[time-nuts] Adjusting HP 5065A frequency

Magnus Danielson magnus at rubidium.dyndns.org
Mon Oct 22 18:08:06 EDT 2012


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 

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.


More information about the time-nuts mailing list