Direct sampling mode frequency range?

Adam Nielsen a.nielsen at shikadi.net
Thu Jan 30 06:06:56 UTC 2014


> > Yes, and it has nothing to do with Nyquist because the sample rate
> > you receive is only 2MHz or so as per normal.  Direct sampling mode
> > does not capture a wider band than normal, it just uses a different
> > method to tune the receiver.  So the oscillator frequency is
> > irrelevant, and Nyquist doesn't come into it any differently with
> > direct sampling than with normal sampling.
> 
> Mmm, AFAIK, the rtl ADC work at 28.8 MHz and then the IF tuning is
> done by an internal digital DDC (which will also decimate down to 2MHz
> or so to ship via usb).

Good point, that may be correct but I'm not familiar enough to say for
sure.  But either way, direct sampling doesn't impose any additional
limitations than you already have with normal sampling.

It's also worth noting that there seems to be a way to do direct
sampling without any hardware modifications, by exploiting some
undocumented behaviour/bugs in the tuner chip(s?).  I hadn't seen any
mention of this on any of the mailing lists so I thought I would
mention it.  Seems to be plenty of info about it on Google!

Cheers,
Adam.




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