Hi Dan. Both the B200 and the E312 use the Analog Devices AD9361 RF integrated transceiver. This chip does have an integrated LNA. Perhaps there's some sort of mismatch between your DUTs and this integrated LNA at <1 GHz?
ADI publishes the RX S-parameters: https://ez.analog.com/thread/41208#137929 -Robin On Thu, Feb 1, 2018 at 7:46 AM, Dan CaJacob via USRP-users < usrp-users@lists.ettus.com> wrote: > Hey guys, > > I have put together a noise figure meter application that uses a USRP as > the sensing device. It started off as a way to measure the NF of the USRP > itself. I have a calibrated noise source from an HP 8970B Noise Figure > Meter. To test the NF of the USRP, I connect the head to the USRP input. My > GNURadio flowgraph maximizes the USRP gain and measures a moving average of > the received power while I switch the noise source on and off. The > difference in the received power level, in addition to the ENR table from > the noise source, can then be used to calculate the NF of the USRP itself > using the y-factor method. > > Once you have the NF for the USRP at many frequencies (I test every 50 MHz > from 50 MHz - 6000 MHz), you can modify the same procedure to test the NF > of a Device Under Test (DUT) which is connected between the noise source > and the (now calibrated) USRP. You can use the USRP cal table we generated > in the previous step to derive the NF of the DUT corrected for the NF of > the USRP. > > In short, this all works incredibly well and garners very repeatable > results. One complication is that you will see wild NF at certain > frequencies due to local interference like LTE and WIFI. I've also compared > the results to that which the HP device measures and they're very > comparable. ... Except below ~ 1GHz. > > And here's the issue - I am seeing higher NF for DUTs below about 1GHz and > particularly worse below 500 MHz. I was hoping someone at Ettus might be > able to shed some light on why this might be. Curiously, the USRPs NF > doesn't seem to be too bad, just the DUT. > > I'll note that I am nominally using a B200 for these tests, but I also > tried an E312 just in case the filter banks might help out somehow. I > didn't see a difference - they both had the same problem. > > I have used several DUTs for this test, including LNA boards we have > designed ourselves and a Mini-Circuits ZX60-P103LN+ ( > https://www.minicircuits.com/pdfs/ZX60-P103LN+.pdf). Both seem to exhibit > higher NF when measured with a USRP below 1 GHz. When testing them on the > HP NF meter, the NF is as expected all the way down to 50 MHz. > > I have attached the B200 cal data for your enjoyment as well as the > B200-measured ZX60 NF and the HP-measured ZX60. The HP NF meter only goes > up to 1600 MHz, which is why that data file stops there. I was surprised to > see the B200 seemed to have a better NF than the E312, which averaged 8 dB > NF, by the way. > -- > Very Respectfully, > > Dan CaJacob > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > >
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