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RE: [Discuss-gnuradio] direct conversion compensation

From: Clark Pope
Subject: RE: [Discuss-gnuradio] direct conversion compensation
Date: Sun, 9 Dec 2007 20:19:34 -0500

> Date: Sun, 9 Dec 2007 04:58:36 -0800
> From: address@hidden
> To: address@hidden
> CC: address@hidden
> 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?

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