[time-nuts] Advantages & Disadvantages of the TPLL Method

[Pete Rawson]

Bob,

I might shed some light on this topic, but my comments apply only
to a few measurements made in my shop, not part of any PLL scheme.
My intent was to determine the apparent noise floor of a Minicircuits
SYPD-2 phase detector; a low offset, low conversion loss, DBM.
The SYPD-2 IF port was terminated in 600 ohms in parallel with
0.1uF.

The SYPD-2 was first characterized for DC output sensitivity with
10MHz @+7dBm on RF  & 10.0001MHz @+7dBm on LO. At this
drive level, the IF output is triangular & the "gain" was found to be
800 mV/Rad at 0VDC. I did not perform a tolerance analysis on this
measurement & can only estimate the error at +/- 40mV/Rad.

The 90 deg offset of the SYPD-2 was measured at < 500uV using
+7dBm input power to both RF & LO ports. This measurement was
performed to monitor the 90 deg offset DC for any obvious instability
which might make lower level readings useless; the offset was stable
to +/- 2uV over several minutes.

Finally, the mixer was driven with +7dBm at both ports, but the LO
port signal was delayed by a 90 deg hybrid splitter + a 500ps, DC
to 2GHz, adjustable phase shifter. This configuration allows very
fine (5K part settable resolution) adjustment to reach 0VDC output
from the SYPD-2. The DC output stability was +/- 4uV/ minute after
allowing an hour to reach thermal & mechanical equilibrium
(& having no room temperature transients from my heating system).

If the phase sensitivity can be assumed to scale down linearly,
then 4uV resolution = 5uRad (80fs) @ 10MHz. With more care,
or better equipment, <10fs resolution might be possible; but it
will be difficult. Without knowing the noise type involved, it's
unclear what benefit might be provided by averaging. More
work needs to be done.

Pete Rawson