[ntp:questions] Installing more stable oscillator?

Richard B. Gilbert rgilbert88 at comcast.net
Sun Jul 15 16:38:08 UTC 2007

Pete Stephenson wrote:
> In article <4698E3AF.4020402 at comcast.net>,
>  "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote:
>>It's also possible, and far less hazardous to your computer, to use a 
>>high quality oscillator as an external reference clock for NTP.  HP used 
>>to make a GPS disciplined crystal oscillator, I think it was a Z3816A, 
>>that was used by telephone companies and started appearing on the 
>>surplus market three or four years ago.  These were much in demand by 
>>amateur radio operators as a frequency standard and by time junkies as 
>>NTP reference clocks.
> *nods* I saw one of those on eBay the other day. Very interesting -- it 
> looks like it has an ovenized crystal oscillator and a Motorola Oncore 
> GPS receiver. It's got a variety of outputs, including four PPS outputs.
> Expensive, but nice.
>>The cheapest route I know of is to buy a Garmin GPS18LVC GPS timing 
>>receiver (< $100 US) add a 5 volt power supply and a DB9 connector and 
>>plug it in to a serial port.  Install NTPD with the necessary driver, 
>>start it up, and wait a few minutes for it to synch up.
> I was looking at that as an option, but I was concerned about GPS signal 
> strength in my area due to buildings and a limited view of the sky[1]. I 
> was told that the GPS 18 LVC is a generally good unit, but that it's 
> antenna reception was not fantastic in areas of marginal reception.
> The Motorola Oncore receivers look pretty good, as one can attach an 
> active antenna, which should help out a bit with marginal signal 
> strength. That, and you keep all the expensive bits inside the 
> apartment. I'm debating between the GPS 18 and the Oncore, but the GPS 
> 18 does seem quite a bit simpler to setup as one doesn't need to worry 
> about separate boards and whatnot.

The Motorola GPS receivers are nice but are no longer made by Motorola. 
  Once upon a time you could buy a 12 channel Motorola (Oncore M12+T for 
the timing version) with a supporting circuit board (evaluation board) 
that gave you a rechargeable battery to sustain the receiver's memory, a 
serial interface, and maybe other stuff.
> How good is the GPS 18 LVC as a time source? From what I can read, the 
> leading edge of the PPS pulse is within 1 microsecond of UTC, which is 
> certainly more than adequate for my needs, but it's always nice to do 
> better if possible. :)

I've no personal experience with the GPS18LVC.  The reports I've read 
seem to be generally favorable.  It's usually possible to "do better" 
but, unless you are fabulously rich, doing better has to be tempered 
with some common sense.  You also need to consider what your computer 
and O/S are capable of.  Some can actually synch up within a microsecond 
or two; others keep time to within seventeen milliseconds!  The latter, 
of course, are Windows systems!


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