[time-nuts] LTE-Lite module
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Oct 19 23:57:38 UTC 2014
Guys,
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
photos.
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.
THIS IS IMPORTANT: MAKE SURE TO TURN OFF THE POWER OF THE LTE-LITE BEFORE
SELECTING THE "EXTERNAL TCXO" SWITCH SETTING, OTHERWISE THE FLASH OF THE
PROCESSOR MAY BE FRIED DUE TO THE INSTANTANEOUS FREQUENCY CHANGE ON THE
PROCESSOR!
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
output).
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
considerably.
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
instability
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.
Bye,
Said
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
box.
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,
Charles
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