[Discuss-gnuradio] Spectral Shape of USRP Interpolation Filter

[Ketan Mandke]

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?

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Ketan Mandke