odd results working at 250ksp/s

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David Jacobowitz david.jacobowitz at gmail.com
Tue Oct 22 08:45:10 UTC 2013

Thank you. You and another forum member have helped me understand the
situation. I am beginning to see the attraction of SDR platforms that
integrate an FPGA immediately after the ADC. You can have wide analog
filtering going to an ADC running fast, and then apply digital filters in
the FPGA before downsampling to a lower rate for consumption by

I can accomplish the same completely in software; I'll set the sample rate
high, then filter and decimate, probably in a few stages to keep the
filters reasonable. I tried it this evening with some raw IQ recordings I
made this afternoon and it works pretty well -- on a modern PC.

Dave J

On Tue, Oct 22, 2013 at 1:29 AM, Sylvain Munaut <246tnt at gmail.com> wrote:

> 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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