Marcus, As always, I appreciate your detailed response. I was tossing that question out there just to make sure my interpretation of how the external LO RX input worked was correct. One of the recent tests we ran was comparing signals across daughterboards. We noticed that the residuals between signals on the same daughterboard were significantly less noisy than residuals between signals across daughterboards. We had guessed that was due to the difference in local oscillators used in the RX chain. I was hoping to provide the same reference signal to both external LO RX ports in order to synchronize both daughterboards, but the frequency we're working at is below the spec you mentioned.
We've had a bit of discussion about using the external 10MHz input in the past and I believe the conclusion we came to was that the error I was seeing was characteristic of the expected error in the DDC / DUC. Perhaps working at the lower end of the frequency specification of the N310 is starting to catch up to me.... On Thu, Nov 19, 2020 at 3:33 PM Marcus D. Leech <patchvonbr...@gmail.com> wrote: > On 11/19/2020 04:03 PM, Christopher Flood wrote: > > Just for my own edification, what is the exact role of the signal that is > put into the LO IN RX port? And why does it need to be exactly twice the > desired receive frequency? Wouldn't there be additional resolution for any > frequency greater than twice the desired receive frequency? > > That signal substitutes for the LO signal generated internally by the > AD9371 RFFE chip. > > It has to be at 2X the desired input frequency because the phase-splitter > in that chip uses a "2XLO" architecture, which requires the signal > to be at twice the desired LO frequency. > > Further this functionality is only available for tuned frequencies between > 300MHz and 4GHz, (600Mhz to 8GHz LO input). This is because > for frequencies below 300MHz, the device uses an up-conversion scheme. > > If you're not certain WHAT the external LO *does*, perhaps you don't need > it in your application? > > Normally for phase-coherent applications, the highest-quality (lowest > mutual phase noise) coherence is obtained when each mixer receives > exactly the same LO signal, which is why an external LO input is > provided in the N310. The "second-best" implementation is to have > all the synthesizers share a common reference clock, which is the > scenario when you aren't using the external LO input. > > Now, somewhat orthogonal this is is "where does the device get its > reference from?" If you're building a multi-device system where > some form of phase-coherence is desired, then they all have to share > their reference clock--which is where the external 10MHz > clock comes in. > > > > > On Thu, Nov 19, 2020 at 1:11 PM Christopher Flood < > christopher.fl...@colorado.edu> wrote: > >> Yes, the signal should be at +3dBm, the Vrms is ~300mV into a 50 ohm >> load. The frequency of the signal is also twice the frequency of the signal >> I'm trying to gather data on. However, due to the way the documentation is >> phrased, I wasn't sure if 20MHz is too low of a frequency for the LO RX IN. >> >> I have not tried the init_cals=basic argument. I can give that a shot and >> report back. >> >> -Chris >> >> On Thu, Nov 19, 2020, 12:32 PM Marcus D Leech <patchvonbr...@gmail.com> >> wrote: >> >>> Are you inputting an LO signal at twice the desired frequency and at >>> +3dBm? >>> >>> Have you tried adding init_cals=BASIC to your device ages? >>> >>> Sent from my iPhone >>> >>> > On Nov 19, 2020, at 2:26 PM, Christopher Flood via USRP-users < >>> usrp-users@lists.ettus.com> wrote: >>> > >>> > >>> > Hi all, >>> > >>> > I've seen some discussion about this on the email lists, but I'm still >>> having a bit of trouble. I'm trying to use the LO IN RX ports on the front >>> of the N310 to synchronize the oscillators on the two daughterboards. I was >>> thinking I could generate a signal on one of the TX/RX ports of the SDR and >>> feed that into the LO IN RX ports at the appropriate frequency and power. >>> However, when I test this setup in GNU Radio Companion I don't get any >>> meaningful data, so I must be doing something wrong. >>> > >>> > The input signal that I want to sample is a 10MHz signal that is going >>> into an RX2 port on the front end of the SDR. When I set the device >>> argument "rx_lo_source=internal" and run it, the data I get looks exactly >>> how I would expect. When I change the device argument to >>> "rx_lo_source=external" and run it, I don't get anything that makes sense. >>> > >>> > Am I using the LO IN RX ports correctly? The documentation doesn't say >>> much besides power and frequency ranges. >>> > >>> > Any help or advice would be much appreciated. >>> > >>> > -Chris >>> > _______________________________________________ >>> > USRP-users mailing list >>> > USRP-users@lists.ettus.com >>> > >>> http://secure-web.cisco.com/1rpFFtn1JZDE_g214Q-YS6vXmf7iX-GrSqz5_gIRlzT74BRG0LQVcDxlbCKOBLjxI7GcMNxycsBFMF-Lzacu4VXZpGTxpMgLHyxfkdQ2pzOtNNtecm_gMTYICiRaahesVZWF5a2dj7cjYzaLEjuC4PUBV2IKiJN9B3Z0EGCGPr6nbTyCm-I7dJfEsfYsSIFdFFSQP6iWj2NrE4xYBVKyPCxJT34HfnXS_KoENsd6WDKRct0TsItd4_dk4xBZEWrwNqes0pfV_RJzJFNSWC1tJjpFk1UVK8ySaM22YoapPkrKhJ7xvd9EiKc-aK5WHC2syeWL5YAO5QNCrMxT9JJ-i3tvC2sZAlgAyNSx78qV7M09bkZ1k2hn5qi3jbvK3xggz-HHgjahAsx6zRH86NZDIoSe5_4tc6c0UeE03mYMgALuwY1kwoZr7b8-EjaFkvZRoF8mCGzGXTKfyvJuFHID1yA/http%3A%2F%2Flists.ettus.com%2Fmailman%2Flistinfo%2Fusrp-users_lists.ettus.com >>> >>> >
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