[time-nuts] Some More questions
jgd at johngsbbq.com
Wed Jan 18 01:20:28 EST 2006
On Tue, 17 Jan 2006 17:42:51 -0800, "Tom Van Baak"
<tvb at leapsecond.com> wrote:
>Nice set of questions.
>You can find more info if you hunt around my website:
>My very rough guess is that used Rb are available
>for $100 to $500 and used Cs are available for $500
>Most Rb last longer than a timekeeping hobby lasts.
>Rb tend to be much smaller, require less power, are
>available in much larger quantities (the used market
>is flooded with them; they show up on eBay every day).
>Rb have pretty good short-term stability, although a
>really good quartz will blow them away. It all depends
>on your needs.
Hi Tom and list,
I thought this would be a good time to de-lurk, introduce myself and
ask some questions.
First off, Tom, I'm blown away with your shop and accomplishments. I
thought I had a nice instrumentation shop....
I have a box full of Trimble ACE III OEM GPS receivers and active
antennae. I have one set up more or less permanently on my bench as
the source of a 1 pps signal. Here's what the manual says about the 1
The timing accuracy is ± 100 nanosecond (1 sigma) and is available
only when valid position fixes are being reported. Repeatability
checks of 10 sets of 100 one second samples taken
over a period of 20 minutes showed an average variation of
approximately 100 nanoseconds (not allowing for SA)."
The max rise time of the 1pps signal is less than 20ns.
Elsewhere in the manual it says that the 12.504 mhz crystal oscillator
is spec'ed as follows:
"Tolerance - +-1ppm at 20deg C.
Temperature stability - 2.5ppm/deg C."
I'm trying to determine what to make of these specs in terms of the
accuracy of the 1PPS signal as a timing signal. Not necessarily
accuracy as relates to GMT but to the actual 1.000xxx pps signal. 1E-7
doesn't seem all that good for GPS. Can I assume that the actual
timing accuracy is better? Jitter?
I'd like to use this signal as a gating signal for one of my frequency
counters. Would this be better or worse than the un-ovenized TCXO?
This is an 80s vintage HP counter, sorry I don't have the model handy.
I should be able to improve the receiver performance some by ovenizing
it. I wonder if it would be worth the effort?
Next question. I have a 70s vintage HP ovenized 10 mhz frequency
standard like the one that they sold as an OEM item and as used in
some counters. It's in the oversized metal relay can. I'd have to
open the case to get the model number but I do recall the -01 option,
high stability. I have this unit mounted in a case along with a power
supply and have used it for 20+ years as my frequency standard. It's
been powered more or less all that time and as determined by beating
it against the 10 mhz WWV signal, has settled down and is more stable
I have the patience to watch the scope.
If I divided this down to 1 PPS, would it be a better timing reference
than the GPS receiver? Is there a better method of calibrating this
oscillator than beating it against WWV, assuming the usual compliment
of electronic lab equipment but no special timekeeping instruments?
Next, I really want one of David Forbes' CRT clocks! I sent Dave an
email asking if he's planning on making any more but I haven't heard
back. (you lurking here, Dave?) Anyone have one in good condition
he'd like to part with? Prefer the clear case.
Guess that's enough for now.
John De Armond
See my website for my current email address
Cleveland, Occupied TN
A foolish consistency is the hobgoblin of little minds.-Ralph Waldo Emerson
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