[time-nuts] Subject: Re: GPS 1PPS, phase lock vs frequency lock, design

[Bob kb8tq]

Hi

A lot of commercial designs run through multiple lock strategies as they warm up and
stabilize. What the ultimate lock approach is after they have messed around very much
depends on what the customer is looking for. There are indeed FLL GPSDO’s in some
very fancy systems. 

PLL is the method of choice for cellular setups since time is normally the “tight spec” on 
the list. Since probably 99.9% of all the surplus GPSDO’s came from scrapped cell sites,
it is a good bet that your eBay surplus GPSDO is indeed running a PLL. That by no means
makes it the only approach or best in all situations.  

Bob

> On Jun 25, 2019, at 7:29 PM, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
> 
> Hi Bob
> 
> RRR.
> 
> In fact  all my original GPSDOs in the '90s were FLLs, for radio comms gear, essentially frequency counters with the error driving a PI loop. They were controlling standard XOs. For new engineers, if they want a project to teach them many skills, I recommend a GPSDO project from scratch, there are many excellent disclipines to grasp, oscillators, control systems, measurement.... and  many systematic errors to understand... and the project is only done if the spec is met, and of course the spec can go on being as tight as you want. You could spend your whole life on it.
> 
> glen
> On 25/06/2019 1:16 PM, Bob kb8tq wrote:
>> Hi
>> 
>> In a GPSDO, an FLL can be done with no “cycle slips” between readings. In that case, the I term will indeed
>> correct for long term errors. The net result will be effectively the same as a PLL for long term error. That is by
>> no means to say that *all* FLL’s are done this way. Only that it is one possible implementation.
>> 
>> Bob
>> 
> 
> 
> 
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