Did you make sure to select the external reference when creating the UHD device?
On Tue, Jun 4, 2019, 7:11 AM Erik Heinz via USRP-users < [email protected]> wrote: > 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 <[email protected]> > *Gesendet:* Montag, 3. Juni 2019 17:10 > *An:* Erik Heinz > *Cc:* [email protected] > *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 <[email protected]> *On Behalf Of *Erik > Heinz via USRP-users > *Sent:* Monday, June 3, 2019 3:06 AM > *To:* [email protected] > *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 > [email protected] > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >
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