On Sat, 2010-01-16 at 10:57 -0800, Matt Ettus wrote: > On 01/16/2010 10:46 AM, Eric Schneider wrote: > > Just out of curiosity, was there a design decision made to not use the > > DAC PGAs to adjust Tx gain? > > > The issue is that changing the DAC PGA not only changes the differential > amplitude (which is what you want), it also changes the DC bias point > (i.e. the common-mode voltage). If you change the DC bias point outside > of a narrow range the upconverters don't work as well. >
That makes sense. I don't understand why the DAC would be designed to have have the DC offset calibration affected by the PGA gain, but there are a lot of things I don't understand. :-) (IIRC, the datasheet seems to imply that the the PGAs are suitable for use as transmitter power control) On a related note, I have spent some time recently trying to minimize Tx distortion using an RFX1800 for the purpose of generating test and measurement signals. I noted that there was non-negligible LO leakage and related IM products, which were largely mitigated by using a large LO offset. I did however look into IQ DC offset and imbalance issues. Searching the list archives showed several discussions about DC offset on the Rx side, but little regarding the Tx side. Instead of fiddling with the AD9862 registers directly, I made a simple flowgraph that allowed me to adjust the IQ DC and gain components from a sinusoid source and adjusted them while noting the LO and image levels on a spectrum analyzer. The DC offsets and imbalance seemed nearly dead on. Is there any reason that my (pre-distortion) approach would not work, or would give erroneous results? Some sort of DC leveling in the Tx signal path somewhere? Are Tx side DAC calibration values stored in the DB EEPROM like the Rx ones? > The only daughterboards that really make any use of the DAC PGA settings > are the BasicTX and LFTX, which don't have this issue with common mode > voltage. > > Matt _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
