Re: [Discuss-gnuradio] USRP2 LFTX maximum real baseband bandwidth?

[Matt Ettus]

On 02/25/2010 01:01 AM, TANGUY Philippe wrote:
> Hi all,
>
> I would like to send a real signal with a bandwith from DC to 25MHZ
> thanks to the USRP2 and the LFTX daughterboard.
>
> So, I generate a Real baseband signal and I use only the "I" channel.
> But, it seems that the minimun interpolation of USRP2 is 4 (USRP2faq)
> that's why I use interp=4. So, I have a bandwidth of 12.5 MHz because
> the sampling frequency is fs=25 MS/s ==> fs/2 = 12.5 MS/s. For me, it
> seems good.


If you want a 25 MHz wide signal (0 to 25 MHz), then you should generate 
it at baseband (+/-12.5 MHz), and have the digital upconverter in the 
FPGA upconvert to a center frequency of 12.5 MHz  That will give you 0 
to 25 MHz coverage.

> So, my first question is:
> Is it possible to use an interpolation of only 2 to have bandwidth=25 Mhz?

Using an interpolation of 4 gives you a bandwidth of 25 MHz.  Using an 
interpolation of 2 would give you 50 MHz, but would require 8 bit I and 
Q samples.

> Secondly, I'm not sure but if we use interp=2 we have before the DUC a
> data rate = 50 MS/s * 32 (float32) = 1600 Mbits/s ==> > Gb/s.
> However, if we use interp=2 and "short" format, we have a data rate = 50
> MS/s * 16 (short) = 800 Mbits/s ==> < Gb/s.


We don't have an interp by 2 mode yet, but it could be done.  However, 
as I stated above, I don't think you need it to do what you want to do.

Matt