====== Minimizing Noise Bandwidth in Closed Loop Systems ====== Subject: GPSDO time constant >> >>> Magnus Danielson wrote: >>> >>> For ThunderBolt owners it is pretty straightforward to separately adjust >>> the Time Constant and damping, which is very nice. Use this opportunity! >>> >> >> Richard Moore wrote: >> >> So, Magnus (and Tom), what damping factor do you suggest for a TBolt? >> I'm running a verrry long TC now. If 1.2 is not actually critically >> damped, what value would be? Any guesses? BTW, I really like that >> plot of Tom's that tracks the oven and then gets better from the GPS... >> > > Magnus Danielson wrote: > > Dick, > > Assuming that damping factors match classical analysis of damping, then > the square root of 2 is the answer... 1.414 or there about. > > I would be more conservative than that. I would consider damping factors > such as 3-4 or so. I have no support from measurements on GPSDOs but it > is reasonable that the rise of gain at and near the PLL frequency we see > for other systems will occur and result in similar effects even here. > This gain raises the noise floor and amount of gain is directly coupled > to the damping factor. It's just standard PLL stuff all over again. The > only difference is that we view the result in ADEV or MDEV views. > > Cheers, > Magnus > Hej Magnus For a second order loop, the noise bandwidth is minimised for a fixed time constant by choosing a damping factor of 0.5. Using a damping factor of 1.414 increases the noise bandwidth by about 60%. Using a damping factor of 0.7071 only increases the loop noise bandwidth by about 6%. A damping factor of 0.3 increases the noise bandwidth by about 13%. Bruce