Re: [Discuss-gnuradio] How does Rx Gain affect the incoming signal? GNURadio IEEE 802.11a/g/p

[Derek Kozel]

Hello Christian,

The RX gain value controls the analog gain of the USRP's receiver. If you run uhd_usrp_probe on the command line you will see lines such as the following which show the gain range and step size:

|   |   |       RX Dboard: A
|   |   |   ID: TwinRX v1.0 (0x0091)
|   |   |     _____________________________________________________
|   |   |    /
|   |   |   |       RX Frontend: 0
|   |   |   |   Name: TwinRX RX0
|   |   |   |   Antennas: RX1, RX2
|   |   |   |   Sensors: lo_locked
|   |   |   |   Freq range: 10.000 to 6000.000 MHz
|   |   |   |   Gain range all: 0.0 to 95.0 step 1.0 dB

For every dB of gain you add at the frontend you should see a 1 dB increase in signal power amplitude. I recommend using uhd_fft or the USRP source, Frequency Sink, and a Range widget controlling RX gain to look directly at the signal. This should give you an intuitive sense of how RX gain affects your signal amplitude.

From your table it looks like you are trying to change the RX gain in steps smaller than the USRP supports.

This document looks like a quick intro to power and voltage changes with dB.
https://neurophysics.ucsd.edu/courses/physics_120/A%20Practical%20Guide%20to%20Decibels.pdf

On Wed, Jul 5, 2017 at 10:24 PM, Cristian Rodríguez <address@hidden> wrote:

Hi all.

I'm trying to understand how Rx gain, the paremeter that is set in the USRP Source block, affects the input signal. I would like to have an equation for that.

I'm doing the next experiment:

• I take the input signal from the USRP Source block to a MAG^2 block.
• From MAG^2 block to a moving average with a window of 50K.
• My sample rate is 10M that means that my moving average is calculated over 5ms.
• I'm sending a signal from a transmitter each 5ms.
• Finally I capture the value of the moving average, which is always around an average value.

Now, I change Rx gain and it is what i get:

The value of the output of the moving average is changing very strange. I was trying to write an equation for that, but i wasn't able, i used Rx gain in dB, in dBm as a constant, but i wasn't able to build a relation. In an equation i'm doing as follow:

Then I think that if I include Gtx, my equation results in:

But that doesn't make sense with the data that I get.

I would really greatful if someone can explain where is my mistake.

Thanks in advance.

Best regards,

Cristian

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