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## Re: [Discuss-gnuradio] USRP2+WBX - How to use precisely an 8 MHz band?

 From: Vladutzzz Subject: Re: [Discuss-gnuradio] USRP2+WBX - How to use precisely an 8 MHz band? Date: Wed, 17 Nov 2010 09:08:46 -0800 (PST)

```Matt,
I was trying to explain that using 8 as a decimation rate would complicate
my rational resampler coefficients, not that it would give me less band.
Anyway resampling seems too heavy for my processor and I decided to use
filtering as you suggested.
The problem is that I don't understand what is the difference between using
an 8, 10 or 12 rate decimation (aside from the obvious band size
difference), if and how they affect the measured power values.
I used decimation 8 and the appropriate 8 MHz filter and measured a signal
power of -44.42 dBm.
I used decimation 10 and the appropriate 8 MHz filter and measured a signal
power of -42.1 dBm.
I used decimation 12 (which gives me a 8.333 MHz band) and the appropriate 8
MHz filter and measured a signal power of -47.07 dBm.
As you can see there is a great difference between the 3 results,
considering they are measuring the same band.
I've read that using even decimation values that are not multiples of 4
result in an output of only one halfband (the low rate one), but I don't
know how to interpret this information nor if it's even relevant to my case.
What are your thoughts on this?
Thanks.

P.S.: I don't think my power measuring method is the problem: I unbuffer the
filter's output, input it in a magnitude FFT block (fft squared) after which
I buffer the resulting coefficients, sum them, divide the result by the
buffer size and the sample time and transform the output value to dBm.

--- On Tue, 11/16/10, Matt Ettus <address@hidden> wrote:

Subject: Re: [Discuss-gnuradio] USRP2+WBX - How to use precisely an 8
MHz band?
Date: Tuesday, November 16, 2010, 8:09 PM

On 11/16/2010 09:36 AM, Vladutzzz wrote:
>
> Thank you all, for your replies!
> I am a bit confused about the 10 rate decimation, which just gets one
> halfband (the low rate one) - as explained in USRP2 FAQ. Does this
affect
> the power level that I will later measure?
> I thought about using 8 which is also a multiple of 4, but it is a
very
> unfortunate fraction and I would like to keep as much as possible from
the
> original band,

I don't understand this.  Decimation of 8 will give you more bandwidth,
not less.

> plus the 10 rate will also decrease the load on my processor.
> So, is the 10 rate decimation in fact a good candidate for what I'm
trying
> to do or I'm better of using 8 with a 1.5625 rational resampler?

Why do you need to resample?  Just use a filter of 8 MHz bandwidth.

Matt

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