[time-nuts] How accurate are cheap radio controlled clocks?

David Martindale dave.martindale at gmail.com
Sat Jun 25 20:02:58 UTC 2011


On Sat, Jun 25, 2011 at 12:19 PM, Dr. David Kirkby
<david.kirkby at onetel.net> wrote:
> I've got one of the cheap radio-controlled clocks? I was listing to radio 4
> the other day and herd the time signal. The radio controlled clock was about
> 3 seconds off. I was a bit surprised it was so far off. I'm just wondering
> how accurate these things are.

I have a Casio watch that syncs to WWVB when I leave it near a window
overnight.  There's an indicator on the display that shows if it
synced sometime since midnight today (local time), as well as a mode
that will tell you the last time it did successfully sync.  So far,
whenever it has synced today, the difference between it and a
NTP-synced computer time display has always been a fraction of a
second.  Not zero, but well below a half second.  (And when comparing
to a computer, remember that graphics cards and LCD monitors may add
multiple frames of delay).

If the watch doesn't sync for a few days, its timekeeping slowly
drifts but it still takes days to accumulate one second of error.
When I first purchased it, it was 25 seconds fast but the status
display said that the last successful sync was about 8 months ago, so
the internal crystal is likely good to about 3 seconds per month.

On the other hand, I have often heard over-the-air time beeps from
radio stations that are off by 5 seconds or more, usually late.  I
don't know if they are generated by the studio's own clock that is
hand-set and not very accurate, or if the beep is accurately generated
at the mixing board but then fed through something that causes a
significant delay.  (For example, live radio programs have a
multi-second delay between studio and transmitter so they can bleep
out "bad words" that a caller might say before they are broadcast).
So I wouldn't trust a radio time beep for anything without first
comparing it to another source.

     Dave



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