[time-nuts] LTE-Lite module
SAIDJACK at aol.com
SAIDJACK at aol.com
Sat Oct 18 01:10:01 UTC 2014
Jim, et. al.,
I spent some time today and put together a Divide-by-2 circuit. Attached
are the schematics, I will send some photos in additional mails so we don't
overload the mail system.
* I grab the 3.0V from capacitor C6 on the eval board. That is the
low-noise filtered analog supply. By loading it with the FF, that voltage goes
down to 2.86V.. Using the digital 3.3V supply resulted in excessive spurs.
* I used only two additional components: a cap and a series resistor
* The IC I used was an old Fairchild 74LVX74 SO-14 chip I had laying around
* Notice the nice improvement in phase noise, and the absence of any
* Notice the nice 6dB phase noise improvement compared to using the direct
outptut, even the floor improved to close to my reference noise floor, so
theory meets practice
* I spent less than 45 minutes building this on a small copper-clad board,
using the ground of the board as much as possible
* The output power of the 74LVC74 driving the 50 Ohms input impedance of
the analyzer is pretty low, less than 7dBm, so a nice buffer would help
* Notice how I set the Q output of the unused FF to 0V, and then connect
that pin to ground to use it as an additional ground pin
* While I wired up the 3.0V power to the eval board, I did not even bother
wiring up the ground. I simply used the coax cables as DC ground return
* The LTE-Lite board was powered from a Thinkpad PC via USB cable, and
disciplining to GPS so I did not even use an external low-noise isolated 5V lab
supply or anything like that, just the noise PC's USB port.
In a message dated 10/17/2014 11:32:49 Pacific Daylight Time,
SAIDJACK at aol.com writes:
let me answer through Time Nuts as this may interest other parties as
Yes, using a fast flip flop to generate 10MHz out of the 20MHz TCXO 3.0V
CMOS output from the LTE-Lite module will preserve the phase noise (actually
improve it by up to 6dB due to the 20log(n/m) noise improvement) and will
not add any spurs if you use the clean 3.0V output from the LTE-Lite module
or an external clean power supply (please note the LTE-Lite TCXO RF output
is 3.0V due to the internal 3.3V to 3.0V Low Noise regulator feeding the
TCXO and buffer).
Use fast logic such as 74AC74, 74FCT74, or the like. We do exactly that on
our ULN-2550 boards to generate 50MHz and 25MHz out of the 100MHz, and
using a fast CMOS divider will result in additive phase noise that will be
below the crystal oscillator phase noise floor.
That will result in significantly better phase noise and much lower spurs
than using the synthesized 10MHz output from the board, and one 74' chip
can generate both 10MHz and 5MHz out of the 20MHz LTE-Lite output. This is
exactly what we would do here if we needed a clean 10MHz from the 20MHz
I believe you can order low-noise divide-by-2 blue-top boxes from Wenzel
already packaged-up and connectorized as well.
Hope that helps,
I was one of those looking for 10Mhz but I just thought again now that it
might be just as well to divide the standard 20Mhz output by 2 using a FF.
I think that would preserve all the desirable characteristics of the 20Mhz
signal which I understand to just be square wave at CMOS 3.3v levels
anyway. Is that correct?
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