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

Rob Kossler rkossler at nd.edu
Tue Oct 10 16:33:16 EDT 2017


Thanks Michael,
So, do all "timed" commands sent to the command queue need to be in
strictly ascending order (in time)?  In other words, will a disordered
queue always produce a Late error or does it depend on how much disordered?

Are there any other "timed" commands to worry about besides the following?

   - commands sent in between set_command_time() and clear_command_time()
   - rx and tx streaming commands that include a time_spec

Rob

On Tue, Oct 10, 2017 at 4:11 PM, Michael West <michael.west at ettus.com>
wrote:

> 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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