[time-nuts] Some info on the ACAM TDC and a question to Brooks Shera's PI controller

Magnus Danielson magnus at rubidium.dyndns.org
Wed Oct 20 19:16:40 UTC 2010

Dear Achim,

On 10/20/2010 03:58 PM, Achim Vollhardt wrote:
> Dear Group,
> it might be of interest, that I have some measured data on the ACAM
> TDC-GP2 chip, which I am using for another project in our lab here.
> Single shot resolution of 54 ps rms was quickly achieved, which is close
> enough to the 50 ps quoted in the datasheet for me to believe it :)
> As I am willing to use this device in a GPS-locking device, I have of
> course set my eye on the PI controller from Brooks Shera. He is quoting
> using a second order IIR filter, but as a newbie in digital filters, I
> have problems to write suitable code for it..
> I did find however an easy implementation of an PI controller:
> http://www.atlantixeng.com/pdf-files/PID_Controller_in_FPGA.pdf
> I(k) = (E(k)+E(k-1))*Ki*Ts/2 + I(k-1)
> P(k) + Kp*E(k)
> P(k) and I(k) being the proportional and integral outputs at sample 'k',
> E(k) being the error input. Ts is the sampling rate, Kp and Ki are the
> gain factors for the two controller parts, which results in a time
> constant being Tn=Kp/Ki.
> Is this a suitable approach for a control loop (assuming proper choice
> of gains)?
> Is an IIR filter better suited? As far as I can tell, this is not IIR..

The PI-loop is a type of IIR filter, it is definitly not a FIR filter.

There is two feedback loops, one in the integrator (which just by itself 
is an IIR filter) and then the phase-feedback which also put is well in 
the IIR domain.


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