[time-nuts] FE5680 GPS Disciplined Controller
EWKehren at aol.com
EWKehren at aol.com
Fri Jun 20 09:52:39 EDT 2014
I have stayed quiet over the last six month on your effort but allow me to
make a few comments and recommendations.
First getting 1 E-13 per second accuracy is not possible with a FE5680
because of tuning steps of 6.8 E-13. Even 1 E-12 may not be possible. We are
working on braking 1 E-12 but it is not easy since parameters like
barometric pressure start making a difference and have to be compensated for.
My question is : is your project an time nut project or an Aduino project.
For time nuts a low cost good performing project that can be done with a
vector board and better than 1 E-9 is for instance the PLL version that J.R
Miller has done. As GPS receivers are getting better that circuit will give
better performance.. See attached.
Now if you want to do a digital filter you have to first recognize the GPS
limitations and the source you want to control. Let me start with the easy
part. Exceeding 1 E-10 you will have to monitor GPS for a longer period of
time and if you choose analog control you will not be able to control your
source wether Rb or OCXO without a DAC or other circuits outside your
Aduino. Some Rb's like the FE 5680A have DDS control via RS232 and it will most
likely be a solution. Please take a hard look at that alternative.
On the input side of the filter you want to capture small changes and we
have tried a few and went back to some thing we know that works reliable and
without any adjustments and for good measure we also have a GPS input
filter that is user adjustable something we found very handy. Something you may
want to consider.
With all your effort and help from other time nuts it would be a shame if
the outcome is just a low cost solution not making performance a key
requirement. Looking at the last six month your approach may get better results
because so many true experts have an opportunity to contribute and in the
final analysis create a product that expands the availability of better
performance at an affordable price to time nuts. That is the underlying goal of
our projects.
Think about it, I prefer not to get in to long discussions.
Bert Kehren
In a message dated 6/19/2014 2:13:11 P.M. Eastern Daylight Time,
albertson.chris at gmail.com writes:
I've been working in the same thing BUT I don't want anyone who builds it
to need a PCB. And I want the firmware to load over USB so there is no
need to ship programmed chips or deal with external programmers. I think
I can get the cost below $20. That said I doubt I'll get 1E-13
performance out of my Rb.
My little Arduino based controller has been running now for a couple months
and keeping a crystal in lock. The board has a pins left over for a serial
port that I'll hook up to the Rb.
The trick to getting the cost down is NOT to do a custom PCB. Take
advantage of one of the uP development boards and then for under $5 you get
the USB interface, D/A and A/D, serial ports, timers and quite a bit of
logic all 1/3rd the size of a credit card.
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