Re: [Discuss-gnuradio] HELP! - Problem with radio application deploy

[Aditya Dhananjay]

Tom, that was a very useful explanation. Thanks!

On Tue, Nov 19, 2013 at 7:29 PM, Tom Rondeau <address@hidden> wrote:

On Tue, Nov 19, 2013 at 7:19 PM, Paul B. Huter <address@hidden> wrote:
> I recall that that was what you had mentioned yesterday, but could you
> explain why setting it to 15M will allow me to grab 30M?

Sure.

We're working at complex baseband. The signal you capture at 50 Msps
will be observable from -25 MHz to +25 MHz. Because it's a complex
signal, we have both sides around 0 Hz; also, it means that for a 50
Msps sampling rate, we can "see" 50 MHz of bandwidth.

So to get the middle 30 MHz, you want to define a filter that extends
from -15 to +15 MHz. A low pass filter is a real-valued signal. We
define it from 0 to 15 MHz, but because it's real, it's symmetric
around 0 and thus extends from -15 to +15 MHz.

Also, be careful about using 1000 Hz as your transition band. That's
still going to generate a gigantic filter. Using a 100 kHz transition
band with a 50 Msps sampling rate produces a filter 909 taps long,
which is very, very long.

Use gr_filter_design (launched from the command line) to design a
filter that's suitable. You can probably get away with a transition
band of 1 MHz.

Tom

> On Tue, Nov 19, 2013 at 6:17 PM, Tom Rondeau <address@hidden> wrote:
>>
>> On Tue, Nov 19, 2013 at 6:35 PM, Paul B. Huter <address@hidden>
>> wrote:
>> > I am trying to deploy my radio application (using GRC), and I am running
>> > into a problem. I am sampling at 50MHz and trying to pare things down to
>> > 30MHz using a Low Pass Filter, defined as follows:
>> >
>> > Decimation = 50
>> > Gain = 1
>> > Sample Rate = samp_rate (50M)
>> > Cutoff Freq = 30000000 (30M)
>> > Transition Width = 1000 (1k)
>> >
>> > I get an error telling me cutoff frequency is greater than sample rate /
>> > 2
>> > (30 > 50/2).
>> >
>> > How can I get down to 30MHz? I do intend to look at smaller "chunks" of
>> > data
>> > at a time, so I am perfectly okay with taking data at 50MHz and cutting
>> > it
>> > down later, I just wanted to avoid gather unnecessary data.
>> >
>> > Thanks!
>>
>> Set the cutoff freq to 15M, not 30M.
>>
>> Tom
>
>

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