Re: [Discuss-gnuradio] WBX Dual Receiver

On 02/23/2016 11:34 PM, Justyn wrote:
> Makes sense.
>
> If I can ask a follow up here:
>
> 1) If I instead use 2 USRPs connected via an external reference clock
> and an Ethernet switch for receiving data, will they be
> phase-aligned? If I understand what's going on in the context of the
> ref clock, I think this is a yes.
Assuming that you have an external reference, and 1PPS to
time-synchronize them, and you use timed-commands to properly assert
the phase-resynch logic in the synthesizers, then yes, with the
proviso that WBX in particular uses a 2XLO mixer, which has a
180 deg phase ambiguity--since there's no way to predict or control
the start state of the flip-flop inside the mixer that's used as
a phase-splitter. So they'll either be aligned, or 180deg out-of-phase.

>
> However,
>
> 2) On the TX end, if I use two USRPs connected to an Ethernet switch,
> and synchronized by an external clock connected to the ref clock port,
> is there any way I can guarantee that the first samples of each are
> sent out at the same "time" (I don't know what the appropriate term
> would be here, time, clock cycle, ref clock tick, etc)? I suspect
> that unless there's a mechanism that does this, this is a no.
If you have both USRPs in a common multi_usrp object, then their outputs
will be time-aligned. The same comments above about
using timed-commands for tuning, and the 180deg phase-ambiguity
continue to apply here.

Now the issue is that with GRC, there's no way to use timed-commands
directly, so you end up either coding in naked python/C++ yourself,
or modifying the code generated by GRC to wrap the tuning functions
in UHD timed commands.


>
> --
> Justyn
>
> On 02/23/2016 08:26 PM, Marcus D. Leech wrote:
>> On 02/23/2016 11:22 PM, Justyn wrote:
>>> Hello,
>>>
>>> I'm experimenting with the MIMO capabilities of the N210s with WBX
>>> daughter cards and seeing how far I can get before I need the MIMO
>>> breakout cable or GPSDO.
>>>
>>> One of the first things I'd like to see is if both receivers on the
>>> WBX daughter card can run at the same time (TX/RX and RX2).
>>>
>>> Unfortunately, if I add two USRP source blocks with the same IP to
>>> my flow graph and try it out, I get D's on the output.
>>>
>>> According to
>>> http://files.ettus.com/manual/page_general.html#general_ounotes, it
>>> seems that I'm getting "discontinuous packets" and are subsequently
>>> dropping data (which I can confirm). I assume it's because I'm
>>> receiving data packets with the same source address, and probably
>>> duplicated sequence numbers which GNU Radio thinks are discontinuous.
>>>
>>> Is there a way around this? Is this a bug? It seems if I have the
>>> bandwidth and physical hardware, I should be able to leverage those
>>> capabilities, no?
>>>
>>> This USRP and my workstation are connected through a switch, but if
>>> I add a separate identical N210 with WBX to the switch, I get both
>>> sets of data, so it's not a bandwidth issue.
>>>
>>> --
>>> Justyn
>> There is only a single receive chain on a WBX card--there are,
>> granted, two ANTENNA PORTS that may be selected that the receive chain
>> attaches to, but there really is only one receive chain on a WBX card.
>>
>> The fact that you have two distinct UHD/USRP objects trying to
>> communicate with the same physical USRP is going to create a fair amount
>> of confusion in the lower layers which will result in unpredictable
>> behaviour.
>>
>>
>>
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>


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