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 _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
