Re: News from the OsmoSDR frontier

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Sylvain, thanks for the thumbs up :) On 17.10.2012 19:18, Sylvain Munaut wrote: Yes, I think so, too. Every time the ADC opened the sample-and-hold circuit (low impedance), the high impedance of the E4000 caused a voltage drop. This means that the signal was overlaid with something exactly the frequency of the sample rate. Since this happened on I and Q equally, I assume this to be a pretty maths exercise for any communications engineering student. For the rest of us it is just a signal at the Niquist frequency, which appears either on the outer limits of the spectrum or at zero... At zero we would assume it to be a DC offset - but it isn't and that's why it wasn't easily compensated for or at least our algorithms didn't work very good. Yeppa, the impedance compensating op amps also have a gain of two. To be more precise, they have a I2C controlled potentiometer and the gain can be changed. This also makes it possible to do a hardware DC offset compensation and IQ imbalance compensation. However with the impedance mismatch gone, I think, this is not really needed anymore - the DC offset is quite simple to fix in software and the imbalance as well. Also adders/multipliers are in the FPGA datapath and we can do it there as well. The values are not so big that we absolutely need to do it on the analog side. You can see the DC offset compensation is switched off, but I forgot to also switch off the IQ imbalance compensation. To tell the truth there are some effects in the middle - much lower now, but still. Also routing the SDIO signal over wires or my stack-on-PCB is not a good idea. The noise is everywhere... Yes of course. I will clean up my stuff and do a major check in tonight. Cheers :) Christian - -- - --------------------------------------------------- | maintech # Dipl. Inf (FH) Christian Daniel | | GmbH ### Otto-Hahn-Str. 15 · D-97204 Höchberg | - --------------------------------------------------- | AG Würzburg, HRB 8790 Tax-ID DE242279645 | - --------------------------------------------------- | http://www.maintech.de cd(a)maintech.de | - ---------------------------------------------------