I've purchased some VERY cheap ones from eBay (china/hongkong). Some from
Motorola/ Trimble/ etc, .... Some with SiRF protocol, all with NMEA
protocol. All cheap. :-)
Just read the spec sheet on each before you buy, ...they are often in the
eBay auction, btw.
If it says the 1-pps is 'tied to' UTC time, then they will be accurate,
...mine are within 15-50ns.
Otherwise, ...like David says, ...they are not tied to the GPS clock and can
be off by microseconds.
-Don
--------------------------------------------------
From: "David" <[email protected]>
Sent: Wednesday, September 26, 2012 3:31 PM
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Subject: Re: [time-nuts] usb gps devices
The pulse per second output on the Garmin serial/CMOS interface units
is only specified to be within 1 microsecond. I guess they are
commonly used to synchronize NTP servers where any extra precision
would be lost in network transport anyway.
I have been thinking of picking one up cheap to test since the
specification is so vague. 1 microsecond makes me think that the
pulse per second output is generated from a 1 MHz asynchronous clock.
On Wed, 26 Sep 2012 14:11:10 -0600 (MDT), "Don Latham"
<[email protected]> wrote:
Have any of the 'nuts hacked one of the very simple very cheap GPS USB
devices for your car top to see if there is an available 1 sec tick
inside one of them somewhere? I have one that I intend to look at, but
I'll have to get a scope and teenyweeny probe outside to do it, so, if
there is a readily available something, I'd like to know. Thanks!
Don
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