[time-nuts] GPSDO design

[Bruce Griffiths]

Just how significant is opamp flicker noise?

If the opamp ficker noise can be modeled as

en = k/SQRT(f)

for a typical JFET opamp K ~ 100nV/rtHz

Then the noise of interest is

~ k*SQRT(ln(f2/f1))

where
f1 is the PLL noise bandwidth
f2 is the noise bandwidth of the low pass filter at the EFC input

for
f1 = 1uHz
f2 = 1Hz

relevant noise due to opamp flicker noise ~ 0.4uV rms

which produces  fractional fm modulation of 4E-15 at the output of a 10811A.

In practice an analog PLL with a noise bandwidth as low as 1uHz is not 
likely and the flicker noise contribution will be a little lower than this.

This isn't significant for most 10811A's.

Thus the most significant effects are thermal fluctuations of the opamp 
temperature causing drift and the opamp bias current changes.

Bruce