> in my opinion, you would probably introduce some thermal noise through this > 50KOhm equivalent resistor while the PLL is not operating. Noise could maybe > be reduced if there is a cap to ground on this divider (this cap being part > of > the loop filter).
As long as you don't think it will affect stability when in lock, then I am ok. > The PLL bandwidth depends solely on the phase noise of your 10MHz source > versus your 100MHz oscillator. If the 10MHz source is better at say 100Hz > offset > (better by more than 20dB) then the loop bandwidth should be more than > 100Hz, so that the PLL can actually reduce the phase noise of your 100MHz > oscillator. > > At 100Hz offset, you say you have -68dBc/Hz at 100MHz. This calculates to > -88dBc/Hz at 100Hz offset for the 10MHz source (excluding the PLL chip and > loop-filter noise). You may want to check the noise performance of the PLL on > the > ADI website's PLL simulator. > > So if you have much better than -88dBc/Hz at 100Hz on your 10MHz oscillator > (not hard to achieve, many oscillators have <-140dBc/Hz at 100Hz already) > then you would be wasting performance with a <100Hz loop filter, and you may > want to do a 1KHz or even wider loop filter or so. But if you don't know the > 10MHz source's performance, it is probably best to be safe and use 10Hz, or > 100Hz loop filter BW. > > A good spectrum analyzer (such as HP 8560B/E etc with the phase-noise > software option) should allow you to measure <-68dBc/Hz noise at 100Hz offset > at > 100MHz, so you can check what BW results in the overall lowest noise. > > In short, if you want to maximize your systems performance, then loop > bandwidth depends on the performance of the 10MHz versus the 100MHz > oscillator for > close-in phase noise. > > To do the math, subtract 20log(100/10) = -20dB from the noise of the 100MHz > oscillator to get the equivalent noise energy for the 10MHz oscillator (at the > same frequency offset). > > Hope this makes sense, Yes, I understand all the math. I just don't know what to expect from the 10 MHz references people will plug in. Clearly if they plug in a Fury, I should use a wide bandwidth, but who knows what else they'll use... Thanks, Matt _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
