Hi all, I measured an Ettus OctoClock with a 3120A phase-noise probe (for the 10MHz) and the 1PPS channel skew with a 53230A TIC. The results are decent but not stellar: http://www.anderswallin.net/2016/09/ettus-octoclock-distribution-amplifier/ In particular the AM noise on the 10MHz looks high. Any thoughts/comments? For 1PPS a 200ps skew between fastest and slowest channel is not the end of the world, but I would be interested in the cause. To produce 200ps skew both a 4cm trace-length-difference and/or 200 mV of DC-offset (for a 1V/ns slewrate signal) seem large? Anders (will build better distribution amp as soon as I getroundtoit..)

		Attila Kinali

Hoi Anders, On Mon, 12 Sep 2016 17:54:57 +0300 Anders Wallin <anders.e.e.wallin@gmail.com> wrote: > The results are decent but not stellar: > http://www.anderswallin.net/2016/09/ettus-octoclock-distribution-amplifier/ > > In particular the AM noise on the 10MHz looks high. Any thoughts/comments? I am not surprised. They used a digital clock distribution chip. The chip is not designed for lowest phase noise, neither is the surrounding electronics. And selling that whole thing, which is nothing more than the example circuit in the datasheet, for 1700 is a bit of a rip-off, IMHO. IIRC your design beats theirs already in performance, doesn't it? > For 1PPS a 200ps skew between fastest and slowest channel is not the end of > the world, but I would be interested in the cause. To produce 200ps skew > both a 4cm trace-length-difference and/or 200 mV of DC-offset (for a 1V/ns > slewrate signal) seem large? Not really. They use some unnamed 74x04 instead of a specialized clock distribution chip (like e.g. LMK00101). They have split the PPS distribution into two parts, which are symmetric. You can see that clearly from your measurements. My guess would be that the track lengths on different layers are not equal for all paths, which introduces quite a bit of skew (thanks to \epsilon_r changes). That only one port is so much off probably stems from the need of going around the chip in the hex-inverter package. Attila Kinali -- Malek's Law: Any simple idea will be worded in the most complicated way.

> > > The results are decent but not stellar: > > http://www.anderswallin.net/2016/09/ettus-octoclock- > distribution-amplifier/ > > In particular the AM noise on the 10MHz looks high. Any > thoughts/comments? > > I am not surprised. They used a digital clock distribution chip. The chip > is not designed for lowest phase noise, neither is the surrounding > electronics. > And selling that whole thing, which is nothing more than the example > circuit > in the datasheet, for 1700 is a bit of a rip-off, IMHO. > In fairness my link was to the version also containing a GPSDO. The price I see on the web for only the distribution amplifier is 960 eur. Timetech and/or Microsemi are probably better - at a higher cost. > IIRC your design beats theirs already in performance, doesn't it? > Yes, my prototype of the TADD-1 design with an AD8055 op-amp, powered from a +12VDC battery, and wrapped in alu-foil to shield, looked quite good: http://www.anderswallin.net/wp-content/uploads/2015/12/alufoil_and_battery.png The 6502 is better and I opened it up to find it has an LMH6702 input stage and LMH6609 output stages. I will try that at some point. Anders