> Date: Sun, 9 Dec 2007 04:58:36 -0800> From: [EMAIL PROTECTED]> To: [EMAIL
> PROTECTED]> CC: discuss-gnuradio@gnu.org> Subject: Re: [Discuss-gnuradio]
> direct conversion compensation> > > >> > 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>
Sounds reasonable. It's still not clear to me if the imbalance is just at DC
(because that is where the LO is) or if it varies across the band. For example,
if I have a 6 MHz bandwidth that I direct convert to baseband can I tune a
narrow band DDC (say 30 kHz) around that bandwidth without the performance
degradation?
Your answer implies the imbalance is across the band but I've seen other
references that say to ac couple the ADCs or in the case of OFDM the center
bins are nulled which makes me think the rest of the band doesn't suffer as
much from the imbalances?
Thanks,
Clark
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