[time-nuts] Controlling FEI 5680A

EWKehren at aol.com EWKehren at aol.com
Sun Jan 15 05:11:11 EST 2012

I am staying out of that discussion due to lack of knowledge, My question  
is wether the input circuit is acceptable or if some one has a different  
solution. We have integrated the Shera input including the interrupt counter 
on  the chip, so there are only three interface pins, interrupt,  data out 
and  clock from the PIC to transfer the data. The interrupt count is pin 
selectable,  just like the 5/10 MHz divide. We are presently looking at 
increasing the  counter from 16 to 20 or 24 bits.
I think before going forward there has to be agreement on the input circuit 
 first, because it will influence every thing else. There may be other 
better  solutions, the reason I picked this one is it is simple, low cost, very 
few  parts, solderable and it works.
Bert Kehren
In a message dated 1/14/2012 10:14:26 P.M. Eastern Standard Time,  
magnus at rubidium.dyndns.org writes:

On  01/14/2012 01:32 PM, EWKehren at aol.com wrote:
> I have no expertise when  it comes to filter design or programming PIC's 
> other micro  controllers. But I know what works for me. For 11 years I 
>  been  using Shera controllers with very good results. (I still have some 
> assembled  extra A&A boards, if any one is interested,  please contact me
> off list)

Designing a PI-regulator in digital  is pretty simple and works well.

The core routine that needs to be run  at the steady sample rate is this:

Ph = getPD();
FI = FI +  I*Ph;
F = FI + P*Ph;

where I and P gives the  steering properties of the PI regulator.

There is a few things to  consider, such as the scaling and width.

An implementational benefit of  the above is that the integrator steering 
is done prior to the integrator,  which makes the integrator state FI 
have static dynamics in relationship  to the steering parameter I, which 
is practical as change of I (which is  typically useful to change 
bandwidth) will not require rescaling of FI to  maintain the same frequency.

The relationship between P and I sets the  damping factor of the loop.

The loop bandwidth changes with the square  root of I.

It's not too hard to use a quick track-in mode with higher  bandwidth and 
then scale it to slower mode.

To achieve a quicker  track-in of far-distance, diffrentiating the phase 
over time can be done,  and then feed the integrator loop the scaled 
difference. That way will the  frequency difference measured (complete 
with phase-wraps) steer the  frequency state of the integrator and once 
the FLL is well tracked in the  phase tracking just takes over. The FLL 
part can then be removed to reduce  disturbances.


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