[time-nuts] Phase noise from Allan Deviation ?
kb8tq at n1k.org
Mon Dec 14 16:10:56 EST 2015
One of the reasons the TS2700’s went out of favor is the “quality” of the CDMA signals
available. The design assumption was that the CDMA carriers provided timing as good
as GPS on their over the air systems. After the units had been in the field for a while it
became apparent that the 2700’s were not performing up to expectations. Further investigation
turned up a range of issues that degraded the CDMA timing relative to GPS. A lot of it
boiled down to “we are a phone service not a time service”. System wise, CDMA gets
into trouble at the 10us level. GPS is in trouble at the 100 ns level….
Yes there are all sorts of rules and regulations. In the end it’s “this works fine” that trumps
a lot of them.
> On Dec 14, 2015, at 1:15 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> Tom wrote:
>> The 'unknown' is a Rubidium oscillator locked to CDMA pilot (TS2700)
> There is very little information publicly available on the Symmetricom "BesTime" engine ("BTE"), but after playing with a few TS2700s for quite some time, including monitoring a number of internal signals, several things became apparent. First, the 2700 does not seem to discipline the PRS10. The rubidium runs open loop and the BTE keeps track of the offset and the drift rate from "BTE time" (which is synthesized from all available sources -- however many CDMA signals it is receiving, plus any wireline telco timing signals and the PRS10 -- using a proprietary algorithm to estimate the reliability of each source and outputting BTE time and frequency using DDS). Hobby users won't be feeding the unit any telco timing signals, so the BTE has only the CDMA signals to work from. During holdover (and assuming no telco timing signals), the Rb is the sole input to the BTE, which uses the stored offset and drift to calculate BTE time.
> I found that the TS-2700 is more than an order of magnitude less stable than a Trimble Thunderbolt, even with a full complement of rock-solid CDMA sources. This may vary somewhat, depending on the CDMA equipment in use at any particular location and the diligence of the CDMA operator.
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