[time-nuts] Adjusting HP 5065A frequency

EWKehren at aol.com EWKehren at aol.com
Mon Oct 22 18:38:32 EDT 2012


Magnus
Are you changing or did you change and if you are how much time do you  
have.
Bert
 
 
In a message dated 10/22/2012 6:24:06 P.M. Eastern Daylight Time,  
magnus at rubidium.dyndns.org writes:

Bert,

On 10/22/2012 04:46 PM, EWKehren at aol.com  wrote:
> Tom
> A good place to start is with a manual of the  Austron 2055 delay if there
> is real interest. Does any one have it in  PDF form?
> As to the disciplining a Rb or specifically a HP5065A, the  filter part is
> the challenge. Having worked on it for the last ten  years always using
> Brooks  loop and developing work around's it  will do every thing except
> temperature  control and barometric  pressure. I have done temperature 
analog but rely
> mainly  on  holding the Rb temperature stable within a tenth degree C.
> Bob  recently put things in perspective when there was talk about a 32 bit
>  DAC. On one side you have GPS and time averaging is required to get full 
 use
> of  it. That is why a Rb is uniquely qualified for it. The  original Shera
> input  implemented on a 1 $ G/A is more than  enough for time capture 
wether
> using a  sawtooth corrected GPS  receiver or a Tbolt.  On the other end a
> LTC1655 is  again  more than enough in an Rb application. You can use it 
16 bit
> direct  or  dither two or 4 bits. Filter is  easy and do not forget that  
the
> Rb has an  additional filter between cell and OCXO. Brooks  V402NE will 
do the
> job, how ever  others and I have not been able  to buy some in the last 
nine
> month. Any one  knows what is going  on?
> What is needed is some one in the group be willing and able to  step up to
> the plate and develop the filter on a PIC with all the  things learned, 
and
> believe me, we are still learning as we adapt a  M100/8600 via a Tbolt .
> I will be willing to help with every thing  except the PIC.
> If you bring out the C field coil directly you have  full isolation and do
> not have to worry about ground loops.
> A  complete unit would cost less than $ 40 and more important be  
assembled
> by any one. I brought it up before but no one responded. If  no response I
> will  still be able to do every thing with the  HP5065A RVFR that was 
given to
> me.

The needed loop filter isn't  all that hard to achieve. Do read the 
Stanford Research PRS-10 manual. The  core PI-filter can be formulated as:

VI = VI + Vd * I
VF = VI + Vd *  P

Where Vd is the phase detector value (may be in number of nanoseconds  or 
whatever), VF is the output frequency control (EFC) value. The I and P  
values is the control parameters and VI is the integrator  state.

Adding a pre-filter for Vd values can be done as in the  PRS-10:

Vd = Vd + (Vi - Vd)* F

where Vi is the raw input and F  is filter control parameter. F can be 
set in power of 2 steps without too  much loss of control, allowing for 
shift-steps, which is what the PRS-10  uses. The PRS-10 runs it all in a 
whopping 6805 if I recall things  correctly, just your off the shelf 8 
bit processor that felt right in the  moment.

You can then allow for some front-end processing to cook up the  I and P 
values from more user-friendly parameters if you  like.

Tossing in a FLL functionality on top of it for improved lock-in  
performance isn't hard either.

There is a few scaling issues and  most things is about getting a stable 
timing and such, but it's not all  that hard. If you want to do PPS there 
is a little more attention to  lock-in details naturally.

Cooking up an open "lock-it-all" isn't that  hard thing to accomplish 
with a bit of knowledge and experience. One might  even have a bit of fun 
and cook up a Kalman filter, which is essentially a  self-tuning PI 
regulator.

PS. Sorry for not making you aware in  advance that I was changing planes 
in Miami this Friday. It was a busy  week in  Atlanta.

Cheers,
Magnus

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