10. Again referring to our ideal PID controller, our control variable u(t) in the case of most low-cost GPSDOs is the control voltage that we can apply to the OCXO to vary its frequency. For Lars' DIY GPSDO and for the STM32 GPSDO a 16-bit PWM DAC is used to generate a control voltage Vctl between 0 and 4V. This is an extremely economical solution but it does the job.
11. A complicating factor for programming the PLL or FLL algorithms for a GPSDO is that we are dealing with a discrete correction process, in other words changes in the control variable are applied at discrete time intervals which can be fixed or variable. For example, the STM32 GPSDO with its very crude FLL algorithm used a fixed interval of 429s to change Vctl (except during the initial calibration). In older GPSDO designs using a purely analog circuit the control variable is applied continuously. 12. Consequently the program of an FLL or PLL loop for a GPSDO has two decisions to make every second: a) what is the size of the correction to be applied to the control variable iow what is the new value for Vctl ? and b) Should the correction that was just computed be applied now, or should we wait and apply a different correction later ? 13. The programmer (in this case myself) has to decide whether to use a P, PI or PID loop, the optimal values for Kp, Ki and Kd, the use of a fixed or variable "time constant" (the delay between changes to the Vctl), and any processing (filetring, averaging, removal of outlying values, etc) of the measurements from the frequency counter or the TIC. 14. More precisely in my case, there is an extra complicating factor because I am trying to merge the FLL and PLL control loops into a "hybrid" FLL/PLL control loop. How to make the best use of the information from the two measurement ? _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
