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I am trying to build an FMCW radar using a USRP N210 with a CBX40 daughterboard. I generate a sweep from 1.5GHz to 5GHz. According to their respective datasheets, the daughterboard and the USRP should work fine up to 6GHz. However, as the frequency increases, the output power from the TX/RX port decreases significantly, even though the transmitted power and gain are constant in GNURadio.

I attach a frequency domain graph of the received signal's power when connecting a wire between them. The wire could introduce some losses, but still the power decrease with frequency is very evident. enter image description here

I would appreciate if you could tell me the reason why this is happening, and if you know the way to solve it.

Thanks in advance,

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2 Answers 2

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That's an expected phenomenon: Real-world physical systems tend to be frequency-selective (i.e. not constant over frequency), and "at large scale" low-pass systems.

This applies to amplifiers, mixers, oscillators, and even transmission lines and connectors.

So that's normal.

Ettus even publishes exactly such measurements at https://files.ettus.com/performance_data/cbx/.

When you see a device that doesn't have such behaviour, there's either something very expensive going on – that's why high-end spectrum analyzers are way more expensive than USRPs – or someone measured this and then applied correction for that. Usually, it's a mix of both.

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  • $\begingroup$ Thanks a lot Marcus, I've done another measurement, and, as you say, the results are the expected ones. I've compared the power levels with the ones in the document you shared and they are very similar. Thank you very much one more time! $\endgroup$
    – Fati
    Oct 21, 2020 at 12:48
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In addition, the $S_{11}$ parameters of the both TX/RX antennas should be typically less than -15 dB for a better measurement in the frequency interval that you sweep. Then, you will have a better observation about how much the received signal power decreases as the operating frequency increases. Otherwise, your measurements will be affected by the characteristics of the antennas.

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