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 <rkoss...@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@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 >> >> >> _______________________________________________ >> USRP-users mailing >> listUSRP-users@lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> >> >> >> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> >> >
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