Re: [Discuss-gnuradio] What am I really seeing?

[Tom Rondeau]

On Sun, Nov 17, 2013 at 8:51 AM, Paul B. Huter <address@hidden> wrote:
> Thanks for the response.
>
> I am using a USRP2 N210 with the shortwave receiver daughterboard. When I
> was playing around, I was getting an error when I set the sample rate on the
> USRP source to 30MHz, and was told it had something to do with dividing 100
> by 30 and not getting an integer (you get 3.33333), so it was recommended I
> use 25 or 50, with 25 giving more resolution on the results. If I am setting
> the rate to 50MHz on the source and 30MHz on the FFT sink, and I am actually
> seeing (and ultimately recording with a file sink) the whole shortwave
> spectrum, I will be satisfied.
>
> Thank you.
>
> Paul B. Huter

So you set the sampling rate to 50 Msps, which produces complex 8-bit
samples, and so it represents 50 MHz of spectrum.

When you pass this to the FFT sink, you will see all 50 MHz. You're at
complex baseband, so you'll see -25 to +25 MHz. Even if you set the
sampling rate of the sink to 30 Msps, you are still seeing 50 MHz. The
sampling rate to the sink does nothing but set the x-axis. So if you
set it to 30, the x-axis will represent -15 to +15 MHz, but a signal
at -15 MHz will actually be at -25 MHz.

You have to go through a sample rate changing block, like a decimating
FIR filter (which only gives you integer decimation rates), the
rational resampler block (which gives you a rate change of
interp/decim where interp and decim are integers), or the fractional
resampler or PFB arbitrary resampler (where the rate you set is a real
number). See the manual for how to use any of these blocks
(http://gnuradio.org/doc/doxygen/index.html).

Tom



> Andrew Davis <address@hidden> wrote:
>
> Hello, first I think you meant sample rate, not size. Who said you cant use
> 30Mhz, what kind of hardware are you using? Lastly the FFT algorithms do not
> deal with frequency in that way, you are still seeing 0-50Mhz spectrum, just
> the label now says 50Mhz.
>
> Andrew
>
>
> On Sat, Nov 16, 2013 at 10:11 AM, Paul B. Huter <address@hidden>
> wrote:
>>
>> I have a GNU Radio setup to look at shortwave. I have my sample size set
>> to 50MHz (since I was told I can't use 30). If I set my FFT to 30MHz with a
>> baseband of 15MHz, am I actually seeing the shortwave spectrum (0-30MHz)?
>> Also, any suggestions on setting the FFT Size?
>>
>> Thank you!
>>
>> Paul B. Huter
>>
>>
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