Oscilloscope and logic analyser notes: Difference between revisions

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==Trigger mode==
==Trigger mode==
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Triggering is synchronizing capture with some event, which is great to put events of interest on the screen.


The most typical thing to trigger on is a rising (or falling) edge.
Triggering refers to synchronizing capture with ''something''.
 
Why? Well.
 
 
It is fundamental to "we show you what is happening live" that, the more time detail you want to show,
the more that becomes too much, and goes by too quickly.
 
 
As a side effect, 'things that are consistently there' is a lot easier to show - given some further assumptions.
'''Historically, triggering''':
* assumes that the thing you want to see happens continuously in basically the same way
and
* prefers that it has a fairly well defined start
and
* prefers that there is a time within which we wouldn't need/want to trigger again
 
 
 
The more that those assumptions are true {{comment|(E.g. a bunch of digital communication matches that ''fairly'' well, and e.g. most music does ''not'')}},
the easier it is to get a useful thing you want on screen,
and in the same position, because aside from aligning at the start of what we call it event,
we are also ignoring some of what is happening on our input.
 
 
Note that part of this that came from restrictions of early scopes not having any memory - so the easiest way to visualize well was to wait to do a sweep, entirely live with what is on the input, until a certain event.




Single - after the next trigger, sample a bit, show, and then pause everything (regardless of what the signal does after)
: press run to get it back to continuous sampling


Normal - update only when the condition is met
'''Normal''' - update only when the trigger condition is met, and nothing otherwise
: where that condition is often "has gone above a voltage threshold"
: e.g. useful
: e.g. useful
:: for rare events, if you can get it to trigger only on each event
:: for relatively rare events, if you can get it to trigger only on each event
:: see the ''last'' trigger (e.g. a crude "what happened when I powered off" sort of use)
:: or even as a "update only while I hold the probe to something happening" (though this is more fragile in that you might see the bit where you made poorrt contact just before lifting contact)
:: or even as a "update only while I hold the probe to something happening" (though this is more fragile in that you might see the bit where you made poorrt contact just before lifting contact)
:: sometimes to see the ''last'' trigger (e.g. a crude "what happened when I powered off" sort of use - somewhat fragile as there may be other messy things going on that trigger it again)
: while e.g. for a squarewave this functions essentially the same as auto.
: while e.g. for a squarewave this functions essentially the same as auto.


Auto - basically means trigger continuously (regardless of the condition set)
 
: how quickly depends on the timescale, which is better than nothing, but is unlikely to synchronize with anything, so looks messy
'''Auto'''
: still useful to see where the waveform is, e.g. to move in the trigger level
: sort of a normal-with-timeout: "whenever there is no real thing triggering" then trigger after a certain time
: the amount of time to wati depends on the currently set timescale
: note that this fallback does ''not'' kick in whenever you are triggering faster than that
: more practically:
:: while not actively triggered, it is unlikely to synchronize with ''anything'' (even e.g. a very regular sine wave), so looks messy,
:: but e.g. useful to see where the waveform is, and move in a trigger level and then get that thing to show more stably
:: (older scopes might put threshold and auto on the same knob - then you don't get a threshold during auto, only during normal mode)
 
 
'''Single''' - after the next trigger, sample a bit, show, and then pause everything (regardless of what the signal does after)
: press run to get it back to continuous sampling
: can only be done on memory scopes (but all modern scopes are)




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There may be some further options, like
There may be some further options, like


* Delay - after the condition is met, wait some time to actually trigger
* '''Delay''' - after the condition is met, wait some time to actually trigger
: on a memory scope you can set this ''negative'', amounts to mean "look back some more"
:: e.g. useful if the interesting part happens some time after the triggering part
: on a memory scope you can set this ''negative'', amounts to mean "look back some time before the trigger"
:: e.g. useful if you are specifically interested in seeing all of what triggered it




* Holdoff - only trigger if the last trigger was at least some time ago (holds off to rearm the trigger)
* '''Holdoff''' - only trigger if the last trigger was at least some time ago (holds off to rearm the trigger)
: in analog scopes some of this was just necessary, e.g. while and after the beam back while blanked, and you could extend that time.
: in analog scopes some of this was just necessary, e.g. while and after the beam back while blanked, and you could extend that time.
: not useful for very regular signals, but e.g. consider trying to look for the start of a pulse train repeated with some interval
: not useful for very regular signals, but e.g. consider trying to look for the start of a pulse train repeated with some interval




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Trigger type
* '''Trigger type'''


Edge - makes more sense on block waves, digital communication
Edge - makes more sense on block waves, digital communication

Revision as of 15:45, 13 May 2024

📃 These are primarily notes, intended to be a collection of useful fragments, that will probably never be complete in any sense.


See also the nice aggregation at http://opencircuits.com/Oscilloscope



The things below are roughly ordered from cheaper to more expensive

What do you need?

Sampling rate and bandwidth

Rise time

UI

Memory - at all

Memory depth (how much)

Triggering

Math features

Decoding

Poor man's oscilloscopes

PC sound card

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

One of the simplest oscilloscopes, one you already have, is a sound card.

This will give you a 2 channel, 16-bit ADC often 48000 sample/sec, too slow for many fancier things things, but measuring some slow baseband signals for basically free (a few cents worth of resistors for some protection) is pretty nice.

Sound cards now tend to have a pretty low noise floor.



Bandwidth

Note that running your sound card 96 or 192kHz does nothing to increase your bandwidth, because all sound cards have lowpass filters on their input, meaning you should assume content above ~15kHz is attenuated and above ~20kHz is gone.


You sometimes find faster ADCs in PC hardware, such as a 448kHz (not a typo) ADC in bt878-based TV cards, with ~360KHz usable bandwidth(verify), though it takes more work to make this into something usable. (what about a lowpass?(verify) possibly not, if it's meant to be able to sample 8 channels?)



Voltage

Since consumer audio assumes it'll rarely go above ~0.5V, and most things you want to measure is higher-voltage, you'll at least want to make a factor-20ish voltage divider.

A few more components can provide more protection and/or convenience - consider (e.g. switchable division or potmeter voltage divider, buffering, protection diodes, decoupling)

See pages like http://xoscope.sourceforge.net/hardware/hardware.html



See also:

Microcontrollers

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

...such as the Arduino's Atmega chip, the PICs, and others.


If they have an ADC, you can use it as a lowish-speed scope. Consider e.g.

Arduinoscope (AVR based software scope)

...but note that performance is on the same order as a sound card.


Even without an ADC, they can be a (somewhat faster) logic analyser. Consider e.g.

Bus pirate (PIC based logic analyser)


Arduinoscope

It's not going to have the precision, regularity, or memory depth, or interface features of even a relatively cheap DSO, but it can be a pretty good debugging tool, given it's software for something you already have.


One of the first, and simplest to get started with, was https://github.com/konsumer/arduinoscope (GUI for win, lin, osx), which is fed with ASCII lines of values - not very efficient, but it's an easy way to visualize what's happening on a pin, and with just a few lines of Arduino-side code.

The arduino IDE itself has basically adopted that same approach since version 1.6.6 (2015), with its "Serial Plotter" (see the tools menu), that reads numbers written as text on the serial port.



There are various other projects that tried to be a little fancier than that, in particular getting a little more speed.

AVR-wise, the 9.6ksamples/sec that libraries default to is for compatibility with more arduino variants, and most AVRs can easily be run at ~77ksamples/sec.

Higher is possible, but using/moving the data fast enough becomes an issue, and above ~77ksamples/sec the resolution loss starts becoming noticeable.

It seems the combination FTDI UART and AVR USART (both also run at higher bitrates than typical in Arduino), and AVR code needing some time, will in practice probably transfer no faster than ~1.5mbits/s. If you are willing to give up bit depth for speed, you can get on the order of maybe 150k or 300ksamples/sec.



Toy-like DSOs that fit certain purposes

Somewhat better

USB / mini oscilloscopes

USB logic analysers

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.
  • Logicport one
  • ZeroPlus Logic Cube
  • Bus Pirate
  • USBee


USB scopes

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

'DSO' is a description -'Digital Storage Oscilloscope', not a brand or type. You see the term on toys as well as professional gear.



Many cannot use all their sample memory at the top speed(verify)




DSO Nano

  • One probe (so no probe-based triggering, but there's level and edge triggering).
  • 12bits ADC at 1Msamples/sec. Don't count on seeing unaliased / detailed signals faster than 200kHz.
  • Can store waveform to SD storage
  • USB connection can be used to charge battery, read stored waveform (from SD storage), upgrade firmware
  • USD ~100


DSO Quad - basically an improvement over the DSO Nano

Desktop memory scopes

On probes

On (not) damaging your oscilloscope

See also


Practical use notes

Trigger mode