[time-nuts] Practical considerations making a lab standard with an LTE lite

SAIDJACK at aol.com SAIDJACK at aol.com
Sun Nov 23 15:46:32 EST 2014

Hi guys,
this is the kind of lively discussion I was hoping for! I enjoyed  this.
Some comments (these are my opinions only):
* Thanks much for Tom publishing the plots, and spending a lot(!) of time  
evaluating and helping improve the units significantly. Tom's unit was a  
pre-production unit. We added RTV (some units black, some a combo) to the TCXO 
 production units based on his suggestion of the successful TP  
modification. The RTV will help keep airflow away, but additional shielding will  help 
even more.
* There is a point where thermally stabilizing the unit does not help  
anymore. I suspect that point is reached shortly before burying the unit 50  
meters underground :) At the point of diminishing marginal returns the GPS  and 
loop noise will be larger than the thermally induced phase offsets. Also  
local heating from the GPS receiver (which is not constant) will swamp 
external  thermal effects at some point. For us in our lab, the point of 
diminishing  returns is reached when we simply slide the unit into its ESD packaging, 
then  put some pink ESD padding on top of it. With that simple  shielding 
we can get ADEV at 5x to 8x its rated 1ppb performance out of most  units.
* Temperature changes are typically not the problem with TCXOs, simple  
airflow and convection turbulence is what causes most of the phase drift  
problems. As shown by Tom simply putting a layer of TP on top of the unit made a  
huge difference in stability by keeping convective flow away from the TCXO, 
 while it probably did nothing for temperature insulation. These convective 
flows  are very fast and high-frequency so inside the GPS loop time 
constant, whereas  temp changes are usually easy to low-pass dampen to the point 
that the  GPS loop will hide them.
* Actively heating the units' enclosure to some stable temperature is  
counter-productive in my opinion for two reasons: first higher temperatures  
cause convective airflow inside the enclosure. We want as little convective 
flow  as possible. Second CMOS slows down at higher temperatures, and noise 
levels go  up with temperature. As mentioned before temperature changes (other 
than instant  changes such as when the sun almost sudden hits the enclosure) 
usually are  easily low-pass filtered to be slower than the GPS loop time 
constant which is  below a couple 100 seconds, so keeping the enclosure at 
some high temperature is  probably going to make things worse. There are other 
items to consider such as  the AT-cut TCXO crystal probably has its most 
stable operating point at  around 25C, and the lifetime MTBF of electronics 
typically gets cut in  half with every 10C Degree increase in temperature.
* The 10MHz units have a different RF output than the 20MHz units. The  
20MHz units have a 50 Ohms series-terminated and buffered RF output, while the  
10MHz units have the TCXO output drive the MMCX connector directly without  
series impedance matching. Both drive the line with 3.0V CMOS levels. This 
means  the cable on the 10MHz unit should be kept as short as possible, and 
that  impedance matching for maximum power-transfer is not required nor 
desired. The  suggestion that Charles made for checking the impedance by 
progressively loading  the output more and more is valid for Sine Wave outputs, but 
not for CMOS  outputs as implemented on the LTE Lite. One issue is that the 
TCXO is driving a  1.8V CMOS input through a capacitive voltage divider, 
and if you load  the TCXO so much that its output voltage goes to 1/2 the 
no-load voltage then  the input of the processor will likely not get enough 
voltage range to  operate properly.
I mentioned 1M Ohms input impedance simply for convenience as it is a  
standard input impedance as Charles mentions. You can significantly reduce that  
impedance since the 10MHz TCXO can drive a handful of mA no problem, and 
the  20MHz buffered output can drive 20mA or more. This means a 1K Ohms load 
is also  no issue as it would load the output only with 3mA, however(!) the 
more you load  the CMOS output the more heating will happen in the 3.0V 
linear regulator close  to the TCXO and inside the 10MHz DIP-14 TCXO. This will 
cause load-induced  instability.
The best input for the LTE-Lite output is simply a 3.3V or 5V powered CMOS  
gate. No input termination resistance required. Cable lengths should  be 
kept short (less than a foot) to prevent ringing and loading the TCXO output  
for more than a couple of nanoseconds as the edges traverse into the  coax. 
I like to put a weak pull-down of 470K to 1M on those CMOS gate  inputs so 
the input does not float when its not connected to anything. There is  
absolutely no need to load down the output with 100 Ohms, 1K, or even 10K. For  
CMOS inputs, the only thing that makes a difference in phase noise seems to be 
 the rise/fall time and voltage swing. The faster swing and higher  voltage 
the better. Loading down the output will reduce this voltage swing  and 
slow down the internal CMOS switching process, and thus increase phase  noise.
Simply use a 74AC04 buffer as your 10/20MHz receiver/buffer, and keep  the 
coax feed to less than 1 foot and you should see the performance we had  
measured in our plots. No need to over-design this. Using a couple of the 
74AC04  gates in parallel with each going through its own series resistor to  
increase the drive power then combining the outputs of several of the 74AC04  
gates, then adding a series capacitor to get rid of the DC offset will  allow 
you to drive 50 Ohms terminated systems properly. If you use four gates in  
parallel then use 180 Ohms or 200 Ohms series resistors on each gate and 
that  will combine to close to a perfect 50 Ohms output impedance with very 
high  current drive capability. Any noise on the power supply for that IC will 
 end up as AM noise on the output signal, so an ultra low noise LDO is 
suggested  (that's NOT a 7805 regulator!).
* On the MMCX cables, we were informed that the factory sent a number of  
incorrect straight versus right-angle cables by accident. I sincerely 
apologize  for that. They will work just as well electrically, but may not 
mechanically fit  into the Hammond enclosure we mentioned. If you like please let us 
know and we  can replace your straight cables with RA cables for your.
In a message dated 11/23/2014 11:26:37 Pacific Standard Time,  
dave.martindale at gmail.com writes:

Did you  use one-ply, two-ply, or three-ply TP?

More seriously, your LTE-Lite  differs in a couple of respects from the
batch of "production" ones, or at  least my example.  Your TCXO seems to be
in a metal package (shiny  gold colour) and open to the air, if I'm
interpreting the photo on your  LTE-Lite page correctly (and also the photo
that Said posted in his  divide-by-two document).  The production units have
the TCXO in a  solid black package, probably black epoxy, with a blob of RTV
rubber on  top.  So the "production" units are probably already somewhat
better  shielded against drafts.

(Thanks for doing the tests, particularly for  those of us who can't do
these tests ourselves.  I can only watch the  1 PPS of the LTE-Lite wander
with respect to the 1 PPS from my old  Thunderbolt (Piezo oscillator), and
look at the worst-case variation, but I  have no way of knowing how much of
the drift is due to each  GPSDO).

- Dave

On Sun, Nov 23, 2014 at 11:24 AM, Tom Van Baak  <tvb at leapsecond.com> wrote:

> The short-term performance is  10x worse if you don't shield the TCXO from
> air, even if the ambient  air is "still". I suggested Said sell the 
> with some sort of  engineered shield in place. Instead each of us will 
> the problem  in our own way; which is ok for a dev kit.
> For plots and  photos showing performance with, and without, and with
> insulation  see:
> http://leapsecond.com/pages/LTE-Lite/
> The difference is  dramatic, especially if you are used to working with
> OCXO where this  sort of effect does not occur.
> The insulation may be found in  convenient rolls at many local stores. I
> used TP, which for this  application is an acronym for Thermal Paper.
> /tvb
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