[time-nuts] Mains frequency

[Tom Van Baak (lab)]

Doc,

I measure mains time & frequency with a picPET all the time. In fact that's one of the reasons I designed it. If you're having any trouble contact me by email.

/tvb (i5s)

> On Nov 16, 2013, at 11:23 AM, Bill Dailey <docdailey at gmail.com> wrote:
> 
> 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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