Good point, and an example of how good digital filtering (helped with upsampling) can make the design of the analog filter much easier :)
Reference the digital audio battles of the past century when 1 bit D/As running very fast started replacing the expensive 16 bit audio DACs running at 44kHz. Didier On August 17, 2016 5:25:39 PM CDT, Magnus Danielson <mag...@rubidium.dyndns.org> wrote: >Hi, > >I agree. > >There is however a subtle detail, how they leak out over time. > >At one time we had to lock an 155,52 MHz oscillator up to 8 kHz, this >for a 2,48832 Gb/s link, which needs to pass the SDH STM-16 jitter and >wander specifications. The first attempt at that PLL was using a 4046, >and the charge-pump was being used. The charge-pump has dead-time, and >well, they thought it was good to only push the EFC here and there. >What >this meant was that they created a triangle-waved frequency modulation >of low rate, which then created phase modulations as it went through >the >integration of the oscillator. The scale-up factor made this quite >noticeable at the actual bit-rate. It made the point that you need to >update often to keep deviations limited, and when doing it at a higher >frequency, they are easier to filter out. > >In essence, you need to think what each comparison or update creates as > >a step response and how it is averaged out over time. > >In this regard a PWM is a really bad signal, as it can push the >strongest amplitude at the lowest frequency, which becomes hardest to >filter. For one design I needed to increase the resolution, so I made >an >interpolation but with inversed spectral density to that of PWM, to >push >the highest amplitude to the highest frequency so that filtering >becomes >easier. Turned out to be quite easy and work well. > >High update rates can be very useful even if the bandwidth of the loop >is low. The bandwidth only limits how low the updaterate can be, but >the >phase-noise purity makes update rates and smoothing mechanisms >interesting. > >Cheers, >Magnus > >On 08/17/2016 11:53 PM, Bob kb8tq wrote: >> Hi >> >> You can update the EFC a billion times a second. Update rate and >bandwidth are not the same thing. If you want good ADEV, the loop >better not have a bandwidth greater than 0.01 Hz. GPS ADEV is pretty >awful at 1 and 10 seconds. It is starts to be good past a few thousand >seconds. Yes, older modules are a bit worse than newer ones. Also >sawtooth correction can make things a bit better. >> >> Bob >> >> Sent from my iPad >> >>> On Aug 17, 2016, at 2:51 PM, Nick Sayer via time-nuts ><time-nuts@febo.com> wrote: >>> >>> Updating the EFC more quickly reduces the ADEV, though. I find that >the fiddly part of tuning a GPSDO design is balancing the ADEV against >phase control. If you want keep an iron fist on the phase, you can only >do so by constantly swatting around the frequency. >>> >>> I won't say that getting more frequent phase feedback is a bad >thing, but if you're trying to get the PLL time constant to be longer >rather than shorter that it won't help a lot. >>> >>> Sent from my iPhone >>> >>>> On Aug 17, 2016, at 9:57 AM, Peter Reilley ><preilley_...@comcast.net> wrote: >>>> >>>> You can get crystal oscillators that have a frequency control >signal and are more >>>> stable than the run of the mill oscillators. Changing the GPS >oscillator would >>>> require modifying a very tightly populated circuit board. Perhaps >not possible. >>>> >>>> What about some of the SDR (software defined radio) projects that >aim to >>>> implement GPS functionality? If you used the GPS chipping rate >(1.023 MHz) >>>> to dicipline the 10 MHz oscillator then you are less sensitive to >crystal instabilities. >>>> You are updating the crystal one million times a second rather than >once per second. >>>> This is assuming that the chipping rate of the transmitter is just >as good as the >>>> 1 PPS signal. This info from here; >>>> https://www.e-education.psu.edu/geog862/node/1753 >>>> and here; >>>> https://en.wikipedia.org/wiki/GPS_signals >>>> >>>> Even using the 50 bits/sec data rate of the GPS signal would allow >updating the >>>> GPSDO faster than the 1 PPS signal. >>>> >>>> Pete. >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >_______________________________________________ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. -- Sent from my Moto-X wireless tracker while I do other things. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.