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

Robert Benward rbenward at verizon.net
Sat Jun 19 17:44:05 EDT 2010


Hi Pete,
I calculated about 0.5uV/V for 10fs, so we are on the same page.  I will 
need to read this several times for it to sink in, but I much appreciate the 
calculations and technical reply!

Thanks!

Bob


  ----- Original Message ----- 
  From: Pete Rawson
  To: Discussion of precise time and frequency measurement
  Sent: Saturday, June 19, 2010 5:08 PM
  Subject: Re: [time-nuts] Advantages & Disadvantages of the TPLL Method


  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


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