[sdiy] Measuring Vpp

[John Speth]

> The scope I have isn't great, more just for visual display of waveform
than
> accurate measurements.   I can't afford to get a proper scope, so I'm
looking
> for another way to calibrate outputs to 10Vpp and 20Vpp.  I looked up
how to
> do it with a DMM and it seemed more complicated than I had hoped. Does
anyone
> here have a suggestion of how to do this (easily) using a DMM, decibel
> levels, or freeware?

A calibrated scope is nothing more than knowing the voltage difference
between graticule hash marks.  You can calibrate that yourself with a
known reference voltage for vertical and a known clock for horizontal
(time).

The voltage reference can be attained by getting a voltage reference IC,
wiring it up, and sending the voltage to your scope.  Then use some tape
with carefully drawn hash marks.  Stick the tape to your scope screen and
adjust your vertical gain so that you get a nice calibrated scale (for
example, 1V per division).  Now you have a calibrated scope.  You'll need
to recalibrate every time the vertical gain changes and maybe even every
time you turn on your scope.  Be sure to verify your reference with your
DMM for sanity.  You might need an op-amp circuit to gain up your
reference voltage.

JJS