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Re: [Discuss-gnuradio] two WBX boards synchronization

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] two WBX boards synchronization
Date: Tue, 09 Dec 2014 19:41:59 -0500
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On 12/09/2014 07:32 PM, Nemanja Savic wrote:
is there any way to synchronize those two clocks sources?
Not on a USRP1.

On N200, there's support for "timed commands", which allows both synthesizers to be locked to the same phase when they're re-tuned, but that'll
  leave you with a 0/180deg ambiguity, because the *mixer* used on the WBX uses a 2XLO scheme, and the LO phase splitter can't be forced into
  a specific state on tuning.

Synthesized LOs have this inherent property.  Even when two synthesizers share a common reference, it's unpredictable what their phase will be
  when they lock to the LO.  More so for so-called fractional-N synthesizers, which are what's used in nearly everything these days.  In the case of
  the WBX and SBX, they use a synthesizer from ADI (ADF4351) that has a fairly-rare "phase resynch" feature which allows the chip to bring the
  resulting LO into a particular phase relative to the reference clock, using a hardware synchronization signal.  That signal, and the FPGA code to
  make it work, is only available in the N2xx and X3xx family.  The USRP1 FPGA codebase has been utterly-frozen for many years, and there's
  *zero* room to do anything fancy, without leaving stuff out.

The usual way around this is to use a calibration signal that is common to both receivers, and use that to calibrate-out the "per run" phase

On Tue, Dec 9, 2014 at 4:51 PM, Marcus D. Leech <address@hidden> wrote:
On 12/09/2014 04:42 AM, Nemanja Savic wrote:
Shouldn't relative phase be constant and 90 degrees for example if transmitted wave had circular polarization?
The samples will be time-aligned, but those samples will have been derived from two independent analog downconversion
  chains, which means that the synthesized LO will have a different relative phase (between the two sides) every time.

On Tue, Dec 9, 2014 at 10:38 AM, Nemanja Savic <address@hidden> wrote:
Yes, the platform is USRP1. What is relative phase in this case if they are alligned in time?

On Mon, Dec 8, 2014 at 4:19 PM, <address@hidden> wrote:

You haven't stated which USRP motherboard platform, but I assume you're talking about the USRP1, given the 4RX image.  Yes, the samples
 will be aligned in time, but the relative phase will be random every time you re-tune or start a new session.



On 2014-12-08 06:03, Nemanja Savic wrote:

Hi all guys,

I am about to make a receiver with two WBX daughterboards. I want to receive simultaneusly with horizontal and vertical antenna. For this purpose I want to use 4RX FPGA image. My question signals obtained in this way, from two daughterboards, alligned in time?

Many thanks,

Nemanja Savić

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Nemanja Savić

Nemanja Savić

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

Nemanja Savić

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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