[time-nuts] 1 pps sync'ing

[Bob kb8tq]

Hi

If the 1 PPS is from something like a Maser ( so it’s not all over the place)
*and* the 100 MHz is from something like an OCXO (so it also is reasonably 
stable), this all gets down to PLL design. 

Since it’s 1 pps in, you need a process that will resolve the input phase
reliably to << 1 ns. That is not trivial, but it can be done. You also need 
a stable way to get from 100 MHz to 1 pps. The “just do it with an 
FPGA” does not cover all the issues there…. there are a number of
delays that are going to vary.

Once you have that worked out, there *will* be some time period over
which you will achieve your 1 ns number to some level of confidence. 
There will never be an absolute guarantee.

Bob

> On Jun 29, 2018, at 9:13 AM, Martyn Smith <martyn at ptsyst.com> wrote:
> 
> Hello,
> 
> 
> 
> My colleague Steve asked a question about wanting to generate a 10 MHz and
> 100 MHz squarewaves with both rising edges aligned to a reference 1 pps
> input (to within 5ns).
> 
> 
> 
> We already have a product that can do this (at 10 MHz), but it is very
> complicated, and Steve's job is to fine an easier approach.
> 
> 
> 
> I'm not trying to achieve 1-5 ns to UTC as that's not possible as already
> pointed out, just 1 to 5 ns to a reference 1 pps (from Hydrogen Maser).
> 
> 
> 
> We definitely need all rising edges synchronised.  Of course, we will have
> to be very careful with cabling lengths etc otherwise we will lose all this
> synchronisation.
> 
> 
> 
> So starting at 100 MHz and dividing down is a good idea, except the 100 MHz
> needs to be synchronised in the first place, which brings me back to the
> first problem.
> 
> 
> 
> Our existing product generates 48 bit BCD time code all synchronised to a 1
> pps inputs.  This was designed ages ago for a customer.  So, the last bit of
> the 48 bit is 100ns in time or 10 MHz in frequency. 
> 
> 
> 
> So we could use this board and then lock the 100 MHz to the 10 MHz.  
> 
> 
> 
> But it's not very elegant!!
> 
> 
> 
> Anyway, the research continues.
> 
> 
> 
> Best Regards
> 
> 
> 
> Martyn 
> 
> 
> 
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