Oh, sorry, didn't mean to imply that! FFT interpolation might work well (if you can live
with the sinc sidelobes).
If the bandwidth is already constrained to 20MHz/23MHz, then there would be no sidelobes - correct?
I do have a question, though: Why do you go for a 4600-FFT? why not simply a 23 FFT?
Larger n yields greater efficiencies, right? Doing lots of small calls isn't necessarily as efficient as doing less large calls, so long as you can handle the latency and the processor can stay fed with data. I figure ~4000 samples is a good compromise, but maybe larger ones work out better, too.
For reference, here's some performance benchmarks for FFTW (randomly chosen processor):
Single precision, non powers of 2, 1d transforms are some good benchmarks to look at for something like this. Obviously some numbers are better than others, but the general trend seems to skew towards the mid-thousands as being a decent number to target for maximizing throughput.
Brian