Re: Measuring transmission power from USRP B210

[Marcus D. Leech]

On 11/09/2020 11:39 AM, 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. 

Could we perhaps see a minimal flow-graph that demonstrates the issue?

Generally, setting the baseband magnitude to 1.0 at the same times as setting to MAX gain can cause linearity issues.