[time-nuts] yet another GPSDO design, or so

EWKehren at aol.com EWKehren at aol.com
Sun Jun 27 13:58:55 UTC 2010

 the faster counter also has the jitter, no change, as long as it is  not 
tied to the input frequency. The 24 MHz is not unique, the 100 MHz is same  
technology just four times faster and thus gives me smaller steps on the D/A 
and  since I use it on Rub. the full range of the 18 bit covers the full 
tuning range  of the Rub.
In a message dated 6/27/2010 9:05:12 A.M. Eastern Daylight Time,  
stanley_reynolds at yahoo.com writes:

I have  been thinking about a faster counter also but the Shera board was 
depending on  the jitter in the 24 Mhz clock to average out the +- count. The 
faster  clock would reduce the need for this but without the right amount 
of jitter we  lose the benefit of this average.


----- Original  Message ----
From: "EWKehren at aol.com" <EWKehren at aol.com>
To:  time-nuts at febo.com
Sent: Sat, June 26, 2010 8:14:02 PM
Subject: Re:  [time-nuts] yet another GPSDO design, or so

What you want is  basically a Shera Board. That design has been around for  
quite some  time and has served me very well. I have a total of six running 
including two controlling Rubidium. There are in my opinion a couple  of  
problems: not every 4066 works on the design the 18 bit D/A is  very hard 
find  and now expensive and the single step of the D/A  is intended for a 
E-13  frequency step. I have build a input  section that counts 100 MHz in 
stead  of  24 MHZ making the unit  create steps of 4.3 E-14 which works 
better on 
my Rubidium's and Datum FTS  1000.  Also it eliminates the 4066's. Since I  
do not know how  to write code that was my solution. I have also designed a 
later   version Shera, with less IC's and a low cost dual D/A but I do not 
have  the  programming skill. 
If you contact me directly I will send you a  copy of the QST Shera 
my design and the D/A data sheet.. I am  sure you can replace the PIC with 
an  Atmel device.

In a  message dated 6/26/2010 1:16:09 P.M. Eastern Daylight Time,   
attila at kinali.ch writes:


I recently had a look at the  data sheet of the LEA6-T  GPS module
from ublox, which now features a  second time pulse output  that
is capable of delivering a 10MHz  signal, synchronized to  GPS.

After thinking quite some time quite  some time about  building
my own GPSDO and struggling with the  question how to  synchronize
a 10MHz signal to a 1Hz signal that has  some substantial  phase
noise, the new LEA6-T module seems like to  make things a  lot
easier. Although the LEA6 specs do not say anything  about how
the  timepulse output is generated or how it is  synchronized
to GPS, i assume  that it will either have some jumps or  phase/frequency
noise due to  oszillator and synchronization  imperfections.

But, it should be  possible to use the LEA6-T  together with
some OCXO and a PLL setup to  stabilize the OCXO to get  a high
quality frequency  standard.

Unfortunately, my knowledge  in that field is rather limited,  thus
before starting to make wrong  design decisions i'd like to ask
for  some advice here.

My  basic idea is to feed the 10MHz output of the  LEA6-T and
the 10MHz  OCXO into a current output PFD, do some  low-order
filtering of the  output signal. Feed that into an ADC which
is  read by a uC which in  turn controls an DAC that sets over
some amplifier  stage the EFC  input of the OCXO.

As PFD i thought about using a ADF4002  from  Analog, which
is actually an PLL, but allows to bypass the input   dividers,
so that it can be used as pure current output PFD.

I'm  not  yet sure what kind of output filter i want to use.
I probably  have to add  at least one low noise opamp there,
to isolate the PFD  output/filter from  the ADC. I'm also
not sure what filter frequency i  should use here. It  will
have to be below 10MHz for sure, probably in  the lower 
kHz range,  but how low is the question. The lower the  easier
gets the ADC stage and  the less work has to be done in the  uC,
but using a low frequency filter  either means using an  active
filter (noise) or high value R or L (again  noise,  especially
the L might couple in 50Hz noise from the enviroment or   show
microphone effects).

The ADC will be either a low-noise  16bit  type or a 24bit
type. This will largely depend on the sample  rate to  be
used and the availabilty of the ADCs. Any good  advices
on what to use  here? Should there be some form of  signal
conditioning done? If, what form  of conditioning would
you  advise me to use?

As a uC i thought about  using a AT91SAM7  variant from Atmel.
I know these beasts (and their bugs)  pretty well  by now
and already have some code ready for those.
I thought  about  clocking the uC with a 40MHz crystal that
is synchronized to the   10MHz OCXO using a PLL. This would
allow me to generate quite   precise+accurate digital signals.
Unfortunately, there doesnt seem to  be  VCXOs at 40MHz available
so that means that i'd have to build one  by  hand.

The loopfilter is going to end up in the uC as it is  easier
to  build such low frequency filters digitally than in  analog.
I havent put  much thought into how that filter should  look
like, as this can be easily  changed later.

The DAC will  probably be a 16bit type (there does not  seem
any higher resolution  DAC with sane specs and still  reasonable
availability). The amplifier  for the DAC output will be a  two
stage amplifier. One stage that adds  an (adjustable) offset
and one  stage that adds the (again adjustable)  amplification.
This approach is  choosen as the needed EFC range will  probably
much lower than the full  range. Hence the resolution of  the
DAC can be enhanced by producing only  values within that  range.
The disadvantage here is that it requires   calibration.

A rough guestimate is that the whole thing will  probably  cost
less than 500CHF (including PCB production, but  excluding  OCXO).
Yes, i know, i could get a Rb frequency standard for  that  money
on ebay. But where is the fun in that? ;-)

Beside  whether this  setup makes sense, the two biggest questions
i have are,  what OCXO to use.  Are the ISOTEMP 134-10 that are
available on ebay  "good enough" for such an  application?
Or shall i look for something  better/different?

And the  other is, how do i amplify and  distribute the 10MHz
signal i get out of the  OCXO to be used by other  devices
with minimal phase  noise?

Thanks for your  help

Attila Kinali

If you want to walk fast, walk   alone.
If you want to walk far, walk together.
-- African   proverb

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