RE: Measuring transmission power from USRP B210

[Bernd Schleicher]

Hi Moses,

 

I would try to reduce the task to something that >must< work and then gradually increase the complexity.

• Maybe even start with checking the spectrum analyzer with another signal source. Do you have a programmed calibration (power) offset?
• Put a DC block at the input of the spectrum analyzer.
• Reduce sample rate
• Reduce amplitude of the signal source in flow chart. à Does the spectrum change? View wider spectrum. Do you see intermodulation products, harmonics, other spurs?
• Reduce gain of the USRP sink in flow chart à Does increasing gain follow a linear relation, i.e. 1 dB increase in gain value gives 1 dB increase of output power?
• Change center frequency à Is it the same at all center frequencies or does it have a frequency slope?
• …

 

Best regards, Bernd

 

 

 

 

Sent: Dienstag, 10. November 2020 09:12
To: Moses Browne Mwakyanjala <mbkitine@gmail.com>; GNURadio Discussion List <discuss-gnuradio@gnu.org>
Subject: Re: Measuring transmission power from USRP B210

 

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Hi

I did not tried to calibrate my USRP source up to now but I'm interesting by this subject.

• I saw in Ettus USRP manual  that :
• "The TX reference power level is defined as such: When a 0 dBFS signal is transmitted, it will leave the RF output at the selected reference power level. In other contexts, this value is sometimes referred to as "peak power level", because it's the maximum power that can be transmitted."
• It seems that on some USRP, this reference power level can be set/changed at runtime
• Given example clearly states that the source level in GRC is not directly related to the power level at URSP output in dBm
• Looking at How to set the trasmit power of USRP? on NI forum (see Sara answer)
• "Since these are uncalibrated devices, a particular gain value doesn't correspond to a particular power level"
• take a look at Sarah's reference b200_rev4_TX_FE1.pdf ‏859 KB page 61 : it seems that the output power for a B200 at 2.2 GHz, with 0  dB gain (1 in linear) is close to -5 dB for a zero gain : Could that explains your -6 dBm?
• These references do not give a clear answer. I don't clearly understand given explanation.

Regards

On 09/11/2020 17:39, Moses Browne Mwakyanjala wrote:

Hello everyone,

I'm in the process of testing a GNU Radio transmitter. The transmitter will be connected to a solid-state power amplifier (SSPA) for near-Earth satellite communications. The SSPA expects an input of 0dm from the USRP TX port. 

 

The first step is thus to measure the power from the USRP. To that end, the flowgraph generates a complex sinusoidal signal with a unit magnitude. The UHD:USRP Sink block is set to maximum gain (i.e. normalized gain of 1.0). Unfortunately, the maximum power I can see from the spectrum analyzer is around -6.93 dBm as shown below. To my understanding, the USRP can transmit up to 20 dBm as reported in the "External Connections" section at this Ettus link :

https://files.ettus.com/manual/page_usrp_b200.html.

Could anyone explain why I can't achieve a 20 dBm (or anything around it) power level? 

Thanks in advance,

Moses. 

 

 

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