Re: [Discuss-gnuradio] direct conversion compensation

[Peter Monta]

> 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?

If you tune the DDC to, say, +500 kHz, then the output of the DDC will be
contaminated with a small amount of signal from -500 kHz, even though
the DDC filter itself may have very large rejection at -500 kHz.  The damage
is already done pre-DDC, so it won't vary with the DDC output bandwidth.

With a narrow bandwidth, though, a correction consisting of just scalar
differential-amplitude and differential-phase will be very accurate.  These
values will vary slowly across the 6 MHz.

>  
> 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?

Hmm, no, it would seem imbalances still matter at AC.  For example,
if the ADC for the I arm had gain 1 dB higher than the ADC for the Q arm,
one would expect a large imbalance at all frequencies.  AC coupling might
paper over the DC imbalance, but at 1 MHz, say, the I and Q still are 
imbalanced
by 1 dB.

Cheers,
Peter