Hey Tom, I'm still not sure if we're talking about the same thing. Above 500 MHz, one LO is used to mix your signal down to DC. Below 500 MHz, two LOs are used, first to mix up, then to mix down. However, the first LO stage (for < 500 MHz) won't cause IQ imbalance or LO leakage in your signal band. Because the LO feeds into two separate mixers, those may be imbalanced, and because the LO frequency is necessarily close to the center frequency, the LO can leak into the signal band.
Which frequency are you using, and have you tried adjusting the gain settings? --M On Fri, Jun 6, 2025 at 7:17 PM <tommyt...@w5tech.com> wrote: > 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 > _______________________________________________ > 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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