Hi In a normal PLL the discrete components on the PCB play a big factor in how the loop behaves. There are some chips that allow the current out of the detector to be varied a bit, even on those the discrete R’s and C’s are still the “big deal” in how the loop behaves. There will be a significant impact if the loop output is 1/3 of what the design was intended to use. In general, that impact will not be a good one.
Bob > On Sep 13, 2019, at 7:01 PM, Forrest Christian (List Account) > <[email protected]> wrote: > > I suspect someone figured out some of the side effects what you're > describing and adjusted the divisors to better match across all rows. I > just wish SRS would have updated the manual if it was them. > > In my device, the R,N,A values were 6114,3456,29. The R,N,A values in the > table in the docs on row 52 are 2038,1145,31. For the discussion below, > I'm just going to focus on R since the others are effectively related. > > The R values 'around' row 52 are 6400,4219,6257,2038,5971,3933 and 5828 > (rows 49 through 55). It seems logical to me to use 6114 as the R > divisor for row 52 instead of 2038 because the resulting frequency is > closer to the rest of the values in the table. The highest value I can > quickly see is 7258 and the lowest is 1180. That is quite a range in > output frequencies to deal with. If you take the lower values and > multiply them by a integer multiple so that everything is within a narrower > range then I'd expect it would be easier to deal with the side effects of a > now more consistent divided frequency while still permitting the finer > resolution afforded by a larger divider value. If this is the case, I'd > also expect the 1180 'lowest value' to have been multiplied by either 4 or > 5 instead of just 3, and the other lower values in the table to have been > multiplied by an appropriate range to end up closer to 6000. > > Just to add another piece to the big-picture puzzle, the following is from > the manual: > > "The gain of U400’s phase detector may be set (coarsely) by the CPU, and it > is adjusted to maintain roughly the same PLL damping factor as divisors are > changed. This loop has a very low natural frequency (about 10 r/s) and a > damping factor which ranges from 0.84 to 1.19." > > U400 is the same MC145190 which is doing the division. So it sounds like > they're compensating for some of the effects by configuring the phase > detector differently depending on the divisor values. > > > On Fri, Sep 13, 2019 at 11:00 AM Bob kb8tq <[email protected]> wrote: > >> Hi >> >> Ummm ….. errrr …… >> >> The divisors run down to an output port. There has to be a filter at that >> port to >> knock down the noise and make the loop close properly. When you multiply >> the >> divisors by three, you cut the frequency at that port by three. You also >> have a >> significant impact on the control loop….. >> >> Probably a good idea to make sure your board has the “normal” components on >> it that correspond to the numbers in the manual. There may have been a >> running >> change at some point that is not reflected in the doc’s. >> >> Bob >> >>> On Sep 13, 2019, at 12:12 AM, Forrest Christian (List Account) < >> [email protected]> wrote: >>> >>> Mainly wanting to post this to the list so it will end up in the >> archives. >>> >>> So I decided to give calibration/adjustment of my bench PRS-10 a go. >>> >>> What I discovered is that on my particular unit, the frequency was enough >>> off that in order to bring it close to spec, I had to adjust the Mag >> Offset >>> to the lower end of it's range (2300), and even then the set frequency >> was >>> lower than I would have liked. >>> >>> According to the section of the manual under the SP command, if the Mag >>> Offset is at the end of it's range, you can change the frequency >>> synthesizer's parameters by querying the existing SP? settings, finding >> the >>> row in Appendix A which corresponds to those values, then changing the SP >>> value up or down a step (by using the values in the table row just above >> or >>> below the value). >>> >>> In my unit, the SP value set (6114,3436,29) were not anywhere to be found >>> in Appendix A. >>> >>> After some digging, and reading the manual, I discovered that these >> values >>> are used to configure the MC145193 inside the PRS-10. Specifically the >>> first value (R) is used to divide the 10Mhz output. The second two >> values >>> (N, A) are used to divide 359.72Mhz (which is related to the Rb >>> frequency). This second divisor is calculated by (N*64+A). The >> resulting >>> two divided down signals will be very close in frequency, and the >>> difference is used to stabilize the oscillator. >>> >>> After some more work, I discovered that the divisor values currently in >> my >>> oscillator were actually exactly 3 times the value of row 52, that is >>> 6114/3=2038 and (3436*64+29)/3=(2038*64+31). Since it's only the ratio >>> between the divisors which matter, I'm assuming someone at some point >>> decided to use the higher division ratio for some reason. Not sure if >> this >>> was at SRS or in the field. >>> >>> After discovering this, I followed the procedure to move the SP values by >>> one row in the table, and everything seems to have re-centered itself. >>> >>> Hope this helps someone... Even if it's me in the future if I have to do >>> this again. >>> >>> -- >>> - Forrest >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > > > -- > - Forrest > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
