Re: [Discuss-gnuradio] USRP decimation filter performance

[Matt Ettus]

Richard Meston wrote:
> Hi all
>
> Is there information anywhere about the performance on the USRP 
> decimation filter?
>
> I have a setup with an external radio which has a bandwidth of 5MHz 
> around a 10.7MHz IF. I think the attenuation of the radio is quite 
> poor relatively far away from the 10.7MHz centre (e.g. -20dB at 
> +/-10MHz).
>
> If I use the USRP with a decimation of 8, I should get a bandwidth of 
> 8MHz (minus the filter roll-off) I assume – is this correct?

Yes. You get about 80% usable bandwidth due to the filter rolloff.

> If so, then I’m worried about images in my passband – i.e a signal at, 
> say, 18.7MHz will wrap around and be right in the centre of my 
> passband, but attenuated by 20dB by the radio itself. I assume there 
> is attenuation from the decimation process (the CIC/half band filter) 
> – can anyone give me an idea of how much decimation I can expect 
> outside of my passband? (or put me right if I’m completely wrong with 
> all of this!)

About you get >80 dB of attenuation outside of the passband.

If you set the center frequency to 10.7 MHz and the decimation to 8, 
then you are correct that 18.7 MHz would be right in the center of your 
passband. However, the attenuation is well over 120dB in this case.


To clarify, you do NOT need to worry about aliasing into your signal. 
There is no measurable aliasing as long as you are in the center 80% of 
the bandwidth.

You need to determine how wide your signal is. Then set the decimation 
so that it fits your signal. Anything outside that range will be gone. 
Nothing else to worry about.

Matt