Re: [Discuss-gnuradio] Spectral Shape of USRP Interpolation Filter

[Eric Blossom]

On Tue, Apr 24, 2007 at 06:32:49PM -0500, Ketan Mandke wrote:
> This is my first post to the mailing list, so please bear with me if it is a
> little lengthy. After running some tests on the interpolation filters for
> the USRP, I've found that the filter has a significant loss at the Nyquist
> frequency. From my measurements, I observed about 16 dB of attenuation from
> DC to Nyquist.
> 
> The following describes the equipment, parameters, and setup of the
> experiment:
> 
> The data was collected using a National Instruments spectrum analyzer. The
> USRP board, with a FLEX400 daughter board, was connected to the NI spectrum
> analyzer through an SMA cable and a series of attenuators. We generated a
> complex exponential with fixed amplitude, with *usrp_siggen.py*, and swept
> it from DC to Nyquist and observed the received power on the NI spectrum
> analyzer.
> We observed from the frequency response of the received signal that the
> interpolation filter for the USRP had about 16 dB of attenuation (from DC to
> Nyquist). For this particular experiment, the following parameters were
> used:
> 
>   - Interpolation Factor = 64 (i.e. sample rate = 2 MHz)
>   - Carrier Frequency = 420 MHz
>   - Attenuation = 24 dB (4 x 6 dB attenuators)
>   - Transmitted Waveform Amplitude = 1000
> 
> A figure of our results can be found at:
> http://hydra.ece.utexas.edu/internal/
> 
> So after all that explanation, here are my actual questions:
> 
> 1) Shouldn't the interpolation filter have a spectral response that is
> almost flat from DC to Nyquist? This would require that the upsampled
> sequence occupy some excess bandwidth, but I don't really see any problem
> with such a design.
> 
> I realize that one solution might be to oversample my transmitted signal in
> software before sending it to the USRP. However, I was hoping to avoid doing
> additional interpolation.
> 
> 2) Also, is there any way to modify the shape of the interpolation filter?
> In particular, has anyone implemented other interpolation filters and a
> mechanism for selecting from such a set?


What were the command line arguments you specified to usrp_sigggen.py?

What command line parameter were you sweeping?

Eric