[time-nuts] LTE-Lite module

SAIDJACK at aol.com SAIDJACK at aol.com
Sun Oct 19 19:57:38 EDT 2014

I spent some hours at the lab today and connected an external DOCXO to  the 
unit (with 10MHz CMOS output) and did some experiments. See attached  
My setup was so simple, it was almost trivial: I used a BNC T-splitter to  
split the 10MHz CMOS output and feed that both into the LTE-Lite and into 
the  TimePod. Then I used a coax to clip adaptor cable to grab the EFC of pin 
1 of  the DIP-14 socket. I added one resistor, and ran the units.
The results are encouraging. First, this OCXO has a control range of -22Hz  
to +22Hz (0V to 5V), and a mean EFC voltage of about 2.7V. That means that 
the  EFC control is both ~10x less sensitive than the standard TCXO, and 
that  the EFC voltage is very close to the max upper control voltage of the DAC.
To reduce the required DAC voltage I simply soldered a 27K Ohms resistor  
from the EFC pin to the 4.7V Reference output pin of the OCXO, that pulls up 
the  DAC output a bit due to the internal DAC impedance. On an 10811 that 
would be  solved by simply setting the mechanical adjustment so the nominal 
EFC voltage is  around 1.5V (50% DAC setting).
Then it took about one hour for the unit to settle down (the loop has no  
prediction or speed-up, so it takes a long time to add ~1V to the DAC  
The good news is that the Phase Noise of the OCXO is preserved, and the  
ADEV is also preserved, but the bad news is that the internal TCXO is now  
running about 100Hz too high, and that is causing a spur at about 100Hz  offset 
etc (see attached plot). I think that spur is either coming through the  
EFC line, or through the 10MHz coax, so it might be as simple as adding a 
choke  to the EFC line to remove the RF noise originating from the 20MHz TCXO, 
and  using an isolating splitter on the 10MHz line to prevent any noise 
coming from  the board to get into the OCXO output.
Here is the beauty of the setup: because the OCXO is more than 10x less  
sensitive to the EFC voltage changes than the TCXO, the time constant  is 
automatically increased by about the same length which is exactly what you  
want. It makes sense if you think about it, if you have a 1Hz error, it would  
take the loop ~10x as long to generate the EFC voltage required to correct 
for  the 1Hz error if the EFC sensitivity is 9Hz per volt instead of 100Hz per 
volt.  I also tried an OCXO with only a +/-2Hz tuning range, and that one 
did not lock  at all, the loop was way too slow to follow the frequency 
changes that the OCXO  made due to retrace and aging. 
One caveat: the loop is now so slow that it initially had difficulty  
following the aging curve of the DOCXO (the DOCXO had not been powered on in  
many months), so this will only work really well once the OCXO settles down 
into  slow aging after a couple of days, and if it is not exposed to large 
thermal or  airflow changes.
I will send the ADEV plots after some hours of testing, they will likely  
end up peaking at the low xE-011's which is 10x better than the TCXO and 
about  the performance of the particular OCXO I used.
CAVEAT EMPTOR: Your mileage may vary considerably depending on your OCXO,  
this is not trying to imply that any type of performance for the LTE-Lite  
module is possible, I simply hooked up an OCXO I had, and I am presenting the 
 raw data I am seeing on my particular setup. Your results may vary  
So in summary:
1) No loop time constant adjustment seems to be required as the 10x lower  
EFC sensitivity results in about the same increase in time constant which 
puts  it at a sweet spot for typical OCXOs.(to be proven once I get a number 
of hours  of ADEV)
2) The nominal EFC of the oscillator should be set to be around 50%  DAC 
voltage, or 1.5V. I cheated by adding a 27K Ohms resistor to pull up the DAC  
voltage externally, and that probably is also adding some phase noise and  
3) You need to insulate the 20MHz RF coming out of the LTE-Lite as best as  
you can to reduce the spurs caused by harmonic mixing of both oscillators, 
It  may be as easy as adding a good RF low-pass filter to the DAC EFC, and 
an  isolating splitter or buffered splitter with good isolation between the 
two  10MHz output ports of the external OCXO.
In a message dated 10/19/2014 04:53:22 Pacific Daylight Time,  
csteinmetz at yandex.com writes:

Bill  wrote:

>How tough would it be to mate the 10Mhz version up to a  really good 10811?
>*   *   *   I was  thinking of throwing the LTE-Lite and the 10811 in a  

Unfortunately, to get the best out of the local oscillator, the  
control PLL must be carefully adjusted so that the oscillator itself  
controls the stability at averaging times (tau) where it is better  
than the GPS (generally, up to tau of several hundred to maybe 
several  thousand seconds), and the GPS controls the stability at 
longer tau.   The LTE-Lite has fixed (non-adjustable) loop parameters 
that cross over to  the GPS at much lower tau than is appropriate for 
a good OCXO (but well  suited to the installed TXCO).

The other day Said (I think) mentioned  some hacks that may sort-of 
improve the ability of an LTE-Lite to  discipline an OCXO, but that's 
all they are -- very approximate  hacks.  There is really no way to 
properly mate an OCXO to the  LTE-Lite control loop, which would 
require adjusting the PLL loop gain and  the location of the loop's 
poles and zeroes (and possibly even adding new  poles and 
zeroes).  That would need to be done by changing the PLL  parameters 
internal to the LTE-Lite, which are inaccessible.  Without  such 
reprogramming, the LTE-Light can never get the best out of an  OCXO.

Best  regards,


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