Jim, Thank you very much for your mathematical treatment of the I/Q balancing matters. That is really the weak point of the otherwise excellent SDR-1000. To me personally, it means so much that as soon as I realised this more than about 3 years ago I refused to transmit before manually correcting the I/Q balance at the frequency. You know, I am mentally sick in this respect long time! Since the early days of SSB when the signal was generated by home made equipment I had for "convenience" even front panel controls for phase and balance adjustments!
Now that I have been experimenting with "Active Integrating Quadrature Sampling Detectors" (ISD for short and simplicity) I can clearly see a slanting noise floor graph on the display, raising up towards the increasing negative frequencies, i.e. towards the "DC-frequencies". When using 96 or 192 kHz sampling there is a narrower or wider hump visible on the display at around 11 kHz. I have been thinking that the hump is due to higher phase and amplitude erros at low IF-frequencies and due to sampling pulse errors at higher RF-frequencies. Just came into my mind, do we actually need any lowpass filters after the DDS and do we need sinus and cosine outputs? Would just accurately controlled quadrature 180° wide clocking pulses do the trick with a lot less of critical filter components? Then there is some more complication in the QSD circuits. Most of them sample the signal in four short 90° narrow slices when it can be handled by two 180° wide pulses. That is the way I'm sampling in my IDS experiments. Please, wise answers to my silly questions! 73, Ahti OH2RZ On 27/08/06, Jim Lux <[EMAIL PROTECTED]> wrote: > Put some analysis of the variability of the DDS LO filters out on my website. > It's a draft still: comments appreciated. > > http://home.earthlink.net/~w6rmk/sdr1000/index.htm > > > Take home message so far.. At least from the DDS LO side of things, you > don't need a huge number of calibration points across the HF band in order > to get good image rejection. {of course, this depends on what you think > "good" is...<grin>} > > While there can be pretty substantial differences in phase between I and Q > sides due to component variations, once you know what the difference is > (e.g. by calibrating), the difference doesn't change a lot over the entire > range. > > If someone has any information on the temperature characteristics of the > components (in particular, what's the temperature coefficient of the caps), > I can roll that into the analysis. > > Jim Lux, 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 > _______________________________________________ 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
