[time-nuts] LTE-Lite module

Bob Camp kb8tq at n1k.org
Sat Oct 18 13:43:15 EDT 2014


Hi

For a lab reference, “clean” is a relative term. Most (as in every one I’ve ever seen) instruments expect a dirty signal on the reference input. They phase lock an internal oscillator to clean it up. Past some (unfortunately variable) offset, the reference signal has no impact on the instrument at all. In most cases, that offset is below 50 Hz in order to reject power line induced spurs on the reference signal. Yes, phase noise inside 10 or 20 Hz may matter. ADEV at 1 sec and longer is probably a better thing to look at. 

How good does it need to be? Most counters are quite happy with an ADEV at the 1x10^-11 level at 1 second. VNA’s and spectrum analyzers will be happy with something even less stable. Synthesizers will (ultimately) pass along what ever is on the reference to the output. Your specific test application will dictate if a 1x10^-12 wander at 100,000 seconds on your synthesizer is important or not. 

Bob

> On Oct 18, 2014, at 9:34 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:
> 
> On 17 Oct 2014 19:33, "S. Jackson via time-nuts" <time-nuts at febo.com> wrote:
>> 
>> 
>> Hello Jim,
>> let me answer through Time Nuts as this may interest  other parties as
>> well.
>> 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)
> 
> Hi Said,
> 
> I am only looking for a good clean 10 MHz reference for my lab to feed into
> instruments like my SA, VNA, signal generator etc. Would I be right in
> concluding the best way to achieve this is to use the 20 MHz version and
> the simple divide by 2 that you showed?
> 
> I was going to place an order for the 10 MHz version, despite the long lead
> time, but if I understand you correctly I would get better performance in
> less time by going for the 20 MHz version and a ÷2.
> 
> The other thing I am not so sure about is what the specification of the
> external TCXO/OCXO needs to be. I gather it is 3.3 V, but does it need to
> generate a sine or square wave? What amolitude? I was wondering if there
> would be some advantage in using a 10 MHz OCXO, such as an HP 10811A rather
> than the inbuilt TCXO. Without knowing what your board expects to see, it
> is impossible to know what to type to add.
> 
> Dave
> 
> 
> 
> 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
>> LTE-Lite board.
>> 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,
>> Said
>> Hi Said
>> 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?
>> Thanks
>> Jim
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