[time-nuts] Building a GPSDO & trouble using Jupiter-T

EWKehren at aol.com EWKehren at aol.com
Mon Jan 30 13:34:58 EST 2012

Using a $ 2 G/A  (socket included) and a $ 2 PIC, four will fit on a  mini 
board and there are 3 boards to the order that is $ 5.17 per board.   Board 
is laid out, waiting for uP pin assignment and will be able to deliver in  a 
week board and G/A.
Bert Kehren
In a message dated 1/30/2012 1:00:04 P.M. Eastern Standard Time,  
albertson.chris at gmail.com writes:

On Mon,  Jan 30, 2012 at 5:16 AM, Bob Camp <lists at rtty.us> wrote:
>  Hi
> The GPS ops signal (or 10 KHz signal)  you will be  trying to track will 
be moving by 10's or 100's of ns per second. 10 ns per  second is 10 ppb. 10 
ppb at 10 GHz is 100 Hz. You need to smooth this out or  your LO will be 
moving all over the place.

Up at the top of this thread  I said you need to have a long loop
constant for exactly this  reason.   It does not matter of the GPS has
a 1PPS 100PPS or  1000PPS the problem is GPS itself.   GPS simply does
not have  good short term stability.   In the short term the OCXO will
be  better.

The reason we build GPSDOs is to combine the best of both, the  short
term characteristics of an OCXO and the long term characteristics  of
GPS.     If you try and build a GPSDO using a very short  loop filter
time constant then you have a GPSDO with same   characteristics of GPS
averaged over your short time constant.

In  other words you select the time constant based of your frequency
stability  requirements.

Look at the first blue line in the top graph.  This  is very much what
a Jupiter GPS has.   If you need 10E-10 you  need a 100 second constant
.   The line is straight to for 10x  better accuracy you need 10x
longer time constant.

I think using an  analog PLL you can get to about the 1E-10 level.  I
think a non-expert  could build a good 100 second RC filter.

Doing better is going to  require a longer time constant then is
reasonable.   a uP based  PLL can do much better and likely with effort
equal the performance of a  Trimble Thuderbolt.     But then you can
simply buy a  Thunderbolt  for $150.

I'm just starting work on a PLL that (I  hope) can be built using just
an Aduino as a controller and an 4046 chip  (or the 74hct9046) as the
phase detector.  The Arduino costs more than  a bare uP chip but I
don't want to have to build a PCB.

Chris  Albertson
Redondo Beach,  California

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