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
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
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.
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
one would expect a large imbalance at all frequencies. AC coupling
paper over the DC imbalance, but at 1 MHz, say, the I and Q still are
by 1 dB.