[USRP-users] Phase coherence problems using TVRX2

Jason Roehm jasonr at 3db-labs.com
Wed Jun 13 14:10:41 EDT 2012


The receivers inside of the TVRX2s contain individual local oscillators 
that can be fed with phase-coherent references. These LOs are typically 
implemented using fractional-N frequency synthesizers, which have a 
phase offset ambiguity (there are multiple stable lock points, each with 
their own phase offsets). This means that if you have a few TVRX2 
devices that are fed with coherent references, the output phase of their 
various local oscillators will not necessarily be in phase with one 
another, as you observed. This is a pretty common issue, not limited to 
the TVRX2. One way of working around it is to include a calibration 
source into your system that you can selectively enable to measure the 
channel-to-channel phase offset.

Jason

On 06/13/2012 02:06 PM, Lucas Caldeira de Oliveira wrote:
> Hi,
>
> I'm working on a project that uses a linear array with four antennas 
> for direction of arrival measurement. In this scenario, 
> phase-coherency between each antenna is necessary.
>
> I'm using one USRP rev4.5 with two TVRX2 rev1.1. I found that a random 
> phase offset is occuring every time i execute the program. After LO 
> lock, the phase offset remains constant.
>
> Inside FPGA, the phase accumulator is the same for four DDCs. These 
> signals between the input of ADC and inside GNURadio are phase 
> coherent. This random phase offset is occuring inside TVRX2.
>
> I'm using the last version of UHD and GNURadio.
>
> Could you give us information about this problem? Is it possible to 
> achieve phase coherency between these signals? Is there any limitation 
> in TVRX2 about this?
>
> Thanks,
>
> Lucas
>
>
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