A good source for what is actually going on with power line frequency is
the web site of the University of Tennessee, which in partnership with
Oak Ridge National Labs has a mains frequency monitoring program at
http://fnetpublic.utk.edu/. The "Table Display" page shows frequency
data for the United States and some other parts of the world. My station
is #853 in the "Western Interconnection."
The frequency bounces around seemingly at random but within ±0.1 Hz
maximum and usually half of that. The random pattern of the frequency
shifts certainly could be used to identify a point in time at which a
recording was made. By the looks of the data, I doubt that small
measurement errors such as those being discussed would affect results.
Jeremy
On 4/6/2016 6:21 PM, Jay Grizzard wrote:
Since it seems to be a week for new projects on time-nuts... ;)
So I've been wanting to set up a power line frequency monitor for a while,
and now(ish) seemed to be a good time for me.
So initially, I was planning on doing a simple design that was posted here
a couple of years back, which basically works out to:
mains -> simple 9v ac/ac power brick -> dropping resistor -> picPET
I have a good 10MHz reference to feed the picPET, so this seems like it
would make a good first shot. But, of course, I eventually want to do
better than just a first shot. So, I have questions!
Q1: Assuming the schmitt trigger in the picPET triggers at a consistent
point in the waveform, the frequency at any given cycle is easy to
calculate: 1.0 / (timestamp2 - timestamp1) ...but, is there a better
way? That method just feels... naive, for some reason.
Q2: What are the sources of noise in this design? Assuming the picPET is as
accurate as my 10MHz reference is, I can think of a few potential places
that phase noise could creep into the measurements:
- Whatever is in the power brick beyond the transformer (I don't think a
step down transformer alone would add phase noise, right?)
- The dropping resistor will slowly change the amplitude of the waveform
(and thus the point in the cycle that the schmitt trigger fires) due to thermal
and aging effects, if we're measuring anything that's not the exact zero
crossing
- The point at which the schmitt trigger in the picPET fires will change
over time for the same reasons. Also potentially due to picPET input voltage,
depending on how the comparitor is built
- Am I missing any?
Q3: The open-ended question: How do I improve on this? I suspect the main place for
improvement will be in the trigger, but I'm not sure where to go with that. Most designs
I've seen involve a schmitt trigger, generally with reference voltages set by things like
voltage dividers. This seems dubious at best, to me, since that means the reference
voltage will be affected by the same effects I'm calling out above. Is there a *specific*
design (rather than "make a zero crossing detector!" or something similarly
vague) that someone can point me to, that would minimize this kind of trigger noise?
Q3.1: Is there a better way to get mains voltage down to something I can work
more directly with? I saw at least one design that just used a couple of
megaohm resistors inline -- does that introduce appreciably less phase noise
than random AC/AC power brick?
I apologize if any of this is overly basic. I've actually read everything I
could find both in the time-nuts archives and the internet at large about this
kind of project, but I've still found myself left with the questions above.
I appreciate any comments / feedback / pointers!
-j
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