[time-nuts] Mains frequency

[Chris Albertson]

The signal is 120 volts.  You hardly need to amplify it.  Clip it with a
diode to +- 9 volts so as not to blow up your serial port.  But I'd use a
transformer for safety. The zero crossing detectors are built into the
RS232 interface.    You take advantage of the RS232 spec which has a DCD
pin input of about +-9 volts that is already set up to find a leading edge
of a pulse and cause a very low latency interrupt.  The system software
already will capture the time all inside a kernel level interrupt handler.

The jitter turns out to be on the order of a single digit microseconds.
 Good enough for measuring a 60Hz signal.

I guess if you want to see transients depends on the purpose of the
experiment.  Are you looking at local AC power quality or wanting to
measure the grid.  The grid is well monitored, just use FNET and you get
real-time data for all of North America.   I think the reason for measuring
it yourself is to see local power quality and things load switching inside
your own building, that's transients.



The other way to measure AC with zero added equipment is to treat it as an
audio signal and after reducing it to 1 volt run it into an audio interface
And then use FFT.   This will let you see very small spikes and noise.   It
depends again on your purpose for doing this.




On Sat, Nov 16, 2013 at 1:18 AM, Magnus Danielson <
magnus at rubidium.dyndns.org> wrote:

> On 11/16/2013 09:52 AM, Chris Albertson wrote:
> > Your method tosses out a lot of data.  You can't see transients.  Ideally
> > rather then record a 1 second average you'd record the time of EVERY zero
> > crossing.  It sounds like a lot of data but not really.   You only record
> > 32 bits 60 times each second.  That is 240 bytes per second.
> But you want it filtered to avoid the transients. Those are really not
> that interesting when you measure the grid.
>
> Also, if you use the event trigger method you probably want to use an
> amplifier to increase the slew-rate such that noise does not convert
> into time jitter.
>
> Cheers,
> Magnus
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-- 

Chris Albertson
Redondo Beach, California