The attenuator in the AD9361 transceiver chip used on the B210 is pretty
accurate. For a certain frequency, all you need is one measurement. Then
the output power will track pretty well with the gain setting. BTW, the
TX gain range on the B210 is 0 to 89.8 in 0.2 dB steps. Above around 80
or so, wide band signals start to get distorted with 3rd order IMD.
You have to connect your B210 directly to the spectrum analyzer,
preferably with a 20 dB or more attenuator just to be safe.
However for OFDM (or any noise like signal), you have to compensate for
the spectrum analyzer resolution bandwidth. Here's an example. The
signal is 5.83 MHz wide OFDM and the resolution bandwidth is 30 kHz. The
compensation would then be 10 log 30000/5830000 = -22.9 dB. So the
approximate -61 dBm reading on the spectrum analyzer is really -61 dBm +
22.9 dBm = -38.1 dBm of average power.
OFDM power1
If you have a spectrum analyzer that can automatically measure channel
power, it's easier and more accurate.
OFDM power2
The spike to the right of the main signal is the DC offset shifted out
of band with UHD offset tuning.
Ron
On 04/24/2018 11:45 PM, Prabhat Kumar Rai wrote:
Dear Marcus & Vitt,
Thanks for your suggestion.
As I am new in this field I don't know how to calibrate spectrum
analyser with usrp. Is there any video tutorial or any website link
you can provide will be helpful? I have searched on internet but
didn't get a thing for calibration.
Suggestion from other members is also welcome.
Regards
Prabhat
On Tue, Apr 24, 2018 at 3:03 PM, Müller, Marcus (CEL) <[email protected]> wrote:
You'll have to calibrate that yourself. There's no fixed formula for
all possible individual devices, and all possible signals (with all
possible PAPRs, which is especially relevant for OFDM).
Best regards,
Marcus
On Tue, 2018-04-24 at 13:05 +0530, Prabhat Kumar Rai wrote:
Hello,
I am using tx_ofdm.grc file to transmit a signal via USRP B210. I am
fixing my system as center_freq 1.8GHz, BW 20MHz, Sample rate 1.92M,
Gain 20 dB, when I am checking the gain in the spectrum analyser it
comes out to be -77dBm, I know that channel is giving me around 30 dBm
attenuation still I am unable to compute the actual transmit power of
usrp.
I know that only way to know transmit power is using the combination
of frequencies, sampling rates, master clock rates, analog bandwidths
and gain but don't know how and where to use them, all of them are
already in a usrp_sink block.
Is there any other way to compute actual transmit power of Usrp or any
other formulae???
I have to use that power for RSSI calculation.
Regards,
Prabhat
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