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From: | Nemanja Savic |
Subject: | Re: [Discuss-gnuradio] two WBX boards synchronization |
Date: | Wed, 10 Dec 2014 09:58:17 +0100 |
On 12/09/2014 07:32 PM, Nemanja Savic wrote:Not on a USRP1.is there any way to synchronize those two clocks sources?
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
difference.
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:The samples will be time-aligned, but those samples will have been derived from two independent analog downconversionShouldn't relative phase be constant and 90 degrees for example if transmitted wave had circular polarization?
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?
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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:
Many thanks,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?
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Nemanja Savić
_______________________________________________ Discuss-gnuradio mailing list address@hidden https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Nemanja Savić
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Nemanja Savić
-- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
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Nemanja Savić
-- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
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