[time-nuts] GPSDO using 100Hz

[Bob Q]

I have a design based on the Motorola MT12+ receiver that uses a CMOS 
XOR-based PLL.  I will send you the file with write up and schematic.
Bob Q.
----- Original Message ----- 
From: "WarrenS Email" <warrensjmail-one at yahoo.com>
To: <time-nuts at febo.com>
Sent: Sunday, November 23, 2008 2:18 PM
Subject: [time-nuts] GPSDO using 100Hz


> This is my first listing so don't know if I'm doing correctly.
>
> Question is: Has anyone done any work using the 100Hz GPS output, instead 
> of the 1 Hz output?
> The reason I ask is because I am in the process of cleaning up my SIMPLE 
> GPS Freq Phase lock tracker breadboard that does about the same as most, 
> but in a different way. By using just a few basic standard  74HCxx IC 
> logic gates and Flip-Flops with RC's I phase lock a low accuracy VCXO to 
> give me a high accuracy 10MHz reference. This is something I built for my 
> own use, to check the accuracy of my 10 MHz freq standard and to check the 
> accuracy of  the 60KHz WWVB signal.
>
> I found out that the most important trick to keep the GPS tracker simple 
> (GPSDO), fast and low power, and do it without the usual microprocessor 
> stuff,  was to use the 100Hz output instead of the 1HZ output from the 
> Motorola Oncore receiver board.  Basically this allows simple logic 
> circuits to give 10 to 100 times better results, such as phase noise of 
> around 1 ns instead of 100ns from second to second, cold turn on and full 
> accuracy lock of the tracking osc in well under a minute (with GPS left 
> on). It also has the ability to track the GPS phase errors at about a 10 
> Hz rate which gave me some new interesting insight, such as the 1 sec 
> sawtooth error that the Oncore reports on, that would usually requires the 
> use of a computer to process, is no longer very relevant.
> I found out the 1 second reported phase error is just an alising artifact 
> of a higher freq phase dither that can be completely filtered out with a 1 
> second Time constant RC. (i.e RC replaces microprocessor).
>
> It generally takes about an hour Tracking time constant to get good 1e-10 
> Freq stability using the 1 Second GPS signal, and more like 3 to 24 hours 
> as not degrade a really good Oscillator.
> I am getting such good results with the simple high speed RC PLL, I am now 
> doing some test to see if there are new ways to reduce the Tracking time.
> It would seem it should be possible to get 1e-10 in more like a 100 
> seconds instead of an hour. This would then reduce the required stability 
> of the tracking Oscillator by an order of magnitude or better. The simple 
> PLL tracks the 100Hz within about 1 ns and can lock up in 10 seconds, and 
> the GPS signals themselves don't seem to vary by more than about 10 ns 
> with 10 minute and 1Hr average times. (from 
> http://tf.nist.gov/service/gpstrace.htm)
> I have not found any GPS signal accuracy data for shorter average times, 
> so that is what I'm presently plotting.
> One of the other many benefits of fast GPS tracking may be the possibility 
> of a more mobile Disciplined Oscillator.
>
> If anyone has done any work using the 100Hz Oncore GS signal to do 
> frequency tracking,  I would be very interested in further discussions.
>
> Warren
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