[sdiy] Trying to establish confidence in my test equipment

[harrybissell]

"Rykhaard D.A.M.I.A.N." wrote:

> Hey Harry!  Lissou...... leesewjus ....... them funky patterns!
> HARRY???  How do I do that on my scope?  :O  I've got 2 single trace
> scopes.  Is it possible with one of them?  (A B&K and a Leader).  I
> have 2 probes now, if that'd help. ?? :)
> I'd like to learn that tuning trick! :O :) :)

Lissajous patterns.

You need to be able to drive the scope X axis directly (external
input, replacing the sweep)

The X axis will be the reference frequency. It must be stable, and its
much nicer if it is a sinewave (easier to interpret) but a triangle
is OK.  A square is really useless...

The Y axis is the normal input. Again a sine wave is the easiest, as
above.

If the X and Y frequencies are perfectly in tune, the display will be a
circle,
oval, or straight line.  It will not move.

If they are close. the pattern will 'tumble' around between these
shapes...
at the beat frequency.  It is easy to pick out even a sub Hz movement
this way.

If you double the input (Y) frequency... there will be two peaks in the
X axis
and the wave will look like a figure eight lying on its side... or a
straight
line. It will not move.  If the frequency is not perfect, the wave will
tumble at
the difference frequency, as before.

If you half the frequency, the figure eight will be vertical (not
horizontal)
and will not move, unless there is a difference then it will tumble (as
above)

You can easily see low integer ratios, like 1:1 2:1 3:1 4:1... or 1:2
1:3 1:4...

or even fractional rations (a fifth will be 3:2).

If they are precisely in tune, the display will not tumble.

Now... it can be HARD to tell for some ratios (or odd waveforms) what
the ratio
is.  I hook up the signals to a small amp as well, so I can hear the
relationships.  When you hear it getting near an octave, THEN look at
the scope
to fine tune it.

For setting up a VCO, set the reference oscillator to the midrange of
the VCO
calibeation setting, and adjust to see like two octves up, and two
octaves down.

As long as the reference oscillator will hold the frequency, and you
don't touch it... the VCO can be calibrated 'ratiometrically' and the
absolute frequency
is not important. It will be 1/4, or 1/2, or 1/1, or 2, or 4 times the
reference
frequency.

I usually use a good digital tuner these days... AND the scope to prove
that
the octave relationships are correct. I can't trust the tuner
completely. :^P

H^) harry