek 511 and 513 scopes, and was revolutionary in its day (the late 1940s).

Single sweep is straightforward to add to this by making the transition from "holdoff" to "armed" require a button push, so the sweep can't be triggered until the user wants it to be. One of the first scopes to feature a recognisably modern dual timebase setup was Tek's 535 in the mid 1950s. Attached is a picture of my own 535A showing the timebase A and horizontal mode settings. You can see the single-sweep setting on the bottom right knob, and the corresponding 'ready' light which comes on when it's waiting for a trigger, and the reset button which arms it for another sweep.

Even on a non-storage scope like the 535A this mode is useful. It's surprising what the human eye can pick up from even a brief glimpse of a trace, and frequently the thing I want to know is whether the trigger event happened or not - if the 'ready' light goes out, I know my event of interest happened.

An analogue storage scope can store the trace electrostatically-deflected on the CRT for easier viewing, but there are other options. The legendary 1GHz Tek 7104 has an image intensifier built in to the CRT so that even nanosecond single sweeps are visible in normal room light, and the truly astonishing 7912AD writes the trace on to a special target in the CRT which is then read rather like a video camera, and worked on by some amazing 1970s digital image processing to digitize it. The result is a single-shot sample rate of up to 100GS/s - try that with a regular A to D converter! I use mine for capturing very short events such as static discharges.

Chris

[Al (astral highway)]

Hello Chris,

Thanks for posting such a detailed reply.

I had no idea that 'scope technology, specifically the sophisticated trigger and hold mechanisms you describe, had dated back to the late '1940s.

I'm not that surprised, given the advances that took place during WWII, (which must have been a springboard for what happened a few years later) but nevertheless it isn't an area that I know very much about.

I'd never heard of Howard Vollum -- is he something of an under-celebrated figure?

Fascinating stuff!

[Argus25]

Al,

Scope time base technology and TV horizontal scan technology once had a lot in common because very early CRT's for TV work were electrostatic types just like scope tubes. Both required the generation of very linear sawtooth voltages for the deflection plates and a rapid fly back and synchronisation to an external pulse. In scopes the problem was exacerbated by requiring linear scans at multiple frequencies for the horizontal deflection plates.

Many of the early H scan circuits were invented and perfected at Cossor. Engineers such as Bedford & Puckle created time bases. Even variable velocity time bases were invented so the beam could be velocity modulated( for TV, but it didn't take off). Thyratrons were also once a popular method in very early scopes & TV's) but lost out to triode & pentode designs. Then came the more elaborate controls of the time base with trigger & hold as noted, with one shot functions.

By the time semiconductors had come along, much of the architecture for TV and scope time bases was already worked out, except that with complimentary transistors yet another generation of designs became possible that were not possible with valves.