[time-nuts] Software Sawtooth correction prerequisites?

[Tom Van Baak]

> Define cheap.
> You can already get essentially single chip TICs with a resolution (and 
> accuracy) better than 100ps for around 100 Euros or so.

Has anyone in the group tried one of these? I would very
much like to see the results.

> However sawtooth correction is cheaper than using a hardware variable 
> delay line.

Good point.

> However the optimal solution doesn't use a TIC at all, just use a good 
> GPS receiver that makes the carrier phase observables available.
> Quite a few of the older receivers used to make the carrier phase data 
> available.

Many of the Oncore VP's did this, but I don't know of
anyone, amateur or commercial, that built a GPSDO
using that feature. That makes me wonder if carrier
phase observables alone is not enough.

> The catch is that the GPS receiver local oscillator must be phase locked 
> to the OCXO.
> Some receivers use a 10MHz crystal oscillator in which case this can be 
> removed and replaced with an external 10MHz signal derived from the OCXO 
> being disciplined.

Right, this is the key. Do you know any OEM receivers that
allow an external 5/10/20 MHz? I know my Z12T does (but that
is far from an OEM receiver).

> Unfortunately there are as yet no relatively inexpensive off the shelf 
> implementations of the GPS carrier phase discipling technique available.
> The calculations involved for maximum performance are somewhat complex, 
> however not too much computing horsepower should be required.

The QL carrier phase GPSDO is unique and probably not
cost-effective. Sounds like it's a custom single-channel GPS
receiver designed from the ground up. If there were an easier
way surely someone would have done it in the past ten years.

> Magnus hopes to remedy this lack of suitable software and hardware 
> sometime in the future.

That would be very nice. Will it be L1 only or L1/L2?

/tvb