Oscilloscope and logic analyser notes: Difference between revisions

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'''How much do you need?'''
'''How much do you need?'''


Consider e.g. retrocomputing repair and most DIY and most microcontrollers.
Consider e.g. retrocomputing repair, and most DIY and most microcontrollers.


Even if things run at dozenz of MHz internally,  
Even if things run at dozenz of MHz internally,
most digital signals that come out will rarely be above a MHz or two.
most digital signals that come out will rarely be above a MHz or two,
 
and most slower than that.
Consider an Arduino.
Aside from the main crystal, the fastest thing you'll see happen on pins is probably 1MHz.


In this regard, if you can find a EUR100 scope but it's limited to maybe 5MHz bandwidth,
In this regard, if you can find a EUR100 scope but it's limited to maybe 5MHz bandwidth,
that would already solve cover a lot of needs.
that would already solve cover a lot of needs.


And you can get a decent 25MHz scopes for EUR200.
And you can get a decent ''new'' 25MHz scopes for EUR200.




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There is a very real difference between sampling rate and bandwidth.
There is a very real difference between sampling rate and bandwidth.


'''Bandwidth''' is largely about whether an analog signal can make it from your device to the oscilloscope's ADC without being changed - can the probes pick it up, can the frontend handle it without distorting it?
'''Bandwidth''' is largely about whether an analog signal can make it through all the parts,
from your device, through the oscilloscope frontend and to the oscilloscope's ADC, without being changed or distorted.


Distortion is a relative measure, though.  
Distortion is not an on-off thing - systems will slowly not be able to meet their specs, which for bandwidth
often means the inability to carry a frequency / speed.


The way we often bandwidth ''basically'' means there is a -3dB attenuation at the specced bandwidth,
The way we bandwidth is frequently specced ''basically'' comes down to that being the point at which its limitations mean there is already -3dB attenuation of signal at that specced bandwidth, meaning the amplitude is at only ~70% of what it should really be (this is why you see reviews use frequency generators. It's useful to check that the waveform look right ''but also'', independently, to check when that attenuation happens, e.g. "I know this is a 1V 50MHz signal and yet it measures 0.7V" then you have a good idea what the limit is.
meaning the amplitude is at only ~70% of what it should really be (this is why you see reviews use frequency generator, not only for 'does the waveform look right' but also when that attenuation happens, e.g. "I know this is a 1V signal and around 50MHz it measures 0.7V" then you have a good idea what the limit is.


You don't have to think about this much before you get near your specced bandwidth (at 1/10th it's maybe 2%)
"so is this a gradual falloff?"


Yes, but mostly not one your need to worry about.
At 1/10th of the bandwidth the falloff is maybe 2%,
so in everyday use, you don't have to think about bandwidth until you actually get somewhere near it, or care about very precise measurements (in which case you need to think about better devices for it).




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'''Sampling rate''' is about whether you sample often enough to see that waveform.
'''Sampling rate''' is about whether you sample often enough to see that waveform.
: Nyquist says ''at least'' a factor two more than the bandwidth you have
: Nyquist says you need to sample ''at least'' a factor two more than the frequency you like to see -- so if this isn't twice faster than the bandwidth, you can't really use that bandwidth (not that it's bad to not get near that bandwidth - you could argue it's a good frontend for this scope).


...yet ''at'' that double rate you can only tell that there ''is'' a signal at that rate, not what shape it has, or even say much about its amplitude.  
...yet ''at'' that double rate you can only tell that there ''is'' a signal at that rate, not what shape it has, or even say much about its amplitude.  

Revision as of 16:08, 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

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