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Re: [Discuss-gnuradio] Reference Clock power level for Ettus N210


From: Attila Kinali
Subject: Re: [Discuss-gnuradio] Reference Clock power level for Ettus N210
Date: Wed, 23 Apr 2014 17:37:44 +0200

On Wed, 23 Apr 2014 15:07:52 +0200
Antonio Petrolino <address@hidden> wrote:


> Using an external 10 MHz reference clock, a square wave will offer the 
> best phase noise performance, but a sinusoid is acceptable.

The difference between the phase noise of a square wave and a sinus
input is negligible in a radio application. Your system will be dominated
by the noise of your input. Unless you are doing detection with very
long integration time (in the order of seconds and longer). And even
there i'm not sure whether it's not still the input noise that's dominating
the performance instead of your reference clock noise. And in that case
you care more about close in phase noise which is dominated by non-linear
mixing of low frequency noise sources into your signal, than what we
commonly refere to as jitter.

To get that down you want to:

1) use sinusoidal input and not square wave
   Square wave has high frequency components, depending on the squaring
   buffer you will have considerable spectral components between 100MHz
   and 1GHz.. or even more. Cables have worse and worse characteristics
   the higher you go. Also small impedance mismatches will lead to
   more severe reflections the higer the frequency is. Also as the
   temperature changes, you will have changing phase shifts for different
   frequency components (cables are non-linear in that regard).

2) Change the clock multiplexer (SY89545) from a LVDS->LVDS type to a
   pure analog part. Or rather, remove it completely and go directly
   onto the AD9510

But that's only the case if and only if you care about long integration
times and the 10th decimal point of performance. 

For all else. Using a good quartz oscillator is more than good enough
and you dont need to care about anything but having approximately the
right signal levels. 



-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
                -- Tirin, The Dispossessed, U. Le Guin



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