[time-nuts] Reasons for still building the Shera GPSDO

[Mark Amos]

Ulrich, 

Thank you for sharing your insights and knowledge with the group.


I can't speak for the people that have and are building the Shera GPSDO.

But here is an abbreviated list of my reasons:

- It's relatively inexpensive

- It uses readily available technology

- The software runs on an easy-to-use and widely available PIC processor

- It is within my intellectual capacity and construction skills

- I thought it would be a good place to start learning about the technology

- Many people are willing to help with problems and provide great advice and suggestions

- It shouldn't be too difficult to replace the Shera board with something else, if I find a better design 
that I can handle.

- It seems to work pretty well (I understand that lots of them are in service in amateur shops around the 
world.)

- Mine meets my need for a stable, accurate master oscillator for my shop.

I spent quite a bit of time researching GPSDO designs for the amateur electronic experimenter before building 
it. If I had found equivalent design and construction documentation and depth of expertise in some other 
design, I would have happily built that. Frankly, as it sounds like there are much better ways to do it, I'm 
surprised one of them hasn't caught on and sparked the same sustained interest as the Shera project - at 
enough to have made it to my short list of potential project designs.

So, although I can't speak for the rest of the people, I suspect many of them may be people like me that are 
learning about GPSDOs and having some fun along the way.

Cheers,

Mark - W8XR

 

 

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Message: 10

Date: Fri, 15 Dec 2006 13:47:01 +0100

From: "Ulrich Bangert" <df6jb at ulrich-bangert.de>

Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS locking

circuit

To: "'Discussion of precise time and frequency measurement'"

<time-nuts at febo.com>

Message-ID: <000001c72047$1f3ae9f0$03b2fea9 at athlon>

Content-Type: text/plain; charset="iso-8859-1"

Tom,

i believe that Bruce as well as me is always referring to what the receiver CAN do i.e. not the raw but 
always the sawtooth corrected signal. That is indeed 2 ns (1 sigma).

> Don't mislead yourself. At 1 s you are limited by GPS 1PPS noise. 

> Having a better TIC doesn't fix this. If your GPS noise is 2e-9 at 1 s

> you don't really need a TIC that is good to 5e-10 at 1 s. So the gain

> isn't as useful as you might think.

Thank you for clarifying this again! While i have been referring to the measurement apparatus's noise floor 
for which my statements are correct, one might indeed get into believing that every increase in resolution 
leads to a increase in performance in a GPSDO. Clearly once that you are below a certain point the GPS's 
jitter is the limiting number. 

I second Bruces's opinion about what is an overshot or not. When ps reolution is ready available then why not 
use it? I attach a online output from my DIY GPSDO from a few minutes ago that shows the M12+'s signal 
properties when measured with abt. 110 ps resolution against a FTS1200. The yellow line reperesents a 
prefiltered version of the sawtooth corrected values (blue). The filter time constant is 1/3 of the loop time 
constant as in a PRS-10. The yellow values are the ones to feed the regulation loop.

What I wanted to explain is the Shera concept noise floor is a factor 20 above what a modern receiver can 
deliver (again inc. the sawtoth correction). And yes, you are right: There were different numbers when this 
concept was thought out! And exactly because different number were there when this concept was thougt out I 
am going to ask why people still built it today.

Best regards

Ulrich Bangert, DF6JB