> On Aug 17, 2015, at 7:29 PM, Dave G4UGM <dave.g4...@gmail.com> wrote:
>
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of
>> drlegendre .
>> Sent: 18 August 2015 00:15
>> To: General Discussion: On-Topic and Off-Topic Posts
>> <cctalk@classiccmp.org>
>> Subject: Re: test equipment / Re: Z80 / Z84C Swap (Doh!)
>>
>> Hey Dave & All,
>>
>> Could you give a little quick kick-start guide to bit depth & sampling rate
>> on
>> DSOs? It's always kind of stumped me, not that I've ever read deeply into
>> it..
>> but how is it that you can get any kind of (vertical, right?) resolution out
>> of 8
>> or even 12-bit samples?
>
> Well the 8-bits is around 5mv on a 1volt signal. It draws a thin line that
> goes up in big(ish) steps. I am off to bed now but will upload some screen
> shots to my OneDrive so you can see them.
It depends on the screen resolution. My Tek 602, a rather early (and very
large/heavy) DSO, does just fine with 8 bits because the screen is fairly
small. Compared to the screen resolution of a typical analog oscilloscope
screen, 8 bits is a pretty good choice.
You can occasionally find DSOs with higher resolution, but there’s a
substantial penalty in sample rate.
> You need at least twice the sample rate as the maximum frequency you want to
> plot, but really to be useful I recon 4 x . So the 6022 samples at 48M
> samples/second and has 20Mhz bandwidth amps so you probably get usefull
> displays up 10mhz (ish).
Nyquist rule: you need 2x the sample rate of the highest signal frequency. Or
more accurately, the bandwidth. That’s the same thing if you start at 0 Hz,
but if you’re sampling a piece of spectrum, you only need 2x the bandwidth of
the band in question, not 2x the frequency of the upper edge. Note also that
you have to consider filter skirts, so you need 2x the frequency at which your
input filter has “high enough” attenuation.
To pick an extreme example, the current record holder DSO has 100 GHz (!)
bandwidth and 240 Gs/s sampling rate.
>
>>
>> Example line of thought - 8 bit sample = 256 possible vertical positions.
>> Even if the screen is low-end (640 x 480) that's almost 2X more height in
>> pixels than samples in an 8-bit sample. So each increment is like 2 pixels
>> tall
>> and seems like it would be awfully blocky and imprecise. Things would seem
>> to get even worse if you try to do maths functions..
>>
>> I must be viewing this quite wrong?
You would not display them as 2 pixels tall. Typically the display machinery
will interconnect the sample points by lines, and the result usually looks
pretty convincingly like an analog scope display (unless you crank the zoom way
up). As for math, that depends. If your math involves filtering, the
effective bit count goes up (the “processing gain” DSP people speak of).
paul
