[Discuss-gnuradio] Setting tx_gain > 20 dB => no effect? (USRP2, XCVR2450)

[Sven Oliver Kreis]

Hi there,

I am using an USRP2 with Gnu Radio Companion 3.3 and XCVR2450 as
daughterboard. I created a simple signal generator with grc.

Signal Source (Sample Rate 12.5M, Waveform: Sine, Frequency: 1,
Amplitude also 1, Offset 0) connected to USRP2 Sink (Interpolation 128,
Frequency: 2,481GHz)

I am also using one micaz node for RSSI Measurement. (using RssiToSerial
and a serial programming board) Distance between SDR and micaZ about
0.5m.

Taking 10000 RSSI Values per dB Step (stopping, changing Gain +1 in
USRP2 Sink, starting again with GRC ) I got the attached result.
(attached max RSSI value, can also attach average value, which looks
similar if requested)

I started at 0 dB Gain, so x= 1 <=> 0 dB GAIN at USRP2 Sink.

Noise floor is at -97 dBm (not in the plot). Starting USRP2 with Gain 0
gets it to about -62/-63 dBm at 0.5m distance.
Increasing gain increases RSSI value as expected. Not quite linear but I
expected that.

At a gain of 20 the RSSI value reaches about -46 dbM. Increasing USRP2
Sink gain over 20 doesn't change anything. It seems to stay at about
-46/-45 dBm.

Printing gain range results in 0 to 30 dB. Also mingain = 0 and maxgain
= 30.

How to get more tx_gain? Something I do wrong there to make gain > 20
take effect?

I can also attach the python code, if requested.

Thx in advance
Sven Oliver

RSSIMULTIMAX.pdf
Description: Adobe PDF document