[time-nuts] Building a mains frequency monitor

[Charles Steinmetz]

Nick wrote:

>At one point, I did try an LM393 instead of a 358. The result was 
>that noise caused excessive false triggering. The 358, so far as I 
>can tell, when acting as a comparator lacked sufficient bandwidth 
>and/or speed to keep up with the noise.

>My results also seemed to be on a par with the published results of 
>other similar investigations (particularly those of tvb).

I'm on record as being in favor of pre-filtering to separate true 
grid phase artifacts from local noise, as much as 
practicable.  However, it would be serendipitous indeed (more like 
miraculous, actually) if the slew-rate limit of an LM358 just 
happened to be exactly the right degree of filtering.  Close enough 
for a science-fair project, perhaps, but not a time-nuts-level solution.

Tom uses a Schmitt trigger input (which, as I pointed out yesterday, 
guarantees that the "non-zero-cross detector" [or, 
"zero-cross-by-proxy detector"] will have AM to PM conversion), and 
(last I knew) he does not filter the input (other than the incidental 
interaction of the input resistance of his pickup with the input 
capacitance of the gate).  A proper comparator with a 0v threshold 
and a few mV of hysteresis, preceded by carefully designed filtering, 
can generate a ZCD output with substantially lower cycle-to-cycle and 
second-to-second phase errors due to local noise that is a 
significantly more faithful representation of the actual grid phase 
and frequency.  If what we're interested in is measuring the grid 
phase and frequency, rather than the incidental local noise that has 
nothing to do with the grid phase and frequency, this is the clearly 
better approach, IMO.

Best regards,

Charles