[time-nuts] Sub Pico Second Phase logger

[Joe Gwinn]

Pete,

At 4:58 AM +0000 12/27/08, time-nuts-request at febo.com wrote:
>
>Message: 2
>Date: Fri, 26 Dec 2008 19:10:47 -0700
>From: "Pete" <peterawson at earthlink.net>
>Subject: Re: [time-nuts] Sub Pico Second Phase logger
>To: "Discussion of precise time and frequency measurement"
>	<time-nuts at febo.com>
>
>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.

What do you mean by "diplexer"?


>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.

I've done similar things, and can also get random errors in the 
femtoseconds, but only over short periods of time.  The key is 
temperature sensitivity.   I would be suspicious of the 90 degree 
hybrid in particular, although the phase detector also has an 
opinion. The splitter phase unbalance is also quite sensitive to 
drive frequency, so the frequency stable sources are essential.

Joe