[time-nuts] Building a mains frequency monitor

David davidwhess at gmail.com
Wed Apr 13 18:23:43 EDT 2016


Tektronix used transformer isolation followed by a low pass RC filter
with about a 1 kHz cutoff; this was fed directly into the trigger
source selector just like any other trigger source.  There is still
enough noise present that the trigger coupling can be used to select
specific features to trigger on.

Modern DSOs often (always?) use an optocoupler circuit for isolation.
I saw an example schematic somewhere and it is pretty simple.  It
produces a roughly linear output referenced to the negative or
positive peak so triggering can still occur over almost 360 degrees of
phase.  I do not know what kind of low pass filtering was implemented.

On Tue, 12 Apr 2016 09:38:13 -0400, you wrote:

>...
>
>I have yet to dredge up the 60Hz line-lock schematic circuit used in more
>than half a century of analog scope trigger circuits. The Tek scopes had
>exceptionally robust trigger circuits.
>
>Tim N3QE


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