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

Robert LaJeunesse rlajeunesse at sbcglobal.net
Wed Oct 20 14:14:13 UTC 2010

Assuming the attached DSPStore file gets through one can see it can be very easy 
to implement a 2nd order IIR filter. Two delays, two multiplies, one summation. 
Simple scaling may be required to prevent overflow, and roundoff errors need to 
be considered. A more detailed reference is Hagemann's article at 

From: Achim Vollhardt <avollhar at physik.uzh.ch>
To: time-nuts at febo.com
Sent: Wed, October 20, 2010 9:58:12 AM
Subject: [time-nuts] Some info on the ACAM TDC and a question to Brooks Shera's 
PI controller

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:


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..

Thank you,
Achim, DH2VA

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