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Re: [Discuss-gnuradio] Regarding Filtering Aliasing in USRP B210

From: Sylvain Munaut
Subject: Re: [Discuss-gnuradio] Regarding Filtering Aliasing in USRP B210
Date: Thu, 7 Aug 2014 11:01:05 +0200

On Tue, Aug 5, 2014 at 10:07 PM, Matias <address@hidden> wrote:
> Hello,
> We are generating a single tone at 875MHz and we are able to see it using
> the USRP and Gnuradio while sampling at 16MHz. However, when tuning to
> 860MHz we see an alias tone near 859MHz; something similar occurs when we
> tune to 890MHz, we see an alias tone near 905MHz.

No amount of baseband digital filtering alone is going to fix that.
It's part of the inherent imperfections of real world radio.

When you tune to 860M, your tone ends up at + 15M at baseband. Which
is outside of what the ADC can sample without aliasing. Now it's going
to be attenuated because there is an analog low pass filter before the
ADC but it's not perfect, you might still see it, especially if you
have a siggen connected directly to it (you'll have a very low noise
floor, no real world noise).

And that +15M tone in baseband will alias to -1M which is where 859M is.

If you want to reduce that effect, the general idea is to :
 - Sample at a higher frequency
 - Use tighter analog filter at the baseband (some SDR have it configurable)
 - Use a digital low pass filter on the baseband data.

In the end, you might end up sampling at 32 Msps but only have 16 MHz
of usable spectrum without alias and a flat response.



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