[time-nuts] Mains frequency

Bill Dailey docdailey at gmail.com
Sat Nov 16 14:23:41 EST 2013


My purpose is to do it with a picpet.  That's it.  So, that eliminates a bunch of the options.  I can decouple the measurements from the pc clock that way.

Doc

Sent from mobile

> On Nov 16, 2013, at 11:26 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 
> 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
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