Tx-power usrp n210 and wbx doughterboard

GNU Radio
1

Hi at all,

I have problem to understand how an USRP used as a trasmitter works in
term
of transmitted power when we fix a tx_gain.

Let me explain better:

  • The WBX Daughterboard is a wide bandwidth transceiver that provides up
    to
    100 mW of output power and a noise figure of 5 dB. (extracted from
    manual)
  • UHD USRP SINK block (in gnuradio) allows to set the tx_gain(dB)

Which is the relation between these parameters???
If I want to set a specific transmitted power???

Thanks in advance,

Ing. Simone Ciccia, PhD Student
Antenna and EMC Lab (LACE),
Politecnico di Torino,
C.so Duca degli Abruzzi 24, 10129 Torino, Italy

[email protected]

2

Simone,

the actual tx power depends on a lot of parameters, and it is not a
simple setting. For one thing, it depends a lot on the actual signal you
are generating. If you need a very specific output power, you need to
calibrate your setup with a tool such as a spectrum analyzer.

In general, the 100 mW (or 20 dBm) is the max possible tx power. The
gain will reduce that. Obviously, the noise figure is not relevant for
Tx.

M

3

As Martin said, it really helps to have a spectrum analyzer. What I’ll
do is
generate a CW tone (sinusoid) of 0.707 Vp; that is 3 dB backed off from
DAC
full scale power. Then I’ll increase the TX gain setting until the TX
image
rejection starts to become poor. This will typically be 15 dBm for low
frequencies (< 500 MHz), and will increase with frequency. These
settings
would be the maximum you want to transmit. If linearity is a concern,
you’ll want to do the same but generate two tones at 0.35 Vp and look at
the
intermod levels, then back off TX gain to maybe -30 dBc intermod.

Once you have that max level set, it really depends on the statistics of
you
signal; peak-average ratio or CCDF (see link). If you stay 3 dB backed
off
from both DAC full scale and TX chain P1dB, you should be OK for most
signals unless you are doing OFDM or other multi-carrier stuff.

http://www.emce.tuwien.ac.at/hfadmin/354059/download/Characterizing_Digitally_Modulated_Signals_width_CCDF_Curves__Agilent_Application_Note_5968-6875E_.pdf

Lou
KD4HSO

Simone Ciccia S210664 wrote

  • UHD USRP SINK block (in gnuradio) allows to set the tx_gain(dB)

Which is the relation between these parameters???
If I want to set a specific transmitted power???

–
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4

I should note that want I really want to do is automate this measurement
and
generate a series of curves that show TX_gain setting vs. intermod, for
several frequencies, at -3 dB from full scale. The state of GPIB
support
under Linux doesn’t seem to be that great though.

I suppose though that you can loop-back the TX to RX via attenuator, and
offset tune the RX. This assume the RX linearity is much better than
TX.

Lou
KD4HSO

–
View this message in context:
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Sent from the GnuRadio mailing list archive at Nabble.com.