RF signal strength variation using NI USRP 2920

[Gowthamkumar]

Hi ,

Is it possible to vary the RF signal strength using USRP 2920? If so, please guide me to do that. I want to transmit RF signal of 900MHz with different GCQ value( shows the signal strength)

Thanks and Regards,

Gowtham Bhogi

[Chris-NI · Accepted solution]

Hi Gowthamkumar,

Unfortunately, since the USRP is not a calibrated device, you cannot specify an absolute power output level.  However, you can set the gain on the device.  The power output will vary depending on the center frequency and bandwidth of the channel you are creating, so it is recommended that you use a power meter to verify absolute power levels.  The gain is also not guaranteed to be linear.  The official caveat is stated as follows:

The USRP devices are not calibrated. The gain value does not represent an absolute gain and does not have linear behavior. Different devices exhibit different gain curves for different carrier frequencies. You may need to experiment to determine the correct gain setting for your application.

With that in mind, you can set the gain (in dB) using the niUSRP Configure Signal VI, which is found in the Tx palette of the USRP API in LabVIEW.

Regards,

[derjan]

For my studies I did some investigations on the USRPs gain-behaviour myself. I found that in addition to what Chris said, the gain is also time-variant. After starting the transmission it can take several minutes until the gain slowly stabilizes. Depending on the actual gain settings, the difference between the start of the transmission and the stabilized value can be as large as 1-2dB.

To measure this I transfered a complex sine signal with constant amplitude and plotted the average received power, relative to some arbitrary level (i.e. do not look at the absolute numbers, just at the differences). I've attached a screenshot of capturing this data for 6 minutes. The received power level drops by about 1.3dB in this case.

It should be noted that this process is so slow that it probably does not matter for the majority of the USRP applications. But if long-term measurements are to be done and varying device gain could skew the measurements, be sure to discard the first minutes of data.

Regards,

Jan D.

Vodafone Chair

TU-Dresden

[Chris-NI]

Great point, derjan!  Since RF circuits are very sensitive to operating temperature, there will indeed be a warm-up time during which power output will vary as the hardware reaches a steady thermal state.

Regards,

[YYYs]

Hi derjan,

would you please so kind and share your VI with us?

Thanks