[USRP-users] Phase Noise for USRP N200 with LFTX/LFRX

Daniele Nicolodi daniele at grinta.net
Mon Dec 8 18:59:36 EST 2014

On 08/12/14 23:11, khalid.el-darymli via USRP-users wrote:
> According to the specs provided in [1], the RF performance of USRP
> N200/N201 is characterized for the WBX daughter-board. Particularly,
> phase noise measurements are given for three offsets from a center
> frequency of 1.8 GHz.
> My question is, are there any such measurements for the LFTX/LFRX
> daughter-board?


others in the thread have replied with the characteristics of the
internal oscillator used to derive the clocks used in the N210. This, if
you do not use an external clock source, tells you how well the samples
acquired by the USRP are aligned in the time domain.

However, for any phase measurement there is a noise contribution coming
from amplitude noise: analog voltage noise and digitization noise in the
USRP but also amplitude noise of the input signal (in a band equal to
your measurement band, around the carrier frequency).

Which (clock noise or amplitude noise) is going to be dominant depends
on you application and on which clock source you use.

Using a (very good) external clock source, I observed a significant
phase noise reduction in my measurements using a linear power supply to
power the N210 instead of the switching power supply.

Also, if you use GNURadio, be aware that most (all?) the functions
dealing with phase computations are implemented using an atan2 function
that trades accuracy for computational speed. The accuracy tradeoff in
the GNURadio atan2 implementation may be too large for your low
phase-noise application (it is for mines).


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