Has anyone experimented with the digital compensation of the I/Q imbalance from direct conversion asics like the one in the dbsrx? I'm interested in the algorithms employed and how successful the compensation can be.
It depends on how detailed a model you're willing to implement and the stability of the hardware (temperature is usually the biggest issue). I haven't looked at the dbsrx, but I've done something similar for a couple of transmitter designs. In rough numbers, a quadrature modulator or demodulator might be natively good to around 40 dB of image rejection. Fixing the LO and applying fixed gain and phase corrections might improve this to 50 dB. The deluxe model would go something like: take your complex-baseband signal; take its conjugate; apply an n-tap complex filter for some n, which includes a scale factor (of around -40 dB) and a frequency-dependent amplitude and phase response; and finally add it back to your original signal. If the filter is right (it would be obtained from some sort of calibration), it will serve to cancel the image response. Image rejection of 65--70 dB is attainable over nontrivial bandwidths. If you can somehow close the loop with a pilot signal, even better rejection might be had with an adaptive canceller continuously estimating the filter mentioned above, tracking out the last smidgen of temperature-dependent bumps and wiggles. For a receiver, this could be something like injecting a comb at a suitable level prior to the receiver, then watching the image frequencies for any tone power, and twiddling the filter to cancel it. Oh, and subtract out the comb before it hits the real receiver. (Pseudonoise could also serve, but it might be less convenient to generate.) Cheers, Peter _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
