Tom, The issue is that the analog bandwidth of the UBX-40 is only 40 MHz. The LO is always the center of your analog bandwidth. See this <https://images.app.goo.gl/Hif742D13MVLg6fV8>diagram. Thus your LO offset is restricted as follows:
LO-offset <= (AnalogBW - SignalBW) / 2 In your case your signal bandwidth is almost 8 MHz which means that your LO-offset must be less than 16 MHz. The fact that it worked for you with WBX-40 may mean that the analog bandwidth roll-off of the WBX is less severe than the UBX-40. If you used UBX-160 daughtercards, you would be able to use large LO offsets. Rob On Wed, Jun 11, 2025 at 6:59 PM <tommyt...@w5tech.com> wrote: > > Hi Martin, > > I have been trying to find out whether the lo-offset applies to the first > LO or the second LO as there is no documentation on how it applies to a > specific radio daughter card. From what I have learned, a lo-offset can > apply regardless of the RF receive frequency is above or below 500MHz. When > the RF Rx frequency > 500MHz the first RF stage uses direct frequency > conversion. Hence, 1st LO is not used. This leads me to believe lo-offset > applies the 2nd LO. > > I tried on Rx frequency below and above 500MHz, same poor constellation > quality issue occurred. That is, the Agilent WCDMA signal passed through > the X310 frequency conversion produced poor constellation output at 800MHz > when lo-offset > 16MHz. When lo-offset at 16MHz or below, the constellation > looks good. Here are the specific details of the test: > > X310 with 2 UBX-40 installed, UHD 4.8 FPGA image installed. A simple > frequency conversion application similar to UHD example app > rfnoc_radio_loop_back is used to perform frequency conversion between RX2 > input at 240.5MHz and Rx/Tx output at 827MHz. An Agilent signal generator > output WCDMA modulated signal to RX2 at 240.5MHz, a R&S spectrum analyzer > monitored Rx/Tx output at 827MHz. The issue is not about the Rx and Tx tune > frequency accuracy or the signal gain. The X310 and UBX40 tuned to the > target Rx and Tx frequency accurately, the Rx and Tx signal gain are within > expectation. The issue is the quality of the WCDMA constellation at the Tx > output. > > At first, I thought the issue might be calibration related, but after > discussed with Marcus, I think another culprit is causing the issue. > > The fact that when lo-offset was decreased from 20MHz to 16MHz resolved > the poor constellation issue indicating it is unlikely due to IQ imbalance > or DC offset, but I don’t know what exactly went wrong. I don’t have such > issue when using WBX-40 boards. > > I have X310 master_clock_rate set to 184.32MHz, DDC down sampling and DUC > up sampling rate set to 7.68MHz. Increasing the sampling rate to 30.72MHz > > lo-offset 20MHz still couldn’t resolve the issue. > > > Tom > _______________________________________________ > USRP-users mailing list -- usrp-users@lists.ettus.com > To unsubscribe send an email to usrp-users-le...@lists.ettus.com >
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