[time-nuts] Another Arduino GPSDO

[Bob Camp]

Hi

The problem is that it can / might / could / will create a “dc bias” in the noise. When you filter it, you get a bump rather than zero. If your GPSDO has a 47 ns wide sawtooth, that could be a pretty big bump. There’s no way to know if the bridge is seconds, minutes, or hours wide. You can make a good guess that hours are a *lot* less common than seconds….

If you are after 0.01 ppb (parts per billion), then a 47 ns offset would need to be averaged for about 10,000 seconds if it’s narrow. If it sticks around for 100 seconds you might need to average for quite a bit longer ….

Bob


On Mar 28, 2014, at 8:15 PM, Eric Williams <wd6cmu at gmail.com> wrote:

> OK, now I understand the reference, but not the significance in this
> context.  The sawtooth error is below the resolution of the counter here.
> It does produce noise on the transition between capture counts that I am
> filtering out, but it should average out over the long-term integration
> that I'm using, right?
> --
> eric
> 
> 
> On Fri, Mar 28, 2014 at 4:41 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
>> 
>> wd6cmu at gmail.com said:
>>> I'm sorry, I don't recognize the reference to "hanging bridges".
>> 
>> I guess it was a good thing I mentioned it.  Sometimes I feel like a broken
>> record.  It gets discussed here occasionally.  There will be lots of
>> comments
>> in the archives.
>> 
>> Starter version:
>>  tvb: Motorola GPS M12+ Sawtooth
>>  http://www.leapsecond.com/pages/m12/sawtooth.htm
>> 
>> Long version, very good:
>>  Tom Clark and Rick Hambly: Timing for VLBI
>>  http://gpstime.com/files/tow-time2009.pdf
>> 
>> 
>> --
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
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