odd results working at 250ksp/s

Sylvain Munaut 246tnt at gmail.com
Tue Oct 22 08:29:28 UTC 2013


Hi,

> I don't understand what is happening here. 1 MHz is an even multiple of
> 250kHz, so maybe I'm getting an image of OAK overpowering the relatively
> weaker SFO signal. But should there not be filters that manage this?

Yes most likely an image.


> I guess I was expecting that if the device is set to 250ksps, then it would
> "close down" filters appropriately to reject signals out of that band. But
> maybe the filters don't work properly below a certain sample rate? Like the
> rtl2832 can sample down to 250ksps, but the 820T tuner was not designed for
> it?

The filters you'd need wouldn't be in the R820T anyway (it has filters
but not that narrow).
IIRC, the rtl2832 always sample at a fixed high frequency but it has
an internal downconverter + FIR filter inside to convert the sample
rate down. But that FIR probably can't handle such a big reduction ...


> Can I control the filters directly?

If you find out, let us know, but AFAIK all attempts so far didn't
succeed to get it more narrow than this.


Cheers,

   Sylvain




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