Hi Marcus, Once again thank you so much for your prompt and detail response. To answer your “direct frequency conversion” question, I like to point out the hardware architecture of UBX-40, the daughter card that we recently switched to because the old model WBX-40 is out of production. I have been evaluating UBX40 to see how it performs compared to WBX40. The UBX40 block diagram shown on website: [UBX 10-6000 MHz Rx/Tx (40 MHz, N Series and X Series) | Ettus Research, a National Instruments Brand | The leader in Software Defined Radio (SDR)](https://www.ettus.com/all-products/ubx40/)
The UBX-40 card uses direct frequency conversion or heterodyne conversion depending on the target Rx or Tx frequency. I thought when direct frequency conversion is used, that is, when the Rx or Tx frequency is above 500MHz, no LO is involved. When the Rx or Tx frequency is below 500MHz, a heterodyne conversion is used (aka with a LO). The UBX-40 block diagram shows that there are two REF input sources, one on the RF front end when the Rx or Tx frequency is below 500MHz, another on the ADC I&Q output. I could be wrong assuming the REF used for the RF front end is the LO. So please bear with me and correct me if I am wrong. The main reason I scrutinized the cal tool is because UBX40 produces less than ideal performance compared to WBX40. When I ran a test application on X310 + UBX40 versus the same X310 + WBX40, UBX40 produced below acceptable WCDMA constellation at the transmitter output. The test ran on the same environment except the daughter card being swapped. The cal files for UBX40 were generated similar to that of WBX40 without tx_offset and rx_offset specified. It is a perplexing issue at this point. Tom
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