minimum sample rates?

I'm trying to write some code that uses a Raspberry Pi not just to capture the samples, but also to do the DSP. The signal I'm working (aviation VOR) is narrow and the DSP is not too hard-core, but it'll be easier if I can sample at a lower rate. Though rtlsdr_set_sample_rate returns no errors and rtlsdr_get_sample_rate returns the same value I gave it (or quite close), it does not appear to be working. I had been working at 250ksps but wanted to try 125ksps or even 62500sps, but none of those seem to work. Is this just a limitation of the hardware? I have a NooElec RTL2832 / R820T device if that matters. Regards, Dave J