Just a couple of sanity checks on this...You have to make sure that 1) You don't already have a GPSDO installed on your B210. If you do, the external reference won't make it in.
2) Assuming you don't have the GPSDO, you have to tell the B210 to use the external reference instead of its own. Also, from past experience, I have found that avoiding an LO offset that is an integer multiple of the sampling rate will avoid some pretty bad spurs. I think these are integer boundary spurs in the PLL. Dan Lundberg From: USRP-users <usrp-users-boun...@lists.ettus.com> On Behalf Of Erik Heinz via USRP-users Sent: Tuesday, June 4, 2019 10:10 AM To: Torell, Kent L <kent.tor...@gd-ms.com> Cc: usrp-users@lists.ettus.com Subject: Re: [USRP-users] B210: 1/f noise and LO offset Thank you for the explanation. I tried using an external reference clock (HP 58503A). Unexpectedly, this made no difference at all. The noise is still exactly the same. Erik Heinz -- ──────────────────────────────────────── Supracon AG Dr. Erik Heinz An der Lehmgrube 11 07751 Jena Tel.: +49 3641 2328-165 Fax: +49 3641 2328-109 Internet: http://www.supracon.com ──────────────────────────────────────── Handelsregister: Amtsgericht Gera HRB 208970 Umsatzsteuer-Id.: DE 216 111 685 Kaufm. Vorstand: Matthias Meyer Vorsitz Aufsichtsrat: Prof. Dr. Michael Siegel ________________________________ Von: Torell, Kent L <kent.tor...@gd-ms.com<mailto:kent.tor...@gd-ms.com>> Gesendet: Montag, 3. Juni 2019 17:10 An: Erik Heinz Cc: usrp-users@lists.ettus.com<mailto:usrp-users@lists.ettus.com> Betreff: RE: B210: 1/f noise and LO offset The phase noise comes from the B210 LO (RF synthesizer), and is present regardess of the frequency offset...you are misled by the log axis of the plot. The close-in noise can be improved by using a high quality external 10 MHz source. The control loop action of the synthesizer translates the reference phase noise to the LO phase noise. You may have a lab standard available; or you could purchase a GPSDO version for the B210 (Ettus part 783454-01). If this doesn't meet your needs, you will need to shift the signal lower in frequency with an external mixer and a high quality microwave synthesizer (e.g. $20K Rhode/Keysight/etc.) that has the phase noise you require. The B210 uses an Analog Devices AD9361 chip, which generates the RF local oscillator signal with a high frequency phase locked loop, then divides it down to the requested frequency. 5 GHz is at the upper end of it's range, so the division is small and the phase noise will be at it's highest. Moving down several octaves will improve the phase noise, dropping 6 dB for every octave (e.g. 500 MHz would have 20 dB lower phase noise than 5 GHz). Kent Torell From: USRP-users <usrp-users-boun...@lists.ettus.com<mailto:usrp-users-boun...@lists.ettus.com>> On Behalf Of Erik Heinz via USRP-users Sent: Monday, June 3, 2019 3:06 AM To: usrp-users@lists.ettus.com<mailto:usrp-users@lists.ettus.com> Subject: [USRP-users] B210: 1/f noise and LO offset Hello everyone, preliminary remark: I am an physicist - so please excuse some possible knowledge gaps in radio engineering :-). I would like to use a B210 to measure the IQ response of superconducting resonators at about 5 GHz. This is done by feeding the resonators a signal from the transmitter with a frequency near the resonance, amplifying by LNAs, coupling to the receiver, demodulating to base band, and recording the resulting IQ signal. The signal bandwidth of interest will be small, in the kHz range. Since noise of the resonators has to be measured as well, I first had a look at the noise of the B210 output without an external signal. The result is plotted in figure 1 (vertical axis is scaled to the output range of the ADC). It seems, below 100kHz the B210 receiver has strong 1/f noise. I would guess this is hardware noise from the mixer. Anyone knows if this assumption is correct? So I thought it would be a good idea to demodulate not directly to base band, but to an IF of may be 100 kHz and do the rest in software. I did a quick test using gnu radio, feeding the output of the USRP source and a 100 kHz signal to a multiplier. This indeed has the desired effect (figure 2, blue curve vs. red curve). If I understand the concept of the B210 local oscillators correctly, the same should be possible directly on the B210, using an LO offset of 100 kHz. The result, however, is completely different (figure 2, green curve). Any ideas why? Thank you. Best regards, Erik
_______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
