[USRP-users] DOA with N310 or X310+TwinRX

Nate Temple nate.temple at ettus.com
Mon Jan 27 12:05:53 EST 2020


Hi Rob, Robert, Sammy:

Generally for this type of application we would recommend the X310+TwinRx.
With the TwinRX, you'll be able to have repeatable phase offsets with a
given gain, frequency, sample rate and temperature of a device/system. The
N310 will have a 180 degree phase ambiguity due to the /2 LO architecture.

It is possible to share the LO across multiple motherboards for a X310/Twin
setup, and with the NI branded X310+TwinRX setup (NI-2955) the LO's are
provided out of the back panel. The chassis for currently shipping and Rev
C, F, G X310's back plate has the holes for the LO cables, but the sticker
needs to be removed. This application note covers the process:
https://kb.ettus.com/Modifying_an_X310_Chassis_for_External_LO_Sharing

You'll also need to provide a splitter and most likely an inline amplifier
to overcome splitter losses. A splitter such as the ZFRSC-4-842+ will work.
https://www.minicircuits.com/pdfs/ZFRSC-4-842+.pdf


@Rob: With the current init process of the N310, yes it is required to
first set the external LO to 5 GHz.

With regards to the offsets you're seeing, I believe you should only see a
possible phase difference of 180* within the two channels on the same DB.
Are you issuing a tune request at the start of streaming?

Regards,
Nate Temple

On Mon, Jan 27, 2020 at 8:20 AM Rob Kossler via USRP-users <
usrp-users at lists.ettus.com> wrote:

> Robert, Sammy,
> I am presently running some tests which compare the X310/TwinRx and the
> N310 with regard to channel-to-channel phase.  In my setup, I have a signal
> source that is split 8 ways (1:8 splitter) to feed the 4 channels of my
> TwinRx and 4 channels of my N310. I have seen some strange behavior of the
> N310 that perhaps Robert has experienced?  Take a look:
>
>    - For the TwinRx (for which I am a more experienced user with LO
>    sharing), I get consistent channel-to-channel phase difference among all
>    channels. This is true regardless of power cycles, re-starts of UHD, etc.
>    - For the N310 (for which I am a beginner when it comes to external LO
>    operation)
>       - it seems more complex to run in this mode (as compared to
>       TwinRx).  In order to get it to work, I have had to disable startup QEC
>       calibration because it seems that the N310 initial cal occurs at 2500 MHz
>       RF such that I would need to have my external LO at 5000 MHz for startup
>       (during the UHD deveice 'make') and then later switch my external LO to the
>       desired RF*2. Is this true?
>       - when I run with either external LO or internal LO, I see
>       inconsistent channel-to-channel phase results even between the two channels
>       of a given daughterboard that share the same LO.  I do not understand how
>       this is possible.  My results over 16 captures (with some re-starts of UHD,
>       device reboots, and switching between internal/external LO) show the
>       following channel-to-channel phase difference between channels 0 and 1
>       which share the same LO: (values in degrees) -77, -19, -77, -19, -77, -19,
>       -19, 39, -19, -19, -77, -19, -77, 39, -19, -19.  Note that there are only 3
>       unique values and the delta happens to be 58 deg, but I don't know what
>       that implies...
>
> Rob
>
>
> On Mon, Jan 27, 2020 at 9:57 AM Robert via USRP-users <
> usrp-users at lists.ettus.com> wrote:
>
>> With external LO its 300 MHz – 4 GHz, check footnote [3] from
>> https://www.ettus.com/all-products/usrp-n310/. LO has to be supplied at
>> twice the carrier freq.
>>
>>
>>
>> Currently we use 4 channels. You can find an example how to do the
>> calibration here: https://github.com/EttusResearch/gr-doa
>>
>> gr-doa was written for TwinRX, but can be adapted.
>>
>>
>>
>> Phase noise behavior of N310 and N320/1 could be different, as N310 uses
>> an RFIC and N32/1 use discrete components. This could be important if you
>> want to operate in the small sample regime.
>>
>>
>>
>>
>>
>> *From:* USRP-users [mailto:usrp-users-bounces at lists.ettus.com] *On
>> Behalf Of *Sammy Welschen via USRP-users
>> *Sent:* Monday, January 27, 2020 3:40 PM
>> *To:* usrp-users at lists.ettus.com
>> *Subject:* Re: [USRP-users] DOA with N310 or X310+TwinRX
>>
>>
>>
>> Thank you for the information Robert! Isn't it 6 GHz? However, 4 GHz
>> would also be sufficient for me.
>>
>>
>>
>> How many channels does your system have?  I suppose you use some
>> algorithm for phase calibration after power cycling? I plan to do the same,
>> so the 180 deg ambiguity should be manageable.
>>
>>
>>
>> I looked at the N32x, however, they cost twice as much and I dont't plan
>> on using 200 MHz of bandwidth. If I have an external LO signal I can feed
>> it to the N310, so the only difference between N310 and N32x in this regard
>> would be that I need to generate the LO externally when using the N310,
>> right?
>>
>>
>>
>> <Robert.Poehlmann at dlr.de> schrieb am Mo., 27. Jan. 2020, 14:42:
>>
>> We use the N310 for DoA estimation, however:
>>
>> -          you are limited to 4 GHz
>>
>> -          after power-cycling you get a 180° ambiguity between the two
>> radios (I do not know if this could also happen when you just change the LO
>> frequency)
>>
>>
>>
>> If you want to have >4 channels, have a look at the new N320/N321. No
>> experience with those, but apparently they can do LO distribution.
>>
>>
>>
>> Also take into account if maybe later in the project you want to be able
>> to transmit, which you cannot do with TwinRX.
>>
>>
>>
>> Regards,
>>
>> Robert
>>
>>
>>
>> *From:* USRP-users [mailto:usrp-users-bounces at lists.ettus.com] *On
>> Behalf Of *Sammy Welschen via USRP-users
>> *Sent:* Monday, January 27, 2020 2:19 PM
>> *To:* usrp-users at lists.ettus.com
>> *Subject:* Re: [USRP-users] DOA with N310 or X310+TwinRX
>>
>>
>>
>> Thank you Marcus! So the N310 would be the way to go? I was unsure since
>> the TwinRX is recommended for phase coherent applications.
>>
>>
>>
>> Marcus D. Leech via USRP-users <usrp-users at lists.ettus.com> schrieb am
>> So., 26. Jan. 2020, 18:57:
>>
>> On 01/25/2020 11:43 AM, Sammy Welschen via USRP-users wrote:
>> > Dear all,
>> >
>> > I am planning a system with 5-10 channels that is capable of DOA
>> > estimation.
>> >
>> > Concerning the calibration of the resulting array, would there be a
>> > difference between a system made up of N310 and one made up of X310
>> > with TwinRX boards? Would there be other important differences that
>> > influence estimation performance?
>> >
>> > As I understand it, the TwinRX allows LO sharing between the boards in
>> > a single X310, but this would not help me if I have two or three X310.
>> > On the other hand, the N310s could be connected to a shared LO.
>> >
>> > Are the following thoughts correct?
>> >
>> > Suppose I turn on my system. Then I have to calibrate phase offsets
>> > between channels in any case. Now I change the center frequency. If I
>> > have N310s without shared LO, I have to recalibrate. Same for the
>> > X310s, since LOs are shared only internally. If I have N310s with a
>> > shared LO, I do not have to recalibrate.
>> >
>> > If I restart the system, I have to recalibrate in any case.
>> >
>> > The devices would by synchronized with PPS in any case and with the 10
>> > MHz reference if no external LO is used.
>> >
>> > What is the better choice for DOA estimation (N310 or X310 with TwinRX
>> > or something different)?
>> >
>> > Thank you very much
>> >
>> > Sammy
>> >
>> >
>> Sammy:
>>
>> Your characterization of the two scenarios is correct.
>>
>> There may be some folks on this list who have implemented DOA schemes,
>> but likely few-to-none who have done it on both X310 and N310
>>    and can comment on the differences they encountered.
>>
>>
>>
>>
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