[time-nuts] Sub Pico Second Phase logger

[Pete]

Joe,

I have been following this thread from the start & learned a lot from it.

Thanks to you and Bruce for the insights.

I thought I would take a look at a simple, passive approach, just to
see what happens. I also limited my attention to stable 5 or 10 MHz
sources & observation windows of <100s. I used a Mini-circuits
SYPD-1 PD with a PSCQ-2-10.5 90 deg hybrid plus an ATM
P1506 phase shifter (delta=0.32ns). There are lots of other items
which will work just as well, this stuff happens to be on hand.

The SYPD-1 also needs a diplexer to terminate the high order
products & low pass filter the desired output; this is a cobble
together item from the parts box. I used a PC logging program
with my HP 3478A to take DC readings from the diplexer output.

Thus armed, I first checked the DVM logging with no RF in to see
if it was stable enough to yield a useable noise floor. This first result
was good enough. Reading 720 points over 100 seconds; the data
was limited to -1, 0 or +1 uV.

Then I applied 10MHz @ to the 90 deg hybrid & connected one
hybrid output to the SYPD-1 LO & the other hybrid output to the
ATM P1506. The ATM output then drives the SYPD-1 RF port.
I measured both SYPD-1 inputs to be +6.4dBm. Now I was able
to set the ATM phase to zero the SYPD-1 output & verify it was
stable over the 100s time I was interested in. The 3478A readings
ranged from -1 to +2uV during this time with STDDEV of 0.6uV 
-again, good enough for me.

Finally, I used a HP 105B to get a quiet 5MHz source & a 1MHz
ext. reference for my HP3336C sythesizer. All this to give me 2
"nearly" identical. phase locked sources to measure the calibration
factor of the SYPD-1/Diplexer output. The 3336C was set to
5,000,000.005Hz & +6.4dBm driving the SYPD-1 LO port. The
105B 5MHz output was attenuated to +6.4dBm driving the RF
port. The resulting 5mHz data was logged & analyzed at the zero
crossings (max gain). The result is a phase sensitivity of 1.22uRad/uV.
The + & - zero crossings were identical and the 1.22uRad/uV factor
is linear (+/- 2 %) up to +/- 20 % of FS. FS = 1.023V

If my math is about right, @10MHz, 1uRad = 16fs; so a noise floor
of +/- 2uV (+/-2.5uRad) equals +/-40fs. I expect that, if 2 sources
are near zero phase, then it should only require <10s to estimate
the actual phase error within 1ps, or less. Anyone agree?

I await & hope for constructive comments and/or corrections.

Pete Rawson