[USRP-users] Using X310/UBX Tx/Rx antenna for single channel pulsed radar

Michael West michael.west at ettus.com
Tue Oct 10 16:11:09 EDT 2017


Hi Rob,

Yes, that would be a problem.  There is a single command queue for both TX
and RX commands to the radio, so something has to collate the T/R switching
commands with the RX streaming command so none of the commands arrive late.

Regards,
Michael

On Thu, Oct 5, 2017 at 7:48 AM, Rob Kossler via USRP-users <
usrp-users at lists.ettus.com> wrote:

> Hi Marcus,
> Still working on the same issue (sporadically).  I was able to get my
> transmit pulse behaving reasonably well (using continuous Tx streaming and
> manually controlling the T/R switch using timed commands).  However, I ran
> into a problem when I tried to simultaneously stream Rx data.  The RX
> streaming reports a single Late command followed by numerous Timeouts.
>
> Since I am able to
> A) run my application in TX-only mode (with separate threads for transmit
> streamer and T/R switching), and
> B) to run in TX/RX mode without T/R switching
>
> I am wondering what is the source of my problem when I try to run in TX/RX
> mode with T/R switching.  I am wondering if the problem could be related to
> ordering of timed commands.  I have made sure that the T/R switching
> commands are sent in time-ascending order, but I have no synchronization
> between these commands and my Rx streaming command (which includes a time
> spec in the meta data).  So, it is possible that my application is sending
> a tone of T/R switching commands (filling up the timed command FIFO) prior
> to sending the Rx streaming command.  Would this be a problem?
>
> Rob
>
>
> On Tue, Sep 26, 2017 at 9:09 PM, Rob Kossler <rkossler at nd.edu> wrote:
>
>> Hi Marcus,
>> Thanks for your response.  I've been away for several days and finally
>> had the opportunity to revisit this today.
>>
>> I modified my code to manually control the TxEnable pin using timed
>> commands in order to pulse a continuously streaming TX waveform (100
>> MS/s).  This worked until I reached a limit on PRF at around 20 kHz (50 us
>> PRI).  When I tried to go faster (e.g., 20 us PRI), the pulse train went a
>> bit crazy - likely from the commands arriving late.  I'm guessing that
>> there is some limit to how fast I can send these GPIO commands while at the
>> same time streaming at 100 MS/s.  The bad news is that this was with one
>> channel.  So, I expect that when I implement with 2 TX simultaneously
>> (e.g., beamforming), I will need to send twice the number of GPIO commands
>> and thus my min PRI will jump to about 100 us (but I haven't tried this
>> yet).  By the way, this was implemented with 3.9.LTS and a single 10Gbe
>> link.
>>
>> The other thing I noticed was that the RF pulse width was about 300 ns
>> shorter than expected for the specified switching times.  The switch data
>> sheet indicates on/off switch times on the order of 45ns.  Thus, enabling
>> and disabling of the switch could account for 90 ns, but this is still much
>> less than the observed shortfall.  Not sure what the cause is.
>>
>> In the end, this may be good enough for my application.  Still, I may try
>> some things in the FPGA.
>>
>> Rob
>>
>>
>>
>> On Mon, Sep 18, 2017 at 5:54 PM, Marcus Müller via USRP-users <
>> usrp-users at lists.ettus.com> wrote:
>>
>>> Hey Rob,
>>>
>>> so, it's probably the good ol' radar bandwidth conundrum: For good range
>>> resolution, you'd typically want high TX and RX bandwidth, but at least on
>>> TX, it feels kinda bad to stream a full 200MS/s to the USRP, just to be
>>> able to turn a sine wave on and off again within a few nanoseconds. And to
>>> confirm your suspicion: Yes, if you use a lower rate than that, the X310
>>> will interpolate to the 200MS/s MCR, and that happens with a low-pass
>>> filter (to get rid of spectral aliasing in the general use case), and that
>>> "washes out" your pulses. So, meh.
>>>
>>> As long as you're not sending constantly, but more in terms of single
>>> pulses or short pulse packets, sending the signal at a full 200 MS/s might
>>> be the right thing to do – the USRP would buffer the sample packets until
>>> the TX timestamp "happens", and there's no unnecessarily high CPU load.
>>>
>>> You could also replace the DUC with a simple "repeat" NoC block. Or with
>>> an upsampler without an anti-imaging filter (ie. a zero-padder), for that
>>> manner. Or, upsample, but use the desired pulse shape as filter.
>>>
>>> 1) well, close reflections are usually very strong with radar. If you're
>>> using an external amplifier, that might be a problem.
>>> 2) There's a the auto-TX/RX switching functionality that you can use to
>>> switch when you start/stop streamers. Also, yes, antenna switches are just
>>> "normal" GPIO, so you can basically look into the daughterboard driver to
>>> see which GPIO gets toggled when you change the antenna, and do the same in
>>> your application.
>>>
>>> Hope that helps,
>>>
>>> best regards,
>>>
>>> Marcus
>>>
>>> On 09/18/2017 01:55 PM, Rob Kossler via USRP-users wrote:
>>>
>>> Hi,
>>> I am interested in implementing a pulsed CW radar using a single channel
>>> (X310/UBX) via the TX/RX antenna port.
>>>
>>> My initial implementation works, but not that well.  In this
>>> implementation, I continuously stream the receiver with antenna set to
>>> TX/RX and I simultaneously send timed transmit bursts for each pulse.  The
>>> USRP automatically switches the T/R switch to transmit during the transmit
>>> bursts and then back to receive when the transmit burst completes.  The
>>> switch time seems good enough for my application.  However, the transmit
>>> pulse doesn't look as expected at the beginning - likely due to start up
>>> filtering in the DUC.
>>>
>>> I am considering a different implementation such that transmit and
>>> receive both run continuously and I just manually "hot-switch" the T/R
>>> switch between transmit and receive using timed commands.  I have 2
>>> questions:
>>> 1) is there a problem with this approach (e.g., possibility of damaging
>>> the device)?
>>> 2) how do I manually control the T/R switch?  (I am expecting I need to
>>> use the GPIO registers, but I can't find the relevant info in the manual).
>>>
>>> Rob
>>>
>>>
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>>
>
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