> >You're correct! No DC path to the QSD. The carrier is coming from the >1/f noise near DC. >That will be taken care of when the TX IF gets implemented.
I don't think it has anything to do with 1/f noise. TX IF won't fix it. Let's for a moment assume that you're feeding in an 11 kHz tone through I/Q (phased so you generate the upper sideband..) You'll get a nice carrier at DDS Freq+11 kHz, and if the image rejection is set up properly, a very small signal at DDS-11 kHz. BUT, you'll also get feedthrough at DDS Freq, and there's nothing you can do about it with an AC coupled system, because the carrier feedthrough in the QSE is entirely determined by the balance of the 4 switches and the DC on their pins, the latter probably determined by leakage current of the electrolytic capacitors. I looked at some measurements I made when I first got my SDR1000s at work and they all have some amount of leakage with the I/Q inputs grounded. In fact, I used this "feature" for a calibration signal, so I could tell the difference between errors due to the DDS and errors due to the sound card. It *might* be that you can create a waveform for I/Q that will compensate for this. After all, if the parts were ideal DC blocks, it wouldn't have the problem, so maybe putting a bias on the audio waveform will have an effect. Jim, W6RMK _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
