So, while waiting for parts to arrive, I have been diddling sketches on anA4 piece
of paper, on how to design the the minimal cost attenuator, for the PIC scope I am planning.
Now I peaked at one scope I have here, and it has an 11 position switch
10mv, 20 mV, 50 mV, 100 mV, 200mV, 500mV, 11V, 2V, 5V, 10V, 20V
Now that sucks in a way, as counting to 5 is very difficult,
as we all think binary these days ;-)
The other thing that is interesting, is that the PIC ADC is 10 bits,
and I have a 64 pixel high LCD.
So I actually only need 6 bits (2^6 = 64 if it escaped you) for display.
That is still better then 2/100 or 2% accuracy, so fine with me.
So 10 bits in, and use only 6, then we can bit-shift, and use the 4 bits shift
to make attenuator steps of 2, 4, 8, 16
This reduces hardware (switching) people!
Then I was thinking:
Why not use binary on the settings? much easier.
So then you get sensitivity of:
10mV, 20mV, 40mV, 80mV, 160mV per division.
And maybe then continue, after switching gain ONCE:
320mV, 640mV, 1.28V, 2.56V, 5.12V per division.
Looks like a need for 2 more steps for higher voltage, 10.24V, and 20.48V / div.
For 64 pixels high and 8 divisions vertical that leaves 8 pixels per division.
makes a max voltage of 163.84V full screen (at 1x probe).
We can have a cursor on the trace, and a volts display, so who cares even if it
switched ranges in octal ;-)
I think you are looking at scope functionality solely in terms of
quantitative precision. This is a mistake.
95% of scope functionality does not come from making quantitative
measurements (except on a very gross scale), but from seeing
waveforms.
It matters very very little exactly what units are on the screen for
the "seeing waveforms" aspect. What matters completely is being able
to comfortably and quickly adjust the scale to match the features in
the waveform you are interested in looking at.
And I feel that having only 64 steps is pretty hopelessly coarse. With
a good analog scope from 40 years ago, I can see ripple of fuzz or
glitches or noise that's way less than 1 percent of full scale. Being
able to see that ripple/fuzz/glitches is very very important to most
of the uses I'm using a scope for.
A scope that only has 64 steps is pretty crappy in those terms. I
mean, I can focus the beam on a 40-year-old analog scope way more
tightly than the 64 steps.
I know, I didn't answer your actual question. I'm just urging you to
step back from an overly-quantitative analysis and think about what
you (if not I!) use the scope for.
Others, I think, don't "look for the picture" the way I do. That's why
the market is flooded with cheap crappy digital scopes. (The expensive
digital scopes do generally do it right!) These are probably the same
people who always write down as many decimal points as their
calculator displays
. I grew up with slide rules and I don't
believe anything after the second or third digit anyway, because I
know none of the inputs are known that accurately!
Tim.
