Date   : Sun, 24 Jan 2016 22:06:02 -0000
From   : blip@... (John)
Subject: Oscilloscopes : Troubleshooting the BBC Mirco

 

I searched this for more information, and after being bamboozled on the 
math (not my strong point) it became quite simple. If you sample at the same
frequency, you are basically hitting the same height of the signal (sine)
wave every time which gives you a digital flat line, right? 

 

The National Instruments page say a doubling of the sample rate gives you
a triangle, which is at least something for granularity.

 

I know that?s over simplifying it, but don?t ask me to look at Nyquist again J

 

In brief: I think you saved me ?12. Virtual beer voucher in the post! I?ll
take a peek at your blog later.

 

It does provide a bit of a setback though ? I was eyeing D.I.Y. kits as something
that could be worked into my computing project as easy and cheap to do for
an end user.

 

Thanks

 

 

John

 

 

 

 

From: Rick Murray [mailto:rick@...] 
Sent: Sunday, January 24, 2016 9:42 PM
To: John; 'BBC Micro Mailing List'
Subject: Re: [BBC-Micro] Troubleshooting the BBC Mirco

 

"Are these hand-held Oscilloscopes any good"

 

In the digital domain you need to have an ADC that can sample at least five 
times the speed of the signal you want to examine. The fastest clock rate 
on a Beeb is 8MHz (although the system runs at 1-2MHz). So you'll need a 
40MHz digital scope to correctly see the 8MHz, or a 10MHz one to trace the 
processor activity. While a digital scope ought to allow features not available 
on analogue models, such as taking a snapshot of the waveform for later 
analysis, in reality the quality of the acquisition circuit and the number
of data points stored mean that the ability to examine signals may be little
more than a gimmick on lower end devices.

 

I recently discussed oscilloscopes on my blog and something that came to 
mind was that a typical non specialist digital model will have an eight 
bit ADC. While you might think having 256 levels would be suitable accuracy,
it is technically insufficient for capturing a correct view of a simple audio
CD passed through a good DAC. CD audio is 16 bit, the scope is eight bit...

It seems to me that while analogue can be limiting and not offer fancy features, 
it is a fairly "what you see is what you get", while there are many varied 
ways in which a digital oscilloscope can blatantly lie to you. So with that
in mind in not sure any of the lower end portable models are worth considering
for anything more advanced than basic fault finding.

That's not to say that all D(S)Os are bad, I just think one needs to understand
how it works to understand it's limitations.

 

 

Best wishes, 

 

Rick.