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
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