I just finished similar project recently. I am using OCXO 9,830400MHZ as
the reference. My MCU driven by this crystal (No PLL). And I was using
opto-coupler as ZCD.
At the begining I was using every crossing (which 120 times per second
in case of 60Hz). However that design was not good. Long live to tom
(/tvb) who did analyses of raw data I provide and gave me very valuable
advises how to improve my project.
The main purpose of my project was to drive the Telechron clock. The
Main measurement was added just for fun. My project has three clocks
inside. Its RTC, driven my simple watch crystal, it is Main clock,
driven my ZDC and it is MCU clock, driven by OCXO.
The MCU still capturing every ZDC events (120 events per second in case
of 60Hz). But I concentrate on full cycles. Also, I implement the switch
how often do the records to the log file. Its either every 64 ZDC events
or once per 8192 cycles. Basically it recording the time difference
between number of ZDC events.
The log looks like this:
# Uptime: 139 hours
# RTC time: 18:00:00
# MCU time: 18:00:00
# MAIN time: 18:00:03
# [Time Stamp] [Period] [1/9830400]
16-04-11 18:00:17.253 [+] 0.01665924072269 163767 -4
16-04-11 18:01:25.500 [+] 0.01666137695315 163788 -21
16-04-11 18:02:33.765 [+] 0.01666656494141 163839 -51
16-04-11 18:03:42.035 [-] 0.01666798909505 163853 -14
16-04-11 18:04:50.300 [+] 0.01666564941407 163830 +23
16-04-11 18:05:58.570 [-] 0.01666748046875 163848 -18
16-04-11 18:07:06.847 [-] 0.01666961669921 163869 -21
16-04-11 18:08:15.097 [+] 0.01666239420575 163798 +71
Time stamp, then "sign" which means if we are bellow or above the
"ethalon", then period, then raw value of capturing timer and then delta
between two readings.
Works great for me.
Regards,
Vlad
On 2016-04-06 21:21, 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
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
WBW,
V.P.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
