USRP2+WBX - How to use precisely an 8 MHz band?

Dear all,
I am trying to receive with my USRP2 + WBX daughterboard a signal band
of
precisely 8 MHz in order to measure it’s power.
The problem is that since USRP2 has a 100 MSample/s sampling and I can’t
just devide it by 12.5 to get the desidered 8 MHz band, I need
to use an FPGA decimation of 100 together with an interpolation of 8 in
order to get the desidered results
(I am not using decim 25 and interpol 2, because it is said in the
specification of USRP2 that both parameters have to be at least 4, and
multiples of 4, in order to get the full band).

Is this the only way of going about this operation?

Doesn’t this eliminate too much usefull information?

Please let me know if you would this in a different way!
Thank you for your time!

Vlad.

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On Thu, 2010-11-11 at 07:46 -0800, Vladutzzz wrote:

Is this the only way of going about this operation?

Doesn’t this eliminate too much usefull information?

Please let me know if you would this in a different way!
Thank you for your time!

I’d sample at 10Msps and use a rational resampler of rate 4/5 to get
8Msps out the other side.

–n

On 11/11/2010 10:46 AM, Vladutzzz wrote:

Is this the only way of going about this operation?

Doesn’t this eliminate too much usefull information?

Please let me know if you would this in a different way!
Thank you for your time!

Vlad.

I’m confused. Interpolation is generally used on Tx, rather than Rx.

What I would do, is specify a decimation of 10, giving you 10MHz of
bandwidth into the
host, and then filter the result down to 8MHz, and measure that with a
complex-to-mag-squared,
followed by a single-pole IIR filter.


Principal Investigator
Shirleys Bay Radio Astronomy Consortium

On 11/11/2010 07:46 AM, Vladutzzz wrote:

multiples of 4, in order to get the full band).

Is this the only way of going about this operation?

Doesn’t this eliminate too much usefull information?

Please let me know if you would this in a different way!
Thank you for your time!

I would suggest decimating by 8 or 10, and then filtering in software to
get the exact bandwidth you need.

Matt

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, 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?
Thank you all.

Vlad.

Matt E. wrote:

(I am not using decim 25 and interpol 2, because it is said in the


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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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