On Wed, 2020-11-18 at 18:27 -0500, Marcus D. Leech via USRP-users wrote: > On 11/18/2020 05:32 PM, Dustin Widmann via USRP-users wrote: > > Hi usrp-users, > > > > I've noticed something strange with an experiment I've been doing. > > The > > experiment is set up like a reflectometer. I'm using an X310 with a > > UBX-160 for transmit and both channels of a TwinRX for receive. In > > short, I put up a tone with the transmitter by applying a DC > > offset, > > tune to a nearby frequency with the receiver (in this case, tuned > > 50KHz > > away) and measure the tone. The problem is that the tone isn't > > being > > observed at the expected frequency on the receiver. > > > > The data with which I noticed it is in the table below, where I was > > only using a 2^17 point FFT (763Hz resolution) > > > > freq target bin/freq actual bin/freq diff > > bin/freq dsp freq discrepancy > > 60MHz 65.536 (50e3) 64 (48,828) 1.536 > > (763) 1160 397 > > 61MHz 65.536 (50e3) 66 (50,354) -0.464 (- > > 354) -61 415 > > 62MHz 65.536 (50e3) 67 (51,117) -1.464 (- > > 1,117) -1282 165 > > 63MHz 65.536 (50e3) 69 (52,643) -3.464 (- > > 2,643) -2503 140 > > 64MHz 65.536 (50e3) 70 (53,405) -4.464 (- > > 3,406) -3724 318 > > 65MHz 65.536 (50e3) 72 (54,932) -6.464 (- > > 4,932) -4945 13 > > 66MHz 65.536 (50e3) 58 (44,250) 7.536 > > (5,750) 6044 294 > > > > The discrepancy looks like it could be explained by the radio's > > claimed > > "dsp tune" not actually occuring. I ran the test over again with > > larger > > FFTs (2^20 points) to see if the noted difference frequency would > > be > > closer tot he dsp frequency... > > > > freq target bin/freq actual bin/freq diff > > bin/freq dsp freq discrepancy > > 60MHz 524.288 (50e3) 512 (48,828) 12.288 > > (1,172) 1160 12 > > 61MHz 524.288 (50e3) 525 (50,068) -0.712 (- > > 68) -61 -7 > > 62MHz 524.288 (50e3) 538 (51,308) -13.712 (- > > 1,308) -1282 -26 > > 63MHz 524.288 (50e3) 551 (52,547) -26.712 (- > > 2,547) -2503 -44 > > 64MHz 524.288 (50e3) 563 (53,692) -38.712 (- > > 3,692) -3724 32 > > 65MHz 524.288 (50e3) 576 (54,932) -51.712 (- > > 4,932) -4945 13 > > 66MHz 524.288 (50e3) 461 (43,964) 63.288 > > (6,036) 6044 -8 > > > > ... and it was indeed much closer. This suggests that the digital > > part > > of the tune isn't occuring. Does anyone have any idea what might > > cause > > this issue / is it a known problem / any alternate theories on what > > might be happening? > > > > > > Relevant infos: > > USRP X310 > > * UBX-160 (1 TX channel) > > * TwinRX (2 RX channel, phase sync, lo sharing) > > UHD 4.0 > > > > Dustin > > > Are you using an external clock, or the on-board clock? > > If I'm interpreting your table correctly, the residual error is under > 1PPM in each case. The absolute error of the system clock will be > distributed over > all the components involved, including the DAC, the ADCs, and the > LO > synthesizers in all the modules involved. I'm not sure that they'll > all > perfectly > cancel each other out, given that the TX and RX synthesizers are > quite different. > > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
Marcus, The error would be within 1 PPM, after accounting for the problem I'm seeing. Instead, what I actually have is errors of up to 6KHz or so (100PPM, if you're so inclined) ; and this error seems to correlate exactly to the dsp tune that UHD is requesting. Dustin _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
