Strange component at Nyquist frequency with USRP X310 and GNU Radio

Hello everyone,

I am using an USRP X310 with two TwinRX daughterboards for an Direction of Arrival application. I have a very simple GNU Radio flowgraph with just two blocks, an UHD USRP Source and a QT GUI Frequency Sink.

I have configured a sampling rate of 100 Msps and try to receive the four channels in a sync manner (there is no overflows issues since I have two SFP+ 10 GbEthrnet cables).

However, when I plot the spectrum of the receive signal it happens that at -50 and 50 MHz (half the sampling rate, i.e., the Nyquist frequency) there is the presence of some tones higher than the noise floor. I don't see this effect if I configure, for example, a rate of 50 Msps. This happens receiving signal from the air, with antennas, and also with a wired connection.

To make a basic test, we connected four matched loads to the RX inputs, and we see this effect too.

I will show you this graphically.

Spectrum capturing with sampling rate of 100 Msps in 2440 MHz. The high power at the Nyquist frequencies can be observed. The same happens for a capture in the 5500 MHz band.

If I use a sampling rate of 200 Msps a curios effect appears. In a capture at 2450 MHz band an important tone with high power appears at -50 MHz. And its power changes with time (the golden trace is the Max hold).

And if I capture with 200 Msps in the 5500 MHz band, a similar high power tone appears but at the right side (at +50 MHz)

Do you know if this beahviour is normal or can occur due the existance of some kind of damage in SDR or the daughterboard?

If this is normal, I guess that a digital low pass filter with a cut off frequency of 40 MHz could solve this (actually the analog bandwidth of the USRP X310 with TwinRX is 80 MHz). Is this correct?

Thanks in advance for you time.

PD. I tried to make a post in the list usrp-users but the mail was rejected.

Anxo Tato Arias Enxeñeiro - Investigador Senior · Área de Comunicacións Avanzadas Senior Researcher - Engineer · Advanced Communications Department (+34) 986 120 430 atato@gradiant.org

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