Re: [Discuss-gnuradio] Frequency Translating FIR Filter

[Tom Rondeau]

On Fri, Nov 22, 2013 at 7:14 PM, Paul B. Huter <address@hidden> wrote:
> I am trying to view the spectrum from 0-1MHz recorded at 50M. I used a low
> pass filter to only record 0-30MHz. My frequency translating filter is
> defined as follows:
>
> Decimation: 50
> Taps: firdes.low_pass(1, samp_rate, 500000, 100)
> Center Frequency: -500000
> Sample Rate: samp_rate
>
> samp_rate: 50M
>
> Any ideas?
>
> Paul B. Huter

Let's take a look at what the frequency_xlating filter does:
http://gnuradio.org/doc/doxygen/classgr_1_1filter_1_1freq__xlating__fir__filter__ccc.html

It filters a signal at the center frequency you set and then moves
that signal to baseband. You've set a sampling rate of 50 Msps and a
center frequency of -50 kHz. Which means you want to get the signal at
-50 kHz moved to baseband. The output signal will be from -500 kHz to
+500 kHz.

You should basically be seeing what you would if you set the center
frequency to 0 since you're only moving 50 kHz out of 1 MHz of
spectrum.

What's the exact problem you're facing, then?

(Also, once again, you're setting a very tight transition band here
that would cause a gigantic filter.)

Tom