|From:||Marcus D. Leech|
|Subject:||Re: [Discuss-gnuradio] two WBX boards synchronization|
|Date:||Tue, 09 Dec 2014 19:41:59 -0500|
|User-agent:||Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:18.104.22.168) Gecko/20111108 Fedora/3.1.16-1.fc14 Thunderbird/3.1.16|
On 12/09/2014 07:32 PM, Nemanja Savic wrote:
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
-- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
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