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.

Vlad.

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 E. [email protected] wrote:

```
From: Matt E. <[email protected]>
Subject: Re: [Discuss-gnuradio] USRP2+WBX - How to use precisely an
```

8

MHz band?

To: “Vladutzzz” [email protected]

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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