Mike Harrison wrote:

> A tip I heard at an MCT seminar is that the amplitude on both sides of
> the xtal should be about the same for most reliable operation -
> obviously you'd need a low-capacitance probe (10M R in series with
> normal probe?)

  Actually, even if you want a low-capacitance probe, you need to
correct its frequency response by providing a capacitance across your
series resistor and adjusting it so the capacitance ratio equals the
resistive division ratio.

  The resistance ratio is that of the series resistor to the input
resistance of the CRO, while the capacitance ratio is that of the
trimmer cap across the series resistor to that of the coaxial cable AND
CRO input.  The input attenuator of the CRO is quite complex with all
these caps and things.  In general, a "10X" probe divides the input
capacitance by ten and so with any other ratio.

  What I have often meant to build is an active probe using one of the
high frequency LM series buffers or UHF MAR devices for this particular
job of monitoring oscillators.  Given that only voltages of 0 to +5 or
perhaps +10 are involved, this should be quite easy.

  Cheers,
        Paul B.