[time-nuts] Brooks Shera

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 25 07:09:15 EDT 2013

The lack of synchronisers when crossing clock domains is a design flaw 
that should be corrected.


EWKehren at aol.com wrote:
> Brooks GPSDO may be 15 years old but is still perfect for today's
> applications. If you look at tvb's Tbolt plot or Ulrich's plots with and without
> sawtooth correction for a day or two the limit is GPS. The basic unit has a
> resolution of 1.73 E-13 in mode 7. Brooks uses a 40 bit filter. I have
> increased  resolution using 100 MHz in stead of 24 and increasing filter time from
> 30 to 60  and 120 seconds. 240 is also an option, I would only recommend
> any thing above  30 seconds if you use a Rb. The only thing wrong is that
> chips that where  available 15 years ago are now hard to find. My answer is a $
> 1 gate array and  if some one will step up to the plate and modify the
> available ASM code to  generate the RS232 code to control a FE 5680A I will make
> a G/A design and a  board design available. While at it also look at putting
> it in a 18 pin PIC. If  interested contact me off list.
> With hundreds of 5680's out there many will appreciate a low cost simple
> solution. Working with the presently available ASM code now out there it will
> be  a very small effort needed to transfer it to the latest version 1.402.1
> when it  becomes available. It will take some one willing and able to pick
> up at the  point where the filter output drives the DAC and develops code
> that reads the  info from the 5680 and generates the correction code. The 5680
> does not control  a DAC but the DDS in the 6.8 GHz loop.
> As to the input circuit, having looked at many alternatives I still thing
> it is the best for a 1 pps input from GPS and its limitations. Hope to have
> some  data available of 1.402.1 driving a Morion in the next couple of days.
> I also have a couple of A&A boards with all original chips and some of  my
> 100 MHz boards with components. Please contact me off list if  interested.
> Bert Kehren Miami
> In a message dated 3/25/2013 2:30:36 A.M. Eastern Daylight Time,
> attila at kinali.ch writes:
> On Mon,  25 Mar 2013 00:43:02 -0400
> "Daniel Schultz"<n8fgv at usa.net>   wrote:
>> Another great ham passes on, I'm sorry I never had a chance  to meet him.
> Definitly a sad day....
>> Is the GPS controller  that Brooks published still useful today, or has
> it been
>> superseded by  something newer?
> That highly depends on what you want and what you  need.
> Shera's design is definitly one of the simplest GPSDOs out  there.
> And with that it defines what the lowest complexity to get  something
> accurate is. If the performance of this circuit is enough for  you,
> i wouldn't go for anything else. Of course, if you need better  performance
> or want to tinker and see what can be done with home made  equipment, then
> you should go for different circuits.
> Attila Kinali

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