Re: gEDA-user: Any DIY USB Scope project on schedule? Or some recommmendation?

[Charles Lepple <clepple@gmail.com>]

On Tue, 23 Nov 2004 13:38:18 -0200, Xtian Xultz <xultz@terra.com.br> wrote:
> I Understand what you say. And what the mega samples per second means? I cant
> understand why having 20 mega samples per second I can measure onle 250k Hz
> of a sine wave...

The Nyquist theorem (20 MS/sec would give you a theoretical upper
bandwidth of just under 10 MHz) assumes ideal sampling, and probably a
bunch of other caveats that I can't remember now.

In reality, you are going to get quantization noise (+/- full scale
divided by 2^N, where N is your number of bits), sampling jitter, and
coupled noise on the input. Setting your sampling frequency to 10x the
highest frequency component of your input is not unreasonable.

Also, real-life anti-aliasing filters (the ones that prevent higher
frequencies from affecting your sampled data) can't have sharp cutoffs
without distorting the input signal (usually manifested as ringing).
If the analog anti-aliasing filter is only 10 or 20 dB/decade, that
still allows a lot of higher-frequency energy in to your sampler. The
oversampling lets you filter out the higher frequencies in the digital
domain.

I can't remember the app note number, but Microchip had a good paper
on their website explaining analog-to-digital converter parameters,
and IIRC they discussed some of these very issues.

-- 
- Charles Lepple